Astronomy Answers
Astronomical Dictionary


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This dictionary explains a number of difficult astronomical words that are also used elsewhere in these web pages. If you find a word in these pages that you do not understand, then you can check if the word is explained in this dictionary.

Some words are followed by an explanation, and some only have references. A reference to another word in the dictionary is indicated as →, and a reference to another web page as ⇒.

If the first sentence of the explanation of a word begins with this kind of letters, then that sentence explains the origin of the word. The language of origin of the word is shown between square brackets; usually this is [Greek] or [Latin].


Letter sections:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z




Aa

A>

the absorption line

absorptio = [Latin] drink

An absorption line is a spectral line (a narrow band of colors) in which an object shines less brightly than in other similar colors.

absorption_lines

the active region

An active region is an area at the surface of the Sun with a lot of magnetic field in the form of sunspots, pores, and plage. Large active regions can grow to be 160,000 km long (equal to four times around the Earth) and can last for two or more months, but small active regions appear and disappear in a matter of days.

active_regions

Adrastea

A moon of 20 km diameter at about 129,000 km from the planet Jupiter. The gravity at its surface is about 0.0005 times as strong as on Earth. The moon goes once around its planet in about 7.2 hours. The moon was discovered in 1979. Also called JXV (Jupiter fifteen). Its provisional designation was S/1979 J1.

Air_Pump

Aitne

A moon of 3 km diameter at about 23,229,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 723.1 days. The moon was discovered in 2001. Also called JXXXI (Jupiter thirty-one). Its provisional designation was S/2001 J11.

the albedo

albedo = [Latin] white paint or color

The albedo of an object is a number between 0 and 1 that indicates which fraction of incoming light is immediately reflected by the object. The albedo of the Earth is about 0.3: the Earth and its atmosphere reflect about 30 percent of the sunlight that hits them. All other things being equal, a planet looks brighter and has a lower temperature when its albedo is greater.

Albert_Einstein

Albiorix

A moon of 32 km diameter at about 16,182,000 km from the planet Saturn. The gravity at its surface is about 0.0006 times as strong as on Earth. The moon goes once around its planet in about 769.0 days. The moon was discovered in 2000. Also called SXXVI (Saturn twenty-six). Its provisional designation was S/2000 S11.

the Almagest

The Almagest is the name by which the best-known astronomical book by Claudius Ptolemy is known since about the 10th century. This book was written by Ptolemy in the 2nd century in Greek (in Alexandria in Egypt) and was then called μαθηματική σύνταξις (Mathematike Syntaxis = Mathematical Treatise). It contains a complete description of a geocentric model for all planets then known and for the Sun and Moon, with explanations for how to calculate their positions and phenomena. The book became the standard book for astronomy, so it was later called Hè Megalè Syntaxis (The Great Treatise). In the 9th century, it was translated by Islamic astronomers into Arabic. The Greek version was forgotten in Western Europe. In the 12th century, Gerard of Cremona translated an Arabic version, which was then called al-mjsṭy "the greatest", into Latin, and so the book became known in Western Europe again, but now by the Latinized version of its Arabic title, the Almagest.

Altar

altitude

Altitude and elevation measure how far something is above some reference plane. In daily life, they are used to indicate how far above sea level something is. In astronomy, they are used for that as well, but also to indicate how far (in degrees) something like a planet or a star is above the horizon. This can be confusing, so the preferred use is for elevation to be used for the height above sea level (in meters or feet), and altitude for height above the horizon (in degrees).

In the horizontal coordinate system, altitude is the coordinate that measures the height above the horizon (in degrees). The other coordinate is the azimuth. Because the true horizon depends on the local landscape and the exact location of the observer, astronomers often use an "artificial" horizon that runs exactly midway between the zenith and nadir. If you read about astronomical altitudes, then you may assume they are measured relative to the artificial horizon, unless the accompanying text says otherwise.

Amalthea

A moon of 172 km diameter at about 181,400 km from the planet Jupiter. The gravity at its surface is about 0.0019 times as strong as on Earth. The moon goes once around its planet in about 12.0 hours. The moon was discovered in 1892. Also called JV (Jupiter five).

Ananke

A moon of 28 km diameter at about 21,276,000 km from the planet Jupiter. The gravity at its surface is about 0.0010 times as strong as on Earth. The moon goes once around its planet in about 633.9 days. The moon was discovered in 1951. Also called JXII (Jupiter twelve).

And

Andromeda

Andromeda is a northern constellation, close to the Milky Way. The official abbreviation is And.

Andromedae

the angle

In geometry, an angle is a difference between directions. Angles are commonly measured in degrees. A full circle is equivalent to 360 degrees: If you turn around completely and end up looking in the same direction as you did before, then you've turned over 360 degrees. A right angle is equal to 90 degrees, and the angles between the sides of an equilateral triangle (one with three sides of equal length) is 60 degrees.

A degree is also written as °, so 31° is 31 degrees. A degree is further divided into 60 arcminutes (also written as 60' or 60 minutes of arc), and an arcminute into 60 arcseconds (also written as 60" or 60 seconds of arc), so one degree is equal to 3600 arcseconds. Astronomical objects often appear so small in the sky that their apparent angular sizes are expressed in arcminutes or arcseconds. For instance, the Sun and Moon have angular diameters of about 30 arcminutes, the planet Jupiter has an angular diameter of about 40 arcseconds, and the planet Pluto of 0.1 arcseconds.

The smallest detail a telescope can possible see (which is called its resolution) is also often measured in arcseconds. It is about equal to 0.13 arcseconds divided by the diameter of the primary entrance of the telescope in meters, or to 130 arcseconds over the diameter in centimeters, or to 5.0 arcseconds divided by the diameter in inches. With a perfect 6-inch telescope, you may be able to see details as small as 0.8 arcseconds - so in such a telescope Jupiter would appear as a small disk, but Pluto would look like a point, just like the stars. Often, the resolution of telescopes is worse than what this formula yields, because the mirrors or lenses are not perfect and because the atmosphere of the Earth tends to blur the images.

angles

anomalistic

anoomalia = [Grieks] deviation, roughness

In astronomy, anomalistic means that it has something to do with the apsides of the orbit of a celestial body. The anomalistic month is the time between two passages of the Moon through its perigee.

anomalistical

anomalistic_month

anomalistic_months

the anomaly

anoomalia = [Greek] deviation, roughness

In astronomy, anomaly is used for different kinds of angles that are important when calculating the position of objects in their orbits. Below, several different anomalies are explained for the orbit of the Earth around the Sun (or rather: of the Earth around the barycenter of the Solar System, but that is almost the same thing), but they are also used for orbits of other celestial objects.

  1. The true anomaly is the angle (as seen from the Sun) between the Earth and the perihelion of the orbit of the Earth. When the true anomaly is equal to 0 degrees, then the Earth is closest to the Sun (or: in its perihelion). When the true anomaly is equal to 180 degrees, then the Earth is furthest from the Sun (in the aphelion).
  2. The mean anomaly is what the true anomaly would be if the Earth moved with constant speed along a perfectly circular orbit (with an eccentricity equal to zero) around the Sun in the same time. Just as for the true anomaly, the mean anomaly is equal to 0 in the perihelion and to 180 degrees in the aphelion, but at other points along the Earth's orbit the true and mean anomalies are not equal to one another. The mean anomaly is often used for one of the orbital elements.
  3. The eccentric anomaly is an angle that is related to both the mean and the true anomaly. You encounter this angle if you solve Kepler's Equation to find the true anomaly from the mean anomaly.

Ant

the antimatter

Antimatter is in some invisible characteristics the opposite of ordinary matter. Antimatter is made up of antiparticles just like ordinary matter is made up of ordinary particles. Antimatter looks and acts just like ordinary matter, but if an ordinary particle and its antiparticle meet then they both turn into energy (according to the famous equation of Einstein (E = mc^2)), and energy can be turned into pairs of an ordinary particle and the corresponding antiparticle.

Antlia

Antlia (Air Pump) is a southern constellation, close to the ecliptic. The official abbreviation is Ant.

Antliae

Aoede

A moon of 4 km diameter at about 23,981,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 758.5 days. The moon was discovered in 2003. Also called JXLI (Jupiter forty-one). Its provisional designation was S/2003 J7.

the aphelion

apo = [Latin] from, Helios = [Greek] Sun

The aphelion is the point in an orbit around the Sun that is furthest from the Sun. The opposite point is called perihelion. The generic word for the furthest point in an orbit around some other object is apofocus.

Apodis

the apofocus

plural apoapsiden; apo = [Latin] from

The apofocus of an orbit of one object around another is the point at which the one object is furthest away from the other object. Apoapsis is the general term for such a point, but there are also many specific terms for specific cases: the aphelion is the furthest point from the Sun in an orbit around the Sun. Likewise, apoastron is linked to other stars, apogee to the Earth, and apojove to Jupiter. The opposite is perifocus.

apparent_magnitude

Aps

apsides

the apsis

hapsis = [Greek] connection

An apsis is a position in an orbit that is at an extreme distance (either a minimum or a maximum) to the central object. The minimal distance is attained in the perifocus and the maximal distance in the apofocus.

Apus

Apus (Bird of Paradise) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Aps.

Aql

Aqr

Aquarii

Aquarius

Aquarius (Water Carrier) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Aqr.

Aquila

Aquila (Eagle) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Aql.

Aquilae

Ara

Ara (Altar) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Ara.

Arae

arccos

Arche

A moon of 3 km diameter at about 22,931,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 709.2 days. The moon was discovered in 2002. Also called JXLIII (Jupiter forty-three). Its provisional designation was S/2002 J1.

Archer

the arcminute

arcminute, plural arcminutes; arcus = [Latin] arch, minutus = [Latin] made small

An arcminute or minute of arc is a unit of angle. An arcminute is the 60th part of a degree. An arcminute is further divided into 60 arcseconds.

arcminutes

arcosh

the arcsecond

arcsecond, plural arcseconds; gradus minutus secundus = [Latin] second diminished degree

An arcsecond or second of arc is a measure of angle. An arcsecond is the 60th part of an arcminute.

arcseconds

arcsin

arctan

Ari

Ariel

A moon of 1157 km diameter at about 190,900 km from the planet Uranus. The gravity at its surface is about 0.0275 times as strong as on Earth. The moon goes once around its planet in about 60.5 hours. The moon was discovered in 1851. Also called UI (Uranus one).

Aries

Aries (Ram) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Ari.

Arietis

arographic

Ares = [Greek] Mars, graphia = [Greek] description

Arographic means it describes the planet Mars. The arographic coordinate system has the martian equator for base plane, and uses the longitude and latitude for coordinates.

arographical

Arrow

arsinh

artanh

the ascending equinox

The ascending equinox or vernal equinox is the equinox at which the Sun moves from south to north of the celestial equator. At that moment, spring starts in the northern hemisphere and autumn in the southern hemisphere. In the Gregorian calendar, the ascending equinox is near 21 March.

ascending_equinox

ascending_equinoxes

ascending_node

ascending_nodes

the asteroid

asteroid, plural asteroids; astron = [Greek] star, -oid = [Greek] -like

An asteroid is a rock that orbits the Sun. The largest asteroid has a diameter of about 1000 km, but most are much smaller than that. If they get small enough, then they are sometimes called meteoroids. Most asteroids orbit the Sun at distances between than of the orbits of Mars and Jupiter.

asteroids

astrologer

astrologers

astrological

the astrology

Astrology is a group of methods to predict the future from the positions of planets and other celestial objects in the sky at important moments of the subject's life. Astrology is not scientific and science says that astrology is nonsense. People who do astrology are called astrologers.

astronomer

astronomers

astronomical

the AU or Astronomical Unit

An AU is very close to the average distance between the Sun and the Earth. Distances between the planets and the Sun are often expressed in AU. 1 AU equals about 93 million miles or about 150 million km.

astronomical_name

astronomical_names

astronomical_units

astronomy

astron = [Greek] star, nomos = [Greek] knowledge

Astronomy is the science that studies everything outside of the Earth. This includes, among other things, the Moon, planets, Sun, stars, black holes, Milky Way, and Universe. People who do astronomy are called astronomers.

Atlas

A moon of 32 km diameter at about 137,700 km from the planet Saturn. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 14.5 hours. The moon was discovered in 1980. Also called SXV (Saturn fifteen). Its provisional designation was S/1980 S28.

atmosphere

atom

atomic_nucleus

AU

Aur

Auriga

Auriga (Charioteer) is a northern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Aur.

Aurigae

Autonoe

A moon of 4 km diameter at about 24,046,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 761.6 days. The moon was discovered in 2001. Also called JXXVIII (Jupiter twenty-eight). Its provisional designation was S/2001 J1.

autumn

autumnal_equinox

autumnal_equinoxes

autumn_equinox

the azimuth

as-sumut = [Arab] the roads

The azimuth is the coordinate from the horizontal coordinate system that indicates the direction along the horizon. The azimuth is measured in degrees, but not everyone uses the same range of azimuth or the same zero point. Sometimes the azimuth is measured between −180 and +180°, sometimes between 0 and 360°, and sometimes with 0° in the south, and sometimes with 0° in the north. For astronomical application it is convenient to set 0° in the south and to measure azimuth between −180 and +180°: that provides the best fit to the hour angle.

Bb

< B>
babylonian_calendar

the barred spiral galaxy

A barred spiral galaxy is a spiral galaxy with a bar-shaped structure through the center.

barred_spiral_galaxies

the barycenter

barus = [Greek] heavy

The barycenter is the same as the center of mass.

Belinda

A moon of 80 km diameter at about 75,300 km from the planet Uranus. The gravity at its surface is about 0.0015 times as strong as on Earth. The moon goes once around its planet in about 15.0 hours. The moon was discovered in 1986. Also called UXIV (Uranus fourteen). Its provisional designation was S/1986 U5.

Berenice's_Hair

Bianca

A moon of 51 km diameter at about 59,200 km from the planet Uranus. The gravity at its surface is about 0.0010 times as strong as on Earth. The moon goes once around its planet in about 10.4 hours. The moon was discovered in 1986. Also called UVIII (Uranus eight). Its provisional designation was S/1986 U9.

the Big Bang

The Big Bang is the beginning of the very strong growth of the Universe, now about 13.7 thousand million years ago. Since that event, the Universe has been steadily expanding. This expansion gives almost all galaxies a redshift.

Big_Bang

Bird_of_Paradise

the bissextile day

bissext- = [Latin] the second sixth

A bissextile day is a day that is inserted every few years to keep the calendar in line with the tropical year, of which the average length is not a whole number of days. In the Gregorian calendar the date of 29 February occurs only in a leap year. This date is nowadays seen as the extra day of a leap year, but in the original Julian calendar as installed about two thousand years ago by the Romans the 23rd day of February was doubled, and the last day of February had the same name in a leap year as in a non-leap year. The Romans referred to the 23rd day of February as the 6th day before the beginning of March, and the extra inserted day was the "second sixth" day, or bissextile day.

bissextile_days

the black hole

A black hole is a thing in which mass is pressed together so tightly that its gravity is so strong that even light cannot escape from it.

black_hole

black_holes

the blueshift

A blueshift is a doppler shift of distinguishing marks (such as spectral lines) in the frequency spectrum of light to greater frequencies, so that yellow light changes in the direction of blue. The opposite of blueshift is redshift.

blue_shift

Boo

Bootes

Bootes is an equatorial constellation. The official abbreviation is Boo.

Bootis

the brightness contrast

A contrast is a difference. A brightness contrast is a difference in how bright two things seem to be. For example, there is a great brightness contrast between the Sun and the sky.

broadband_filter

broadband_filters

Bull

Cc

<A C>
Cae

Caeli

Caelum

Caelum (Chisel) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Cae.

the calendar

Kalendae = [Latin] first day of the month

A calendar is

  1. a method to combine days into larger units of time, such as weeks, months, and years.
  2. a representation of a division of time, for example on paper.

calendars

calendar_year

calendar_years

Caliban

A moon of 98 km diameter at about 7,231,000 km from the planet Uranus. The gravity at its surface is about 0.0021 times as strong as on Earth. The moon goes once around its planet in about 587.5 days. The moon was discovered in 1997. Also called UXVI (Uranus sixteen). Its provisional designation was S/1997 U1.

Callirrhoe

A moon of 8 km diameter at about 24,103,000 km from the planet Jupiter. The gravity at its surface is about 0.0003 times as strong as on Earth. The moon goes once around its planet in about 764.3 days. The moon was discovered in 1999. Also called JXVII (Jupiter seventeen). Its provisional designation was S/1999 J1.

Callisto

A moon of 4820 km diameter at about 1,882,700 km from the planet Jupiter. The gravity at its surface is about 0.1261 times as strong as on Earth. The moon goes once around its planet in about 16.7 days. The moon was discovered in 1610. Also called JIV (Jupiter four).

Calypso

A moon of 19 km diameter at about 294,710 km from the planet Saturn. The gravity at its surface is about 0.0003 times as strong as on Earth. The moon goes once around its planet in about 45.4 hours. The moon was discovered in 1980. Also called SXIV (Saturn fourteen). Its provisional designation was S/1980 S25.

Cam

Camelopardus

Camelopardus (Giraffe) is a northern constellation, close to the Milky Way. The official abbreviation is Cam.

Cancer

Cancer (Crab) is an equatorial constellation, close to the ecliptic. The official abbreviation is Cnc.

Cancri

Canes Venatici

Canes Venatici (Hunting Dogs) is a northern constellation. The official abbreviation is CVn.

Canis Major

Canis Major (Great Dog) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is CMa.

Canis Minor

Canis Minor (Little Dog) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is CMi.

Canis_Majoris

Canis_Minoris

Canum_Venaticorum

Cap

Capricorni

Capricornus

Capricornus (Sea Goat) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Cap.

Car

Carina

Carina (Keel) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Car.

Carinae

Carme

A moon of 46 km diameter at about 23,404,000 km from the planet Jupiter. The gravity at its surface is about 0.0017 times as strong as on Earth. The moon goes once around its planet in about 731.3 days. The moon was discovered in 1938. Also called JXI (Jupiter eleven).

Carpo

A moon of 3 km diameter at about 16,989,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 452.3 days. The moon was discovered in 2003. Also called JXLVI (Jupiter forty-six). Its provisional designation was S/2003 J20.

Cas

Cassiopeia

Cassiopeia is a northern constellation, close to the Milky Way. The official abbreviation is Cas.

Cassiopeiae

celestial_equator

celestial_equators

celestial_meridian

celestial_pole

celestial_poles

Celsius

celsius

Cen

Centaur

Centauri

Centaurus

Centaurus (Centaur) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Cen.

the center of mass

The center of mass of a system is the average of the positions of all objects in the system, each position weighted with the mass of the object. The sum of all the forces from outside the system on the objects of the system is the same as the force on a single particle with the total mass of the system at the center of mass. Forces between the objects in the system cannot influence the center of mass.

centers_of_mass

central_american_calendar

central_american_calendars

the centrifugal force

The centrifugal force is a force that points away from the center of the curve that you're taking. The centrifugal force pushes you to the side of the car away from the center of the curve that the car is taking, pushes you to the outer edge of a merry-go-round that you're on, and pushes your laundry against the inside of the rim of your washing machine when it is spinning fast. The centrifugal force is a virtual force, of which the strength depends on your speed and on the diameter of the curve. The opposite force is the centripetal force.

centrifugal_force

centrifugal_forces

the centripetal force

The centripetal force is a force that points towards the center of the curve that you're taking. If no forces act on you, then you'll keep going straight at the same speed. If you want to make a curve then some force must pull you in the right direction. That's a centripetal force. The centripetal force that keeps planets in their orbits around the Sun is the force of gravity. The opposite force is the centripetal force.

centripetal_forces

centuries

the century

plural: centuries

A century is a period of 100 years. The 21st century contains the years 2001 until and including 2100.

Cep

Cephei

Cepheus

Cepheus is a northern constellation, close to the Milky Way. The official abbreviation is Cep.

Cet

Ceti

Cetus

Cetus (Whale) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Cet.

Cha

Chaldene

A moon of 3 km diameter at about 23,100,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 717.1 days. The moon was discovered in 2000. Also called JXXI (Jupiter twenty-one). Its provisional designation was S/2000 J10.

Chamaeleon

Chamaeleon (Chameleon) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Cha.

Chamaeleontis

Chameleon

Charioteer

Charon

A moon of 1186 km diameter at about 19,600 km from the planet Pluto. The gravity at its surface is about 0.0313 times as strong as on Earth. The moon goes once around its planet in about 6.9 days. The moon was discovered in 1978. Also called PI (Pluto one). Its provisional designation was S/1978 P1.

Chisel

the chromosphere

chrooma = [Greek] color; sphairos = [Greek] ball

The chromosphere is a layer in the Sun that is roughly between about 250 miles (400 km) and 1300 miles (2100 km) above the solar surface. The temperature in the chromosphere varies between about 4000 K at the bottom (the so-called temperature minimum) and 8000 K at the top (6700 and 14,000 degrees F, 3700 and 7700 degrees C), so in this layer (and higher layers) it actually gets hotter if you go further away from the Sun. The density in the chromosphere is much, much smaller than the density of air at sea level on Earth. At the top of the chromosphere there are only about 10 thousand million atoms in each cubic centimetre (100 thousand million atoms per cubic inch). The chromosphere shows up in images taken in the center of the H-alpha spectral line and also (briefly) near the beginning and end of a total solar eclipse.

chromospheres

chromospheric

Chromospheric means it has something to do with the chromosphere of the Sun or a star.

chromospherical

the Chronological Julian Date

The Chronological Julian Date (CJD) counts the number of days since midnight local time at the beginning of January 1st, −4712 on the Julian proleptic calendar. CJD depends on the local timezone, but JD does not. CJD counts from midnight local time, JD from noon UTC.

CJD can have a fractional part. CJD 2451545.75 points at an instant 0.75 days or 18 hours beyond midnight local time at the beginning of January 1st, 2000 on the Gregorian calendar, i.e., January 1st at 18:00 (= 6 pm) local time.

the Chronological Julian Day Number

The Chronological Julian Day Number (CJDN) counts the number of whole days since midnight local time at the beginning of January 1st, −4712 on the Julian proleptic calendar. You get the Chronological Julian Day Number if you round a Chronological Julian Date (CJD) down to the nearest whole value.

Chronological_Julian_Dates

Chronological_Julian_Day_Numbers

Cir

Circini

Circinus

Circinus (Compasses) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Cir.

CJD

CJDN

Clock

CMa

cme

cmes

CMi

Cnc

CNO_cycle

Col

Columba

Columba (Dove) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Col.

Columbae

Com

the coma

komè = [Greek] hair

The coma of a comet is a cloud of water vapor and dust particules around the still frozen center of the comet. The coma develops when the comet gets close enough to the Sun (within about 2.5 AU) that the water ice at the surface of the comet sublimates (changes into water vapor). The coma can grow to be many hundreds of thousands of kilometers (or miles) in size.

Coma Berenices

Coma Berenices (Berenice's Hair) is an equatorial constellation, close to the ecliptic. The official abbreviation is Com.

Comae_Berenices

the comet

komètès = [Greek] with long hair

A comet is a block of ice and dust with perhaps a rock in the middle, that orbits around the Sun. Most comets come from far beyond the furtherst planet and fly through the inner part of the Solar System in a short time. When comets come close enough to the Sun, then they usually develop a coma and one or more tails.

comets

Compasses

the conjunction

When two heavenly bodies are in conjunction, then they are very close together in the sky. When astronomers say something like "Jupiter is in conjunction" without mentioning a second heavenly body, then they mean "with the Sun". In such a case Jupiter is not visible at any time of the night. The planets that are further away from the Sun than the Earth (the superior planets) have one conjunction each synodical orbital period. The planets that are closer to the Sun (the inferior planets, Mercury and Venus) have two conjunctions per synodical orbital period: one when they pass between the Sun and the Earth (the inferior conjunction), and one when they pass behind the Sun (the superior conjunction).

the conjunction spread

The conjunction spread is a measure, introduced on this web site, for how close together planets or other things are in the sky. It is an angle that indicates how far apart the planets or other things are in the sky.

conjunctions

conjunction_spread

conjunction_spreads

the constellation

con- = [Latin] together, with; stella = [Latin] star

A constellation is

  1. a group of stars that form a pattern that made someone think of a certain person, animal, or object, which makes the pattern easier to remember and find again.
  2. an area of the sky, designated by the International Astronomical Union (IAU), in which there is a group of stars that form a pattern as described above. The area of the sky has the same name as the group of stars.

Professional astronomical publications throughout the world always use the constellations as defined by the IAU, but in the past many other constellations were proposed, and in different areas of the world completely different sets of constellations were (and are still) used.

constellations

the continuum

continuum = [Latin] something without breaks

In general a continuum is a collection of all possibilities or values, without gaps.

In astronomy, the continuum is the name for the combination of all colors that an object such as the Sun emits, and also for the broad variation from color to color in how much light is emitted. Broad means: without looking at the little details, such as spectral lines. The continuum is determined mostly by the temperature of the object. The hotter the object is, the brighter it shines. The color at which an object shines brightest also depends on the temperature. Hot objects such as the Sun shine brightest in yellow light; less hot objects shine most in red light, and cool objects shine brightest in (invisible) infrared light.

the convection

convectio = [Latin] bring together, from con- = with, and vectio = carry

Convection is a form of energy transport in which the material with the energy in it moves. The hotter material moves toward the cooler area and the cooler material toward the hotter area. On Earth, you can see convection in a pan of boiling water (where hot water moves up and cooler water moves down), and also in a thunderstorm (where warmer, moist air moves up and forms clouds). You can also find convection in the convection zone of the Sun, and in granulation.

the convection zone

convectio = [Latin] bring together, from con- = with, and vectio = carry; zona [Latin] from zoone = [Greek] girdle

The convection zone is a layer in the Sun that reaches from just below the surface down to about 130,000 miles (183,000 km) below the surface. The convection zone contains about 2/3 of the Sun's volume (up to the visible surface) but only about 1/60 of the Sun's mass. In this layer the energy of the Sun is transported to the surface by convection. The temperature inside this layer is thought to vary between 2.0 million and 6500 K (4 million and 12,000 degrees F, 2.2 million and 6200 degrees Centigrade), and the density between 100 times more and 4,000 times less than that of air at the Earth's surface.

Many other stars also have one more more convection zones, which may also be far below the surface, or even in the core of the stars.

convection_layer

convection_layers

convection_zones

convective

the coordinate

plural: coordinates

A coordinate is one of a set of numbers that together specify a location or direction. There are many different ways to define coordinates. Each different way leads to a different coordinate system, of which many are used in astronomy.

the coordinate system

co- = [Latin] together, ordinatio = [Latin] ordering, arrangement

A coordinate system is a tool that allows fixing of positions by measuring distances in different directions. Those distances are the coordinates. There are two often-used classes of coordinate systems: those that use rectangular (or cartesian) coordinates, and those that use polar coordinates. Polar coordinates use angles (for the direction) and one distance, and usually have a base plane that the angles are tied to. Rectangular coordinates use only distances, measured in mutually orthogonal directions from a common origin. Coordinate systems are often named for the thing in or on which they measure positions, or for the base plane or origin with which they are associated.

Here are a number of polar coordinate systems that are used in astronomy:

Table 1: Coordinate Systems

system object base plane longitude latitude
geographical Earth equator longitude latitude
equatorial sky equator right ascension declination
ecliptic sky ecliptic longitude latitude
galactic sky milky way longitude latitude
horizontal sky horizon azimuth altitude
selenographical Moon equator longitude latitude
heliographical Sun equator longitude latitude
arographical Mars equator longitude latitude
jovigraphical Jupiter equator longitude latitude

The column marked "system" indicates the name of the system; "object" lists the object to which the coordinates apply; "base plane" shows the plane that has a latitude of 0; "longitude" and "latitude" provide the names of the coordinates that correspond to longitude and latitude.

The names of polar coordinate systems that fix positions on a celestial body are often made up of the Latin or Greek name of the body, followed by graphical, for example geographical for the Earth. The names of the corresponding rectangular coordinate systems (with the center of the body as origin) usually have the same first part, but followed by centric, for example geocentric for the Earth, planetocentric for planets in general, or heliocentric for the Sun.

coordinates

coordinate_systems

Nicholas Copernicus

Nicholas Copernicus (1473 - 1543) was an astronomer who wrote a book in which he claimed and gave evidence that all planets including the Earth orbit around the Sun (the heliocentric world view) instead of all (other) planets and the Sun orbiting around the Earth (the geocentric world view) like most people believed until then. Copernicus did still assume (like his predecessors) that all motions in the Universe were along circular orbits. Johan Kepler showed later that the orbits of the planets are far closer to ellipses than to circles. The work of Copernicus and Kepler caused most people to start believing in the heliocentric world view.

Cordelia

A moon of 40 km diameter at about 49,800 km from the planet Uranus. The gravity at its surface is about 0.0008 times as strong as on Earth. The moon goes once around its planet in about 8.1 hours. The moon was discovered in 1986. Also called UVI (Uranus six). Its provisional designation was S/1986 U7.

the core

core, plural cores

The core of the Sun is the centermost part of the Sun, where all the Sun's energy is produced by nuclear processes. It has a radius of about 86,000 miles (140,000 km). It contains about 1/120 of the Sun's volume (up to the visible surface), and about 1/3 of the Sun's total mass. At the very center of the Sun, the temperature is thought to be about 16 million K (28 million , 16 million ), and the density about 150 times that of water. At the outer edge of the core, the temperature is thought to be 9 million K (17 million ℉, 9 million ℃), and the density 34 times that of water. The layer immediately around the core is the convection layer.

cores

the corona

corona = [Latin] from koroonè = [Greek] crown

The corona is the outermost layer of the Sun, starting at about 1300 miles (2100 km) above the surface. The temperature in the corona is 500,000 K (900,000 degrees F, 500,000 degrees C) or more, up to a few million K. The corona is very dilute indeed (less than 1000 million atoms per cubic centimetre or 10,000 million atoms per cubic inch) and cannot be seen with the naked eye except during a total solar eclipse, or with the use of a coronagraph.

Corona Australis

Corona Australis (Southern Crown) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is CrA.

Corona Borealis

Corona Borealis (Northern Crown) is an equatorial constellation. The official abbreviation is CrB.

Coronae_Austrinae

Coronae_Borealis

the coronagraph

corona = [Latin] from koroonè = [Greek] crown; graphia = [Greek] description

A coronagraph is an instrument that can look at the faint outer layers (the corona) of the Sun, by covering the bright solar disk.

coronagraphs

coronal

Coronal means it has something to do with the corona of the Sun or a star.

the coronal hole

koroonè = [Greek] crown

A coronal hole is an area in the corona of the Sun that appears dark in pictures taken with coronagraphs or during total solar eclipses. There are often coronal holes around the north and south poles of the Sun, especially near the minimum of the solar cycle.

the coronal mass ejection

koroonè = [Greek] crown

Also called Coronal Mass Eruption or Coronal Transient, and generally abbreviated to CME. A CME is a huge eruption of material from the solar corona into interplanetary space. They can look like bubbles or loops or stranger shapes. When seen close to the Sun, these CMEs can be bigger than the Sun itself, but they are also extremely dilute. Near the Earth, the material typically has a density of at most about 6 protons per cubic inch (100 per cm^3), which is equivalent to about 19 orders of magnitude less than the mass density of air.

coronal_holes

coronal_mass_ejections

coronas

Corvi

Corvus

Corvus (Crow) is an equatorial constellation, close to the ecliptic. The official abbreviation is Crv.

cos

cosh

the cosmic rays

kosmos = [Greek] arrangement

Cosmic rays are actually particles (mostly helium nuclei, protons, and electrons) that travel through space with a relatively very large amount of energy. Because of their large amount of energy, cosmic rays are about as dangerous as X-rays or gamma-rays. Cosmic rays have many sources. Some are formed in solar flares. Others come from beyond our solar system, such as the galactic cosmic rays.

cosmic_rays

Cosmos

CrA

Crab

Crane

Crater

Crater (Cup) is an equatorial constellation, close to the ecliptic. The official abbreviation is Crt.

Crateris

CrB

the crescent moon

The crescent Moon is the Moon during the period when less than half of the disk of the Moon is illuminated. This period begins at Last Quarter and ends at First Quarter.

crescent_moons

Cressida

A moon of 79 km diameter at about 61,800 km from the planet Uranus. The gravity at its surface is about 0.0015 times as strong as on Earth. The moon goes once around its planet in about 11.1 hours. The moon was discovered in 1986. Also called UIX (Uranus nine). Its provisional designation was S/1986 U3.

Crow

Crt

Cru

Crucis_Australis

Crux Australis

Crux Australis (Southern Cross) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Cru.

Crv

culmination

Cup

Cupid

A moon of 24 km diameter at about 74,800 km from the planet Uranus. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 14.8 hours. The moon was discovered in 2003. Also called UXXVII (Uranus twenty-seven). Its provisional designation was S/2003 U2.

CVn

Cyg

Cygni

Cygnus

Cygnus (Swan) is a northern constellation, close to the Milky Way. The official abbreviation is Cyg.

Cyllene

A moon of 2 km diameter at about 24,349,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 776.0 days. The moon was discovered in 2003. Also called JXLVIII (Jupiter forty-eight). Its provisional designation was S/2003 J13.

Dd

<B D>
daylight_savings_time

the December solstice

The December solstice is the solstice that falls in the month of December (in the Gregorian Calendar). Because not everybody uses the Gregorian Calendar, it seems better to call this solstice by a calendar-neutral name, such as southern solstice.

decembersolstice

the declination

declinatio = [Latin] deviation

The declination is the coordinate in the equatorial coordinate system in the sky that is similar to latitude on Earth. It ranges between −90 degrees at the southern celestial pole and +90 degrees at the northern celestial pole and is zero at the celestial equator. The other equatorial coordinate is the right ascension.

declinations

the degree

degree, plural degrees; gradus = [Latin] step

In general, a degree is a division of a scale of measurement. In astronomy, the following two kinds of degrees are used in particular:

degrees

Deimos

A moon of 12 km diameter at about 23,460 km from the planet Mars. The gravity at its surface is about 0.0003 times as strong as on Earth. The moon goes once around its planet in about 30.3 hours. The moon was discovered in 1877. Also called MII (Mars two).

Del

Delphini

Delphinus

Delphinus (Dolphin) is an equatorial constellation, close to the Milky Way. The official abbreviation is Del.

the descending equinox

The descending equinox or autumnal equinox is the equinox at which the Sun moves from north to south of the celestial equator. At that moment, spring starts in the southern hemisphere and autumn in the northern hemisphere. In the Gregorian calendar, the descending equinox is near 23 September.

descending_equinox

descending_equinoxes

descending_node

descending_nodes

Desdemona

A moon of 64 km diameter at about 62,700 km from the planet Uranus. The gravity at its surface is about 0.0012 times as strong as on Earth. The moon goes once around its planet in about 11.4 hours. The moon was discovered in 1986. Also called UX (Uranus ten). Its provisional designation was S/1986 U6.

Despina

A moon of 150 km diameter at about 52,530 km from the planet Neptune. The gravity at its surface is about 0.0025 times as strong as on Earth. The moon goes once around its planet in about 8.0 hours. The moon was discovered in 1989. Also called NV (Neptune five). Its provisional designation was S/1989 N3.

the differential rotation

differentis = [Latin] different; rotatio = [Latin] rotation

Differential rotation is rotation in which not every part of the object has the same period of rotation. Only objects that are not solid can show differential rotation. In the Solar System, differential rotation occurs in, among other things, the Sun, the giant gaseous planets, and the atmosphere of the Earth.

Dione

A moon of 1125 km diameter at about 377,420 km from the planet Saturn. The gravity at its surface is about 0.0236 times as strong as on Earth. The moon goes once around its planet in about 65.7 hours. The moon was discovered in 1684. Also called SIV (Saturn four).

Dolphin

the doppler effect

after C.J. Doppler (1803-1853)

The doppler effect is the phenomenon that the frequency of a wave depends on the speed of the source of the waves relative to the receiver. When the source moves towards the receiver (or the receiver towards the source; only the relative speed matters) then the frequency is higher; if the source and the receiver move apart, then the frequency is lower. The relative change in the frequency is approximately equal to the ratio of the speed difference and the speed of propagation of the waves, as long as the speed difference is much smaller than the propagation speed.

For sound, the doppler effect changes the pitch: if a car, motor cycle, or train passes by you fast then the pitch of the sounds they produce goes down. The speed of sound is about 340 m/s (1200 km/h) at sea level on Earth, so the pitch of the sounds coming from a passing car change by one standard note (about 6 percent of frequency change) for about each 9 m/s (32 km/h) of its speed relative to you.

A speed measured through its doppler effect is called a doppler speed. The shift of characteristics in the frequency spectrum of waves (such as spectral lines) is called doppler shift.

Astronomers can use the doppler effect to fairly simply determine the speeds of stars and galaxies relative to us, and also of material in the photosphere of the Sun.

the doppler shift

after C.J. Doppler (1803-1853)

Doppler shift is a shift of characteristics of a frequency spectrum (such as spectral lines) because of the doppler effect. A doppler speed can be calculated from a doppler shift. A doppler shift of light to smaller frequencies is called redshift, and a doppler shift to greater frequencies blueshift.

the Doppler velocity

Doppler velocity is the speed in the direction of the line of sight, i.e. directed toward you or away from you. The Doppler velocity is named after the Doppler effect (named after the discoverer, Mr Doppler), which is the effect that the frequency of a tone changes when the speed of the source in the line of sight changes. The same effect occurs in frequencies of light and other electromagnetic radiation and enables us to measure velocities on the Sun. An image showing Doppler velocities is called a dopplergram. Redshift is a kind of Doppler shift.

doppler_shifts

doppler_speed

doppler_speeds

doppler_velocities

Dor

Dorado

Dorado (Swordfish) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Dor.

Doradus

Dove

Dra

Draco

Draco (Dragon) is a northern constellation. The official abbreviation is Dra.

draconic_month

draconic_months

Draconis

Dragon

the dwarf planet

dwarf planet, plural dwarf planets

A dwarf planet is a large spherical object with a diameter of around or just above 1000 km. A dwarf planet is massive enough that its own gravity keeps it round (in contrast to asteroids, comets, and small moons), but too small to clear the neighborhood of its orbit from similar objects (in contrast to planets). A dwarf planet orbits directly around a star (in contrast to moons).

dwarf_planets

Ee

<C E>
Eagle

the Earth

The Earth is the third planet of the Solar System, counted from the Sun. It is a terrestrial planet, with an atmosphere and a moon (the Moon), but without any rings. The Earth has an equatorial diameter of 12,756 km and is number 6 on the list of largest objects in our Solar System.

eccentricities

the eccentricity

The eccentricity of an orbit is one of the orbital elements. It is a number that indicates how much the orbit deviates from a circle. A circular orbit has an eccentricity equal to zero. An elliptical orbit has an eccentricity between zero and one. In this case, the eccentricity is equal to the difference between the lengths of the long and short axes of the ellipse, divided by the sum of those lengths. A parabolic orbit has an eccentricity of one, and a hyperbolic orbit has an eccentricity larger than one.

Orbits with eccentricities less than one are closed, so the objects in such orbits return to the same position regularly. Orbits with eccentricities greater than one are open, which means that objects in such orbits never return to the same position.

eccentric_anomaly

eclipse

eclipses

eclipses_of_the_moon

eclipses_of_the_sun

eclipse_of_the_moon

eclipse_of_the_sun

the ecliptic

The ecliptic is

  1. the plane of the Earth's orbit.
  2. the line where the plane of the Earth's orbit intersects the celestial sphere. This is almost the same as the path that the Sun appears to take between the stars, as seen from the Earth.

The ecliptic passes through the middle of the zodiac. All of the planets and the Moon also stay close to the ecliptic.

ecliptical

ekleiptikos = [Greek] belonging to an eclipse

Ecliptical means that it has something to do with the ecliptic. In the ecliptic coordinate system, positions in the sky are indicated with the ecliptic longitude and latitude. The latitude is measured relative to the ecliptic, and the longitude relative to the vernal equinox.

Edwin_Hubble

egyptian_calendar

Albert Einstein

Between 1900 and 1920, Albert Einstein (1879 - 1955) explained a number of imperfections in the classical theories that were used at the time. He explained Brownian motion of dust particles, which happens because of collisions with invisible molecules of air. Einstein showed that the emission and absorption of light are quantum effects, and he was therefore one of the founders of quantum theory. In 1905 Einstein published the Special Theory of Relativity, which indicates that space and time are not wholly separate but together form a single space-time continuum, and that mass and energy are equivalent (the famous formula (E = m c^2)). In 1915 Einstein published the General Theory of Relativity, which states that gravity comes from the curvature of space-time because of the presence of mass. All of these theories have been tested often and in great detail and have been shown to be correct so far. Because of this, Einstein's fame is deservedly great.

Elara

A moon of 86 km diameter at about 11,741,000 km from the planet Jupiter. The gravity at its surface is about 0.0032 times as strong as on Earth. The moon goes once around its planet in about 259.8 days. The moon was discovered in 1905. Also called JVII (Jupiter seven).

electromagnetic

the electromagnetic radiation

elektron = [Greek] gold or amber; he Magnetos lithos = [Greek] stone of Magnesia; radiatio = [Latin] radiate

Electromagnetic radiation is any kind of radiation that consists of alternating electric and magnetic fields and that can propagate even in a vacuum. Electromagnetic radiation is characterized by its wavelength (or, equivalently, its frequency or energy). Some different types of electromagnetic radiation or electromagnetic waves, in increasing order of frequency and energy and decreasing order of wavelength, are: radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays. In general, the later in the list the type is, the more dangerous it is. Visible light is the only form of electromagnetic radiation that we can see with our eyes.

electromagnetic_force

electromagnetic_forces

electromagnetic_wave

electromagnetic_waves

electron

the electronvolt

An electronvolt is a very small amount of energy, namely the energy that is involved when an electron experience a electric potential difference of 1 volt. This energy corresponds to about 1.6 × 10−19 J. The symbol for electronvolt is eV. The amount of energy that is involved in a typical chemical reaction is of the order of a few eV. Because an eV is so small, the following multiples are also often used: keV (kiloelectronvolt = 1000 eV), MeV (megaelectronvolt = 1000 keV), and GeV (gigaelectronvolt = 1000 MeV). The mass of an electron corresponds to about 0.5 MeV of energy, and the mass of a proton with about 1 GeV of energy, so MeV and GeV are good units of energy for nuclear reactions.

electronvolts

the elliptical galaxy

An elliptical galaxy is a galaxy that appears to have an elliptical shape no matter from which side you see it. An elliptical galaxy contains no arms or bar.

elliptical_galaxies

the elongation

The elongation of a celestial body is a measure for the distance in the sky between that celestial body and the Sun. The following different definitions are used:

  1. the angular distance in the sky between the center of that body and the center of the Sun, as seen from the center of the Earth
  2. the difference in geocentric ecliptic longitude between the center of the celestial body and the center of the Sun

For a celestial body on the ecliptic, both definitions yield the same results.

The elongation cannot exceed 180 degrees by definition. Superior planets can have any elongation up to the maximum 180 degrees, but inferior planets have a maximum elongation that is less than 180°. The maximum elongation of Mercury is about 28°, and that of Venus about 48°.

elongations

embolistic

[Greek] embolos = something put in between

Embolistic means "put in between". An embolistic month is an extra month that is added to the calendar only in certain years, usually to keep a lunar calendar in line with the tropical year. The embolistic day of the Julian and Gregorian calendars is more commonly called the bissextile day.

the emission line

emissio = [Latin] send away

An emission line is a spectral line (a very narrow set of colors) at which an object shines more brightly than at nearby colors.

emission_lines

Enceladus

A moon of 498 km diameter at about 238,040 km from the planet Saturn. The gravity at its surface is about 0.0118 times as strong as on Earth. The moon goes once around its planet in about 32.9 hours. The moon was discovered in 1789. Also called SII (Saturn two).

the energy

Energy is the ability to do things. The more energy something has, the further it can go or the faster it can go or the greater the influence is that it can have. There are many different kinds of energy.

the energy transport

energeia = [Greek] ability to do work; trans- = [Latin] over, portare = [Latin] carry

Energy transport can be done in three ways:

  1. by convection (motion of a material), in which temperature differences make hotter material move to colder areas and colder material to hotter areas, which means that there is net heat (and so energy) transport from hot to cold areas.
  2. by conduction (transport through a material), in which heat travels through a material without making the material itself move.
  3. by radiation, in which energy travels through empty space (for example, as electromagnetic radiation such as light).

All three kinds of energy transport happen when you heat a pan of soup on a fire. The hot gases from the fire go up because of convection. The fire produces heat radiation, which you can block with your hand. The outside bottom of the pan is heated and conduction makes the heat reach the inside bottom of the pan, where it heats the soup. At first, the heat travels up the soup only by conduction, but later (when the soup is boiling) also by conduction.

Epimetheus

A moon of 118 km diameter at about 151,400 km from the planet Saturn. The gravity at its surface is about 0.0010 times as strong as on Earth. The moon goes once around its planet in about 16.7 hours. The moon was discovered in 1980. Also called SXI (Saturn eleven). Its provisional designation was S/1980 S3.

the epoch

An epoch is

  1. the beginning (the start of day 1 of year 1) of a calendar. This need not be the first day on which the calendar is used. For example, the epoch of the Gregorian calendar lies over 1500 years before the first day on which it was used.
  2. the time for which a set of orbital elements is valid. Predictions of positions based on those orbital elements will become less accurate the further from the epoch the time is for which the prediction is made.

epochs

Equ

the equation of time

The equation of time indicates how much you should subtract from the true solar time (as read from a sundial) to get the mean solar time, which runs as regularly as a mechanical clock. The equation of time occurs because the orbit of the Earth around the Sun is not an exact circle, and because there are seasons.

equation_of_kepler

equation_of_time

the equator

aequatio = [Latin] make equal

The equator is equally far from both geographical poles and divides the Earth into a Northern and a Southern part. By extension, there is also an equator in the sky, which divides the sky into a Northern and a Southern part.

equatorial

aequatio = [Latin] make equal

Equatorial means it has something to do with the equator. In the equatorial coordinate system, positions in the sky are indicated with the right ascension (relative to the vernal equinox) and the declination (relative to the celestial equator). The celesital equator is the extension of the Earth's equator into the sky, halfway between the celestial poles, which themselves are the extensions of the rotation axis of the Earth.

equators

equinoci

the equinox

aequinoctium = [Latin] equinox, from aequatio = [Latin] make equal, and noctium, nox = [Latin] night; plural equinoxes

The equinox is

  1. the moment when the Sun crosses the celestial equator. Close to an equinox, day and night have nearly the same length (12 hours) everywhere on Earth. The equinoxes signal the beginning of the seasons of spring and autumn (where those are relevant). The vernal or ascending equinox occurs in March (in the Gregorian calendar) and marks the beginning of spring in the northern hemisphere and of autumn in the southern hemisphere. The autumnal or descending equinox occurs in September and marks the beginning of spring in the southern hemisphere and of autumn in the northern hemisphere. The beginning of the other seasons is governed by the solstices.
  2. the place in the sky between the stars where the Sun is during the vernal equinox. This location is also called the vernal equinox. In the equatorial and ecliptic coordinate systems, the vernal equinox has longitude and latitude equal to zero.

    Because of the precession of the equinoxes, the equinox slowly moves between the stars, so when one quotes ecliptic or equatorial coordinates, one has to indicate relative to which equinox these coordinates are measured. Three equinoxes that are commonly used in stellar atlases and planetary calculations are those of 1950.0 (the beginning of the year 1950), 2000.0 (the beginning of the year 2000), and the equinox of the date (i.e., the equinox of the same date as the coordinates themselves).

equinoxes

Equulei

Equuleus

Equuleus (Foal) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Equ.

Eri

Eridani

Eridanus

Eridanus (River Eridanus) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Eri.

Erinome

A moon of 3 km diameter at about 23,196,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 721.6 days. The moon was discovered in 2000. Also called JXXV (Jupiter twenty-five). Its provisional designation was S/2000 J4.

Erriapo

A moon of 10 km diameter at about 17,343,000 km from the planet Saturn. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 853.2 days. The moon was discovered in 2000. Also called SXXVIII (Saturn twenty-eight). Its provisional designation was S/2000 S10.

the escape velocity

The escape velocity is the lowest speed that you need to have to escape from the gravity of a celestial body without needing a rocket engine or other form of propulsion anymore, if there is no atmosphere or other source of friction to slow you down. Escaping here means that you can get as far away from the celestial body as you want. The escape velocity depends on your location. "The" escape velocity of a planet is the escape velocity that you'd need at the surface of that planet.

escape_velocities

Euanthe

A moon of 3 km diameter at about 20,797,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 612.6 days. The moon was discovered in 2001. Also called JXXXIII (Jupiter thirty-three). Its provisional designation was S/2001 J7.

Eukelade

A moon of 4 km diameter at about 23,661,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 743.4 days. The moon was discovered in 2003. Also called JXLVII (Jupiter forty-seven). Its provisional designation was S/2003 J1.

Euporie

A moon of 2 km diameter at about 19,304,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 547.8 days. The moon was discovered in 2001. Also called JXXXIV (Jupiter thirty-four). Its provisional designation was S/2001 J10.

Europa

A moon of 3121 km diameter at about 671,100 km from the planet Jupiter. The gravity at its surface is about 0.1341 times as strong as on Earth. The moon goes once around its planet in about 85.2 hours. The moon was discovered in 1610. Also called JII (Jupiter two).

Eurydome

A moon of 3 km diameter at about 22,865,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 706.2 days. The moon was discovered in 2001. Also called JXXXII (Jupiter thirty-two). Its provisional designation was S/2001 J4.

eV

event_horizon

event_horizons

exp

the exponential notation

Exponential notation is a method for writing numbers that is used a lot in science because it is convenient for writing the very large and very small numbers that occur often in science. Exponential notation consists of two numbers, the mantissa and the exponent. The mantissa is a number that may be fractional (with a part after the decimal point) and that is usually between 1 and 10. The exponent is a whole number that indicates by which power of 10 you should multiply the mantissa, so by how many places you should shift the decimal point to the left (if the exponent is negative) or the right (if the exponent is positive).

There are different methods in use for linking the mantissa and exponent of a number. The proper mathematical way is to put "times ten to the power of" between the mantissa and the exponent, but this requires the exponent to be written in superscript, a bit higher than normal, and that cannot be displayed properly (or at all) on simple screens. That's why in computer-related texts people usually put a letter "E" or "e" between the mantissa and the exponent.

For example, three million million million, a three followed by 18 zeros, is written in exponential notation as 3×1018 (the 18 ought to be written a bit higher than the 10, if your screen supports this) or 3 × 1018. So, 3.2 × 103 is equal to 3200 and 3.2 × 10−3 is equal to 0.0032.

In general, people use either exponential or normal notation, whichever is shorter. Exponential notation is convenient for describing numbers that are many orders of magnitude different from 1.

Extended_Algorithm_of_Euclid

extended_euclidean_algorithm

Ff

<D F>
Fahrenheit

fahrenheit

Ferdinand

A moon of 12 km diameter at about 20,901,000 km from the planet Uranus. The gravity at its surface is about 0.0003 times as strong as on Earth. The moon goes once around its planet in about 7.9 years. The moon was discovered in 2001. Also called UXXIV (Uranus twenty-four). Its provisional designation was S/2001 U2.

the filament

'fi‧la‧ment, plural filaments; filum = [Latin] thread

In general, a filament is something that looks like a thin thread.

In astronomy, filament is used as a name for (amongst other things)

filaments

the filter

filtrum = [Middle Latin] compacted wool; from feltir = [German] felt

A filter is a device that transmits only certain (ranges of) colors (or frequencies) from the electromagnetic spectrum. A broadband filter transmits a wide range of frequencies, and a narrowband filter only a small range. With a narrowband filter, spectral lines can be measured.

filters

the First Quarter

The First Quarter is a phase of the Moon, equal to

  1. [daily life] the period when about half of the lunar disk is illuminated, about a week after New Moon.
  2. [astronomy] the instant at which the ecliptic longitude of the center of the Moon is exactly 90 degrees greater than that of the center of the Sun.

first_quarters

Fishes

Fly

Flying_Fish

Foal

For

Fornacis

Fornax

Fornax (Furnace) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is For.

Fox

Francisco

A moon of 12 km diameter at about 4,276,000 km from the planet Uranus. The gravity at its surface is about 0.0003 times as strong as on Earth. The moon goes once around its planet in about 267.2 days. The moon was discovered in 2001. Also called UXXII (Uranus twenty-two). Its provisional designation was S/2001 U3.

french_revolutionary_calendar

the Full Moon

The Full Moon is a phase of the Moon, equal to

  1. [daily life] the period when the whole disk of the Moon seems to be illuminated, about two weeks after New Moon.
  2. [astronomy] the instant at which the ecliptic longitude of the center of the Moon is exactly 180 degrees greater than that of the center of the Sun.

full_moons

the fundamental force

plural: fundamental forces

A fundamental force is one of the four forces of which all other forces (outside of elementary particles) are forms. The fundamental forces are (in order of decreasing strength):

  1. the strong nuclear force, which keeps protons and neutrons together in the nucleus of an atom and plays a role in almost all nuclear reactions.
  2. the weak nuclear force, which plays a role in spontaneous radioactivity.
  3. the electromagnetic force, which keeps electrons and nuclei together in atoms, and plays a role in all chemical reactions and in electrical and magnetical forces.
  4. gravity, which tries to keep mass together.

fundamental_force

fundamental_forces

Furnace

Gg

<E G>

galactic

galaktos = [Greek] milk

Galactic means one of the following things:

  1. that it has something to do with a galaxy.
  2. that it has something to do with the Milky Way Galaxy, the galaxy that we are in.
  3. that it has something to do with the location of the Milky Way Galaxy in the sky as seen from Earth.

The galactic coordinate system has the Milky Way (as seen from Earth) for base plane. The galactic latitude is measured relative to the plane of the Milky Way Galaxy, and the galactic longitude relative to the direction to the center of the Milky Way. Because of more accurate information about where the center of the Milky Way is, the old galactic coordinate system was replaced with a new and improved one a few decades ago.

galactic_cosmic_rays

Galatea

A moon of 175 km diameter at about 61,950 km from the planet Neptune. The gravity at its surface is about 0.0033 times as strong as on Earth. The moon goes once around its planet in about 10.3 hours. The moon was discovered in 1989. Also called NVI (Neptune six). Its provisional designation was S/1989 N4.

galaxies

Galaxy

the galaxy

'ga‧lax‧y; galaktos = [Greek] milk

A galaxy is a region of space with millions to thousands of millions of stars and many clouds of gas that are tied together by their gravity. A typical spiral galaxy or elliptical galaxy (two types of galaxies) contains 100 thousand million stars, has a diameter of 100,000 lightyears, and is at a distance of a few million lightyears from other neighboring spiral or elliptical galaxies. Our Solar System is part of the Milky Way, which is a fairly ordinary spiral galaxy.

galaxy_cluster

galaxy_clusters

galaxy_cluster_cloud

galaxy_cluster_clouds

Galileo

the gamma rays

gamma = [Greek] third letter

Gamma rays are a form of electromagnetic radiation with a large amount of energy, and therefore dangerous. Gamma rays are made naturally by material with temperatures of millions of degrees, as occurs in some solar flares.

gamma_rays

Ganymede

A moon of 5262 km diameter at about 1,070,400 km from the planet Jupiter. The gravity at its surface is about 0.1457 times as strong as on Earth. The moon goes once around its planet in about 7.2 days. The moon was discovered in 1610. Also called JIII (Jupiter three).

gauss

Gem

Gemini

Gemini (Twins) is an equatorial constellation, close to the ecliptic. The official abbreviation is Gem.

Geminorum

the General Theory of Relativity

The General Theory of Relativity by Albert Einstein describes how things are with space and time even when forces act upon the observer. The General Theory of Relativity says that the effects of an acceleration do not depend on the cause of the acceleration (not even if that cause is the force of gravity). It follows that gravity bends rays of light and can slow down time, and that black holes can exist. Einstein invented this theory at the beginning of the 20th century. The theory has passed many tests.

general_theory_of_relativity

geocentric

Gè = [Greek] Earth; kentrum = [Greek] center, middle

Geocentric means it has something to do with the center of the Earth.

Geocentric coordinate systems have the center of the Earth for their origin, and are almost always rectangular coordinate systems, but can vary in the directions of the coordinate axes. Oft-used geocentric coordinate systems are the equatorial geocentric coordinate system and the ecliptic geocentric coordinate system.

the Geocentric World View

This is the world view in which the Earth is at rest in the center of the Universe and all other celestial bodies orbit around the Earth. This world view was made popular by the Greek philosopher Aristotle around 350 BC and remained the dominant world view in the West until it was replaced in the 16th century by the Heliocentric World View.

geocentric_world_view

geographic

Gè = [Greek] Earth; graphia = [Greek] description

Geographic means that it describes the Earth. The geographical coordinate system measures the latitude relative to the equator, and the longitude relative to the prime meridian which runs, for historical reasons, through Greenwich in London.

geographical

geomagnetic

Gè = [Greek] Earth; hè Magnetis lithos = [Greek] stone from Magnesia

Geomagnetic means it has something to do with the magnetic field of the Earth.

geomagnetical

the gibbous moon

The gibbous Moon is the Moon during the period when more than half of the lunar disk is illuminated. This period begins at First Quarter and ends at Last Quarter.

gibbous_moons

Giraffe

gpc

granular

Granular means it has something to do with granules.

the granulation

granulus = [Latin] small grain

Granulation covers almost all of the visible surface of the Sun. It resembles rice pudding, with bright "rice grains", called granules, that are separated from one another by a network of dark lanes, called intergranular lanes.

the granule

granulus = [Latin] small grain

A granule is a ball of hot solar gas that has appeared at the surface of the Sun. A typical granule has a roughly round shape and a diameter of about 1,000 km. The gas in a granule cools down when it has reached the surface and flows back down through the intergranular lanes. Granules and intergranular lanes together are called granulation and are a form of convection.

granules

gravitation

gravity

Gravitation or the force of gravity acts between all things that have mass. Gravity is one of the four fundamental forces of nature, and is the most important one at astronomical scales (distances). Gravity is always an attracting force, and never repulsive. Thanks to gravity, we do not float away from Earth, the Earth stays together, and remains orbiting around the Sun. The Sun keeps shining partly because of gravity.

the great circle

plural great circles

A great circle is a special circle at the surface of a sphere (for example of a planet or of the sky). You can recognize a great circle by any of the following attributes:

Some examples of great circles are: the ecliptic, the equator, any meridian.

Great_Bear

great_circles

Great_Dog

greek_calendar

greek_calendars

greenhouse_effect

gregorian

gregorian_calendar

gregorian_calendar_reform

Gru

Gruis

Grus

Grus (Crane) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Gru.

Hh

<F H>
Hare

Harpalyke

A moon of 4 km diameter at about 20,858,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 615.3 days. The moon was discovered in 2000. Also called JXXII (Jupiter twenty-two). Its provisional designation was S/2000 J5.

Hegemone

A moon of 3 km diameter at about 23,947,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 756.9 days. The moon was discovered in 2003. Also called JXXXIX (Jupiter thirty-nine). Its provisional designation was S/2003 J8.

Helene

A moon of 32 km diameter at about 377,420 km from the planet Saturn. The gravity at its surface is about 0.0007 times as strong as on Earth. The moon goes once around its planet in about 65.7 hours. The moon was discovered in 1980. Also called SXII (Saturn twelve). Its provisional designation was S/1980 S6.

Helike

A moon of 4 km diameter at about 21,263,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 633.3 days. The moon was discovered in 2003. Also called JXLV (Jupiter forty-five). Its provisional designation was S/2003 J6.

heliocentric

Helios = [Greek] Sun; kentrum = [Greek] center, middle

Heliocentric means it has the Sun as its center.

Heliocentric coordinate systems have the center of the Sun for their origin, and are almost always rectangular coordinate systems, but vary in the directions of the coordinate axes. Oft-used heliocentric coordinate systems are the equatorial heliocentric coordinate system and the ecliptic heliocentric coordinate system.

the Heliocentric World View

This is the world view in which the Sun is (practically) in the center of the Universe and all planets of the Solar System orbit around the Sun. This world view replaced the Geocentric World View in the 16th century, thanks to the work of Kepler and Copernicus.

heliocentric_world_view

heliographic

Helios = [Greek] Sun; graphia = [Greek] description

Heliographic means it describes the Sun. The heliographic coordinate system has the solar equator for base plane, and uses longitude and latitude for coordinates.

heliographical

Her

Hercules

Hercules is an equatorial constellation. The official abbreviation is Her.

Herculis

Hermippe

A moon of 4 km diameter at about 21,131,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 627.4 days. The moon was discovered in 2001. Also called JXXX (Jupiter thirty). Its provisional designation was S/2001 J3.

the Hertzsprung-Russell Diagram

The Hertzsprung-Russell Diagram is a diagram showing a measure for the brightness of stars (usually along the vertical axis) against a measure for the temperature of stars (usually along the horizontal axis). This kind of diagram was invented around 1910 by Ejnar Hertzsprung and Henry Norris Russell. It turns out that stars concentrate in certain parts of such a diagram, depending on the phase of their life.

Hertzsprung-Russell_Diagrams

Himalia

A moon of 170 km diameter at about 11,461,000 km from the planet Jupiter. The gravity at its surface is about 0.0064 times as strong as on Earth. The moon goes once around its planet in about 250.6 days. The moon was discovered in 1904. Also called JVI (Jupiter six).

Hohmann_transfer_orbit

Hor

the horizon

horizoon = [Greek] boundary

In general, a horizon is a boundary line between an area that you can see and an area that you cannot see. In astronomy, there are at least four relevant meanings of horizon, of which three are closely related. The horizon is:

  1. The line along the sky that is exactly midway between (at 90 degrees from) the zenith and the nadir, at a height of 0 degrees.
  2. The imaginary boundary line in the sky between the land (or sea) and the air, if the Earth were perfectly round (smooth). If your eye were halfway in the ground (or sea), then this horizon would be the same as the one of explanation 1 (though the difference is only important if you are kilometers or miles above the land or sea).
  3. The boundary line in the sky between the air and the things on the ground (or sea). If mountains or buildings are nearby, then this horizon is not a straight line. On a dead calm sea, this horizon is the same as the one of explanation 2.
  4. The horizon of a black hole is the edge of the region around a black hole from where you cannot escape.

If an astronomical text mentions a horizon but doesn't explain which one it is, then you can assume that it is the horizon of explanation 1.

horizons

horizontal

horizoon = [Greek] boundary

Horizontal means it has something to do with the horizon. The horizontal coordinate system uses the horizon for base plane, and the azimuth and altitude for coordinates.

horizontally

Horologii

Horologium

Horologium (Clock) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Hor.

the hour

hour, plural hours; hora = [Latin] hour

An hour is a unit of time. An hour is the 24th part of a day. An hour is further divided into 60 minutes.

the hour angle

The hour angle of a celestial body is the difference in right ascension between that body and the meridian (of right ascension) that is due south at that time. The hour angle is usually measured not in degrees but in hours, minutes, and seconds, just like the right ascension. The hour angle indicates (in sidereal time) how long ago the body was due south.

hours

hour_angles

HR_Diagram

HR_Diagrams

Edwin Hubble

Edwin Hubble (1889 - 1953) was the first person to prove (in 1923) that what we now call spiral galaxies are outside of our Milky Way Galaxy, so that our Galaxy does not fill the entire Universe. He found that far-away galaxies move away from us at a speed that on average is proportional to their distance from us. This relationship is now called Hubble's Law, and the constant of proportionality (the ratio of the Doppler speed to the distance) is Hubble's Constant.

hubble's_constant

hubble's_law

Hunting_Dogs

Hya

Hydra

A moon of 50 km diameter at about 64,800 km from the planet Pluto. The gravity at its surface is about 0.0011 times as strong as on Earth. The moon goes once around its planet in about 38.2 days. The moon was discovered in 2005. Its provisional designation was S/2005 P1.

Hydra

Hydra (Water Snake) is an equatorial constellation, close to the ecliptic. The official abbreviation is Hya.

Hydrae

Hydri

Hydrus

Hydrus (Little Water Snake) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Hyi.

Hyi

Hyperion

A moon of 286 km diameter at about 1,500,880 km from the planet Saturn. The gravity at its surface is about 0.0018 times as strong as on Earth. The moon goes once around its planet in about 21.7 days. The moon was discovered in 1848. Also called SVII (Saturn seven).

Ii

<G I>

Iapetus

A moon of 1469 km diameter at about 3,560,840 km from the planet Saturn. The gravity at its surface is about 0.0228 times as strong as on Earth. The moon goes once around its planet in about 79.4 days. The moon was discovered in 1671. Also called SVIII (Saturn eight).

IAU

Ijiraq

A moon of 12 km diameter at about 11,124,000 km from the planet Saturn. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 438.3 days. The moon was discovered in 2000. Also called SXXII (Saturn twenty-two). Its provisional designation was S/2000 S6.

the inclination

inclinatio = [Latin] slope

In astronomy, inclination is an angle between some direction and a standard plane. Inclination is used as name for

  1. the angle between the orbit of a planet or other celestial body and the base plane of the coordinate system (usually the ecliptic for bodies in the Solar System). The inclination is one of the orbital elements.
  2. the angle that the magnetic field makes with the local surface.

inclinations

Ind

Indi

Indian

Indus

Indus (Indian) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Ind.

the inferior planet

An inferior planet is a planet that is closer to the Sun than the Earth is. Only inferior planets have an inferior conjunction and an elongation that never exceeds a maximum value that is smaller than 180 degrees. The inferior planets are: Mercury and Venus. The opposite of an inferior planet is a superior planet.

inferior_conjunction

inferior_planets

infrared

infera = [Latin] under

Infrared radiation is a kind of electromagnetic radiation with wavelengths just larger than those of visible light (on the red side). People sense infrared radiation as "heat rays", which is an example of energy transport.

the intergranular lane

inter- = [Latin] between; granulus = [Latin] little grain

The intergranular lanes are the darker lanes between the granules in granulation where cooler solar gas is flowing down below the surface.

intergranular_lanes

the International Astronomical Union

The International Astronomical Union (IAU) is the official international organization for astronomy. Among other tasks, the IAU defines standards for use in astronomy. The IAU determines the official name of things in our Solar System outside the Earth, such as for moons, comets, and asteroids, and for craters and mountains on other planets, moons, and astroids. The IAU has also defined the boundaries and names of the official constellations. The web site of the IAU is at //www.iau.org.

Io

A moon of 3643 km diameter at about 421,800 km from the planet Jupiter. The gravity at its surface is about 0.1832 times as strong as on Earth. The moon goes once around its planet in about 42.5 hours. The moon was discovered in 1610. Also called JI (Jupiter one).

Iocaste

A moon of 5 km diameter at about 21,061,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 624.3 days. The moon was discovered in 2000. Also called JXXIV (Jupiter twenty-four). Its provisional designation was S/2000 J3.

ir

Isaac Newton

Isaac Newton (1662 - 1727) was the founder of the science of mechanics. Newton gave the first useful explanation of gravity, in the form of Newton's Law of Gravity. With this he explained how the Moon orbits the Earth. Newton also invented differential calculus and explained many optical phenomena such as the spectrum. The unit of force is named newton (symbol: N) after him.

The three "ordinary" Laws of Newton are:

  1. (The Law of Inertia) An object upon which no forces act moves at constant speed along a straight line.
  2. An object upon which a force acts is accelerated with an acceleration that is proportional to the applied force and inversely proportional to the mass of the object.
  3. (The Law of Action and Reaction) For each applied force there is an equal but opposite reaction force.

Isaac_Newton

islamic_calendar

Isonoe

A moon of 3 km diameter at about 23,155,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 719.7 days. The moon was discovered in 2000. Also called JXXVI (Jupiter twenty-six). Its provisional designation was S/2000 J6.

Jj

<H J>

Janus

A moon of 178 km diameter at about 151,500 km from the planet Saturn. The gravity at its surface is about 0.0016 times as strong as on Earth. The moon goes once around its planet in about 16.7 hours. The moon was discovered in 1966. Also called SX (Saturn ten). Its provisional designation was S/1980 S1.

JD

JDN

jewish_calendar

JI

JII

JIII

JIV

JIX

Johannes Kepler

Johannes Kepler (1571 - 1630) defined the three Laws of Kepler that properly explained the motion of the planets around the Sun for the first time. The three Laws of Kepler are:

  1. Each planet moves around the Sun in an orbit that is an ellipse, with the Sun in one of the foci of the ellipse.
  2. The straight line segment from the Sun to the planet covers the same amounts of area of the ellipse in the same amounts of time.
  3. The third power of the semimajor axis of the orbit of a planet is proportional to the second power of the orbital period of the planet around the Sun.

jovian

Jovian means that it has something to do with the planet Jupiter. A jovian planet is a planet that looks like Jupiter, i.e., a giant gas planet. In our Solar System, the jovian planets are Jupiter, Saturn, Uranus, and Neptune.

jovian_planet

jovian_planets

jovigraphic

Jovi- = [Latin] of Jupiter; graphia = [Greek] description

Jovigraphic means that it describes the planet Jupiter. The jovigraphic coordinate system uses Jupiter's equator for base plane, and uses longitude and latitude for coordinates.

The visible surface of Jupiter is not solid but is made up of clouds. How long it takes for a particular cloud to revolve once around the rotation axis of the planet depends on the jovigraphic latitude: The clouds of Jupiter show differential rotation. Astronomers have therefore defined several different jovigraphic coordinate systems, referred to as "System I" and "System II".

jovigraphical

the Julian Date

The Julian Date (JD) counts the number of days since 12:00 UTC or 12:00 TAI on January 1st, −4712 on the Julian proleptic calendar. Don't confuse a Julian Date (which consists of a single number) with a date on the Julian calendar (which consists of three parts: a year, a month, and a day).

Julian Dates can have fractional parts. JD 2451545.25 indicates the instant of time that is 0.25 days (i.e., 6 hours) past 12:00 UTC or 12:00 TAI on January 1st, 2000 on the Gregorian calendar, i.e., 18:00 UTC or 18:00 TAI.

the Julian Day Number

The Julian Day Number (JDN) counts the number of whole days since 12:00 UTC or 12:00 TAI on January 1st, −4712 on the Julian proleptic calendar. You get the Julian Day Number if you round a Julian Date (JD) down to the nearest whole value.

julian_calendar

julian_date

Julian_Dates

julian_day

julian_day_count

julian_day_number

Julian_Day_Numbers

julian_day_numbers

julian_year

julian_years

Juliet

A moon of 93 km diameter at about 64,400 km from the planet Uranus. The gravity at its surface is about 0.0017 times as strong as on Earth. The moon goes once around its planet in about 11.9 hours. The moon was discovered in 1986. Also called UXI (Uranus eleven). Its provisional designation was S/1986 U2.

june_solstice

Jupiter

Jupiter = [Latin] chief god

Jupiter is the fifth planet of the Solar System, counting from the Sun. Jupiter is a jovian planet, with a very dense and thick atmosphere (of mostly hydrogen and helium), many moons, and narrow rings. Jupiter is the largest and most massive planet in our Solar System. Jupiter has an equatorial diameter of 142,984 km and is number 2 on the list of largest objects in our Solar System.

JV

JVI

JVII

JVIII

JX

JXI

JXII

JXIII

JXIV

JXIX

JXL

JXLI

JXLII

JXLIII

JXLIV

JXLV

JXLVI

JXLVII

JXLVIII

JXV

JXVI

JXVII

JXVIII

JXX

JXXI

JXXII

JXXIII

JXXIV

JXXIX

JXXV

JXXVI

JXXVII

JXXVIII

JXXX

JXXXI

JXXXII

JXXXIII

JXXXIV

JXXXIX

JXXXV

JXXXVI

JXXXVII

JXXXVIII

Kk

<I K>
K

Kale

A moon of 2 km diameter at about 23,217,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 722.6 days. The moon was discovered in 2001. Also called JXXXVII (Jupiter thirty-seven). Its provisional designation was S/2001 J8.

Kallichore

A moon of 2 km diameter at about 24,043,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 761.5 days. The moon was discovered in 2003. Also called JXLIV (Jupiter forty-four). Its provisional designation was S/2003 J11.

Kalyke

A moon of 5 km diameter at about 23,566,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 738.9 days. The moon was discovered in 2000. Also called JXXIII (Jupiter twenty-three). Its provisional designation was S/2000 J2.

Karl Schwarzschild

Karl Schwarzschild (1873 - 1916) discovered in 1916 that the then new General Theory of Relativity of Albert Einstein predicted that there could be places where the force of gravity becomes infinitely great and the curvature of space-time infinitely large. Such places are called Schwarzschild singularities after him. Nowadays we call the surroundings of such a singularity a black hole. The diameter of a black hole is called the Schwarzschild diameter.

karl_schwarzschild

Keel

kelvin

Kepler

kepler

kepler's_equation

kepler's_law

kepler's_laws

the Keplerian orbit

plural: keplerian orbits

A Keplerian orbit is an orbit that is a circle. An object in a Keplerian orbit always has the same distance to the middle of the thing that it orbits around and always has the same speed. Keplerian orbits are named for John Kepler.

keplerian_orbits

Kiviuq

A moon of 16 km diameter at about 11,111,000 km from the planet Saturn. The gravity at its surface is about 0.0004 times as strong as on Earth. The moon goes once around its planet in about 437.5 days. The moon was discovered in 2000. Also called SXXIV (Saturn twenty-four). Its provisional designation was S/2000 S5.

kpc

Ll

<J L>
Lac

Lacerta

Lacerta (Lizard) is a northern constellation, close to the Milky Way. The official abbreviation is Lac.

Lacertae

the Lagrange point

plural: Lagrange points; after J.-L. Lagrange (1736 - 1813)

A Lagrange point is a point in a system of two celestial bodies that orbit their common center of gravity in circular orbits, in which a space ship or other thing of negligible mass can remain for a long time without needing propulsion. Such a system of two bodies in circular orbits has five Lagrange points.

lagrange_point

lagrange_points

the Large Magellanic Cloud

The Large Magellanic Cloud is a small, irregular galaxy that is very close to our own Milky Way Galaxy, at about 179,000 lightyears from us. The Large Magellanic Cloud is in the constellation Dorado and can (sometimes) be seen with the unaided eye from places south of 30 degrees north latitude.

The Large Magellanic Cloud (abbreviation LMC) is called "Nubecula Major" in Latin.

Larissa

A moon of 194 km diameter at about 73,550 km from the planet Neptune. The gravity at its surface is about 0.0035 times as strong as on Earth. The moon goes once around its planet in about 13.3 hours. The moon was discovered in 1989. Also called NVII (Neptune seven). Its provisional designation was S/1989 N2.

the Last Quarter

The Last Quarter is a phase of the Moon, equal to

  1. [daily life] the period when about half of the lunar disk is illuminated, about a week before New Moon.
  2. [astronomy] the instant at which the ecliptic longitude of the center of the Moon is exactly 270 degrees greater than that of the center of the Sun.

last_quarters

the latitude

Latitude is a coordinate that is used to fix positions on a sphere. The latitude of a place is the distance of that place from the equator of the coordinate system, measured in degrees along a meridian. Places at the equator have a latitude of 0; the north pole has a latitude of +90° (or 90° north latitude), and the south pole has latitude −90° (or 90° south latitude). In the sky, latitude is used in the ecliptic and galactic coordinate systems. The corresponding second coordinate is the longitude.

latitudes

laws_of_kepler

laws_of_newton

law_of_conservation_of_energy

law_of_kepler

law_of_newton

the leap year

plural: leap years

A leap year is a year that contains one day more than ordinary years. Leap years are necessary to keep the calendar in step with the seasons.

leap_year

leap_years

Leda

A moon of 20 km diameter at about 11,165,000 km from the planet Jupiter. The gravity at its surface is about 0.0007 times as strong as on Earth. The moon goes once around its planet in about 241.0 days. The moon was discovered in 1974. Also called JXIII (Jupiter thirteen).

Leo

Leo (Lion) is an equatorial constellation, close to the ecliptic. The official abbreviation is Leo.

Leo Minor

Leo Minor (Little Lion) is a northern constellation, close to the ecliptic. The official abbreviation is LMi.

Leonis

Leonis_Minoris

Lep

Leporis

Lepus

Lepus (Hare) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Lep.

Level

Lib

Libra

Libra (Scales) is an equatorial constellation, close to the ecliptic. The official abbreviation is Lib.

Librae

life

light

the lightyear

'lightyear, plural lightyears

A lightyear is the distance that light travels in a vacuum in a year. So, a lightyear is not a measure of time but of distance. A lightyear is not an official unit and it is therefore not specified which definition of year you should use to calculate its length. You may use the length of the Julian year. With that year, a lightyear corresponds to about 9,460,730,000,000 km or 5,878,640,000,000 miles or 9.46073 Pm or 9.46073 × 1015 m or 63241.08 AU or 0.3066 pc.

lightyears

light_year

light_years

Lion

Little_Bear

Little_Dog

Little_Lion

Little_Water_Snake

Lizard

LMC

LMi

ln

log

the longitude

The longitude is a coordinate that is used to measure lccations on a sphere or directions in the sky. The longitude of a location is equal to the distance of the meridian of that location from the prime meridian of the coordinate system, measured in degrees along the equator.

Places on the prime meridian have a longitude equal to zero. On almost all celestial bodies in the Solar System and also in the sky, the longitude is measured from 0° to 360° such that it increases towards the west. On Earth and the Moon, the longitude is measured eastward and westward to 180° from the prime meridian.

In the sky, longitude is used in the ecliptic and galactic coordinate systems. The corresponding second coordinate is the latitude.

longitudes

lunar

'lu‧nar; lunaire = [French] of the Moon; from [Latin] Luna = Moon

Lunar means it has something to do with the Moon.

lunar_eclipse

lunar_eclipses

lunastice

lunastices

lunisolar

Luna = [Latin] Moon, Sol = [Latin] Sun

Lunisolar means it has something to do with both the Sun and the Moon.

Lup

Lupi

Lupus

Lupus (Wolf) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Lup.

ly

Lyn

Lyncis

Lynx

Lynx is a northern constellation, close to the ecliptic. The official abbreviation is Lyn.

Lyr

Lyra

Lyra (Lyre) is a northern constellation. The official abbreviation is Lyr.

Lyrae

Lyre

Lysithea

A moon of 36 km diameter at about 11,717,000 km from the planet Jupiter. The gravity at its surface is about 0.0013 times as strong as on Earth. The moon goes once around its planet in about 259.1 days. The moon was discovered in 1938. Also called JX (Jupiter ten).

Mm

<K M>

Mab

A moon of 32 km diameter at about 97,700 km from the planet Uranus. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 22.1 hours. The moon was discovered in 2003. Also called UXXVI (Uranus twenty-six). Its provisional designation was S/2003 U1.

the magnetic field

hè Magnetis lithos = [Greek] stone of Magnesia

The magnetic field is a force field that is associated with moving electrical charge. A magnetic field can influence electrically charged particles and certain metals by attracting or repulsing them. Almost all solar gas is susceptible to the effects of magnetic field.

Magnetic field behaves as if it is made up of closed magnetic field lines (as you can see if you hold a magnet under a transparent plate with iron filings on it and then gently tap the plate). Magnetic field on the Sun appears to exist in only two forms: either it is so weak that it is swept away by the motion of the solar gas, or it is so strong that it inhibits free motion of the gas (for example in convection). In the latter case, the magnetic field is made up of flux tubes: isolated tube-like things in which the magnetic field is strong, while it is weak or absent outside of the flux tube. Most interesting things on the Sun have something to do with magnetic field: sunspots, pores, plage, filaments, solar flares, and prominences. A famous quote in solar physics, attributed to Robert B. Leighton (around 1970), is

If the Sun didn't have a magnetic field, then it would be as boring as most people think it is.

Solar physicists measure the strength of the magnetic field in units of one gauss (G). The magnetic field of the Earth is at most about 1 G strong. The magnetic field in a sunspot at the solar surface can reach a strength of 3000 G.

magnetic_field

magnetic_fields

the magnitude

In general, "magnitude" means about the same as "importance". In science, a magnitude is usually a logarithmic measure, such as for the strength of an earthquake or the brightness of a star.

In astronomy, the magnitude or apparent magnitude is a measure for how bright something appears to be (at the distance that it has). The brightest stars in the sky have a magnitude of about 0 or 1. The dimmest stars that someone with good eyesight can just make out under good conditions (far from city lights and other lights) without optical aids (such as field glasses or a telescope) have a magnitude of about 6 (are of about the sixth magnitude). The greater (more positive) the magnitude, the less is the brightness.

The scale of magnitudes is a logarithmic scale. A difference of 1 magnitude corresponds to a ratio of about 2.5, and a difference of 5 magnitudes corresponds to exactly a ratio of 100: A star of magnitude 0 appears 100 times brighter than a star of magnitude 5. Stars and other things can also have a negative magnitude. The Sun has a magnitude of about −26.7 and the average full moon has a magnitude of about −12.6. Venus is of about magnitude −4 at its brightest, and the brightest star in the sky, Sirius, has magnitude −1.5. The dimmest things seen so far (with large telescopes and long exposure times) have a magnitude of about +31.

magnitudes

magnitudes_per_square_degree

the main sequence

The Main Sequence is a narrow strip in the Hertzsprung-Russell Diagram where most stars spend most of their lives. A star on the Main Sequence is called a main-sequence star. A main-sequence star gets its energy from converting hydrogen to helium in nuclear fusion reactions.

main-sequence_star

main-sequence_stars

the major axis

The major axis is the longest straight line that fits in an ellipse. One half of the major axis is called the semimajor axis and that one is used often for one of the orbital elements.

major_axes

major_standstill

major_standstills

march_equinox

Margaret

A moon of 11 km diameter at about 14,345,000 km from the planet Uranus. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 4.5 years. The moon was discovered in 2003. Also called UXXIII (Uranus twenty-three). Its provisional designation was S/2003 U3.

Mariner's_Compass

Mars

Mars = [Latin] god of war

Mars is the fourth planet of the Solar System, counting from the Sun. Mars is a terrestrial planet with a very thin atmosphere (of mostly carbon dioxide) and two small moons but no rings. Mars has a diameter of 6794 km and is number 8 on the list of largest objects in our Solar System.

the mean conjunction

A mean conjunction is a conjunction that has been calculated without taking numerous small perturbations into account, so a mean conjunction usually deviates somewhat from the corresponding true conjunction. On the other hand, the formulas for mean conjunctions are usually reasonably simple. The true conjunction is often calculated by adding correction terms to the mean conjunction.

mean_anomaly

mean_conjunctions

Megaclite

A moon of 5 km diameter at about 23,493,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 735.5 days. The moon was discovered in 2000. Also called JXIX (Jupiter nineteen). Its provisional designation was S/2000 J8.

Men

Mensa

Mensa (Table Mountain) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Men.

Mensae

Mercury

'Mer‧cu‧ry; Mercurius = [Latin] the messenger of the gods

  1. In astronomy, Mercury is the first planet of our Solar System, counting from the Sun. Mercury is a terrestrial planet without moons, rings, or an atmosphere. Mercury has a diameter of 4880 km and is number 11 on the list of largest objects in our Solar System.
  2. On Earth, mercury is a metal that is liquid at room temperature.

the meridian

meridianus = [Latin] concering noon

A meridian is

  1. a half-circle (semicircle) that runs across the surface between the geographic poles of a planetary body. Meridians intersect the equator at right angles. Observers on the same meridian have the same solar time.
  2. the half-circle that runs from the equatorial north pole (near the Pole Star) via the zenith to the equatorial south pole. When the Sun goes through the meridian, then it is highest in the sky for the day, and then it is noon. Also called the celestial meridian.

meridians

the meteor

meteooros = [Greek] lifted up, floating in the air

Meteors or falling stars are bits of stone, usually of the size of a grain or sand or smaller, that enter the atmosphere, are heated by friction with the air, and then burn up (usually completely) while they shine brightly. Meteors are visible as fast-moving and only momentarily visible points of light, usually soundless. Most meteors have burned up completely when they are still at great heights (70 km or so above the ground). Sometimes they leave behind a glowing trail that lasts for a little while. If a larger meteor breaks up into many pieces during its fall, then this often yields an explosion of light. Large meteors can appear brighter in the sky than the brightest stars and planets.

In space, before a meteor reaches the atmosphere, it is called a meteoroid. If a meteor does not burn up completely, so that part of it reaches the ground, then such a piece is called a meteorite.

the meteorite

meteooros = [Greek] lifted up, floating in the air

A meteorite is a piece of rock that came out of space and fell to the ground. Before it entered the atmosphere it was a meteoroid, and when it was passing through the atmosphere it was a meteor.

meteorites

the meteoroid

meteooros = [Greek] lifted up, floating in the air

A meteoroid is a piece of rock that floats in space. If it enters the atmosphere then it is called a meteor, and if it reaches the ground then it is called a meteorite.

meteoroids

meteors

Methone

A moon of 6 km diameter at about 194,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 24.2 hours. The moon was discovered in 2004. Also called SXXXII (Saturn thirty-two). Its provisional designation was S/2004 S1.

Metis

A moon of 43 km diameter at about 128,000 km from the planet Jupiter. The gravity at its surface is about 0.0018 times as strong as on Earth. The moon goes once around its planet in about 7.1 hours. The moon was discovered in 1979. Also called JXVI (Jupiter sixteen). Its provisional designation was S/1979 J3.

MI

Mic

Microscope

Microscopii

Microscopium

Microscopium (Microscope) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Mic.

the microwave

mikros = [Greek] small

Microwaves are a kind of electromagnetic radiation with wavelengths between those of infrared light and radio waves.

microwaves

MII

milanković_calendar

Milky_Way

milky_way

Milky_Way_Galaxy

millennia

the millennium

plural: millennia, millenniums

A millennium is a period of 1000 years. The 3rd millennium runs from the year 2001 until and including the year 3000.

millenniums

Mimas

A moon of 397 km diameter at about 185,540 km from the planet Saturn. The gravity at its surface is about 0.0065 times as strong as on Earth. The moon goes once around its planet in about 22.7 hours. The moon was discovered in 1789. Also called SI (Saturn one).

minor_standstill

minor_standstills

the minute

minute, plural minutes; minutus = [Latin] made small

A minute is a unit of time. A minute is the 60th part of an hour. A minute is further divided into 60 seconds.

minutes

minutes_of_arc

minute_of_arc

Miranda

A moon of 471 km diameter at about 129,900 km from the planet Uranus. The gravity at its surface is about 0.0081 times as strong as on Earth. The moon goes once around its planet in about 33.9 hours. The moon was discovered in 1948. Also called UV (Uranus five).

Mneme

A moon of 2 km diameter at about 21,069,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 624.6 days. The moon was discovered in 2003. Also called JXL (Jupiter forty). Its provisional designation was S/2003 J21.

Mon

Monoceros

Monoceros (Unicorn) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Mon.

Monocerotis

the month

A month is a period of time that is (historically) tied to the motion of the Moon around the Earth. Quite a few different kinds of months are in use:

De next table shows the average length of the astronomically determined months at 1 January 2000, and by how much this length changes in a century (derived from Meeus: Astronomical Algorithms, ISBN 0943396352). For example, on 1 January 2000 the average length of the synodical month was equal to 29 days, 12 hours, 44 minutes and 2.877 seconds, and this length increases at 0.019 seconds per century.

Table 4: Month Lengths

Month days d h m s s/century
sidereal 27.32166155 27 07 43 11.558 +0.019
synodical 29.53058885 29 12 44 02.877 +0.019
tropical 27.32158224 27 07 43 04.706 +0.013
anomalistic 27.55454988 27 13 18 33.109 −0.090
draconic 27.21222082 27 05 05 35.879 +0.033

months

Moon

the moon

moon, plural moons

A moon is a celestial body that orbits around a bigger celestial body. The Earth not only has a natural moon (the Moon, 3496 km large, number 14 on the list of largest objects in our Solar System), but also a couple of thousands of artificial moons, launched from the surface of the Earth. Most planets of our Solar System have one or more moons. The biggest planets have more than ten. Another word for moon is satellite. The month is named for the Moon.

moonrise

moons

moonset

mpc

Mundilfari

A moon of 7 km diameter at about 18,685,000 km from the planet Saturn. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 954.1 days. The moon was discovered in 2000. Also called SXXV (Saturn twenty-five). Its provisional designation was S/2000 S9.

Mus

Musca

Musca (Fly) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Mus.

Muscae

Nn

<L N>

the nadir

nadzir as samt = [Arab] opposite the zenith

The nadir is the direction straight down, at altitude −90°. The opposite direction in the sky is called the zenith.

Naiad

A moon of 66 km diameter at about 48,230 km from the planet Neptune. The gravity at its surface is about 0.0012 times as strong as on Earth. The moon goes once around its planet in about 7.1 hours. The moon was discovered in 1989. Also called NIII (Neptune three). Its provisional designation was S/1989 N6.

the names of celestial bodies

The brightest celestial bodies, such as the Sun, Moon, and planets, have their own names in pretty much every language. If a common name is needed, then usually a greek or latin name is used. The following table shows the english, greek, and latin names of a number of celestial bodies, and also the prefixes and postfixes that can be used to refer to those bodies.

Table 5: Planet Names in Languages, Prefixes, and Postfixes

English Greek Latin Prefixes Postfixes
Sun Hèlios Sol helio- -helium
Moon Selènè Luna seleno- -selenium
Mercury Hermes Mercurius mecurio- -mercurium
Venus Aphroditè Venus venero- -venerum
Earth Terra geo- -gee
Mars Ares Mars aro- -martium
Jupiter Zeus Iupiter jovi- -jovum
Saturn Chronos Saturnus saturno- -saturnum
Uranus Ouranos Uranus urano- -uranum
Neptune Poseidon Neptunus neptuno- -neptunum
Pluto Ploutoon Pluto pluto- -plutum
Milky Way galaxias kuklos galaxis galacto- -galactium
planet planètès astèr planetus planeto- -planetum
star astèr siderus astro- sider- -astron

Some examples: geography, selenographical, heliocentric, periastron, apogee, sidereal, astronomy.

name_of_a_celestial_body

narrowband_filter

narrowband_filters

Narvi

A moon of 7 km diameter at about 19,007,000 km from the planet Saturn. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 978.9 days. The moon was discovered in 2003. Also called SXXXI (Saturn thirty-one). Its provisional designation was S/2003 S1.

Neptune

Neptune = [Latin] God of the Sea

Neptune is the eighth planet of the Solar System, counting from the Sun. Neptune is a jovian planet with a very dense and thick atmosphere (made mostly of hydrogen and helium) and many moons. Neptune has a diameter of 49,532 km and is number 5 on the list of largest objects in our Solar System.

Nereid

A moon of 340 km diameter at about 5,513,400 km from the planet Neptune. The gravity at its surface is about 0.0073 times as strong as on Earth. The moon goes once around its planet in about 360.1 days. The moon was discovered in 1949. Also called NII (Neptune two).

Net

the New Moon

The New Moon is a phase of the Moon, equal to

  1. [daily life] the period when the Moon is so close to the Sun in the sky that you cannot see the Moon. The Moon is then roughly between the Sun and the Earth.
  2. [astronomy] the instant of time when, as seen from Earth, the center of the Moon has the same ecliptic longitude as the center of the Sun.

Newton

newton's_law

newton's_laws

newton's_law_of_gravity

new_moons

NGC_292

NI

Nicholas_Copernicus

NII

NIII

NIV

Nix

A moon of 50 km diameter at about 48,700 km from the planet Pluto. The gravity at its surface is about 0.0007 times as strong as on Earth. The moon goes once around its planet in about 24.9 days. The moon was discovered in 2005. Its provisional designation was S/2005 P2.

the node

The nodes of the orbit of a celestial body are the two places where the orbit passes through the base plane of the coordinate system. The node at which the object goes through the base plane from south to north is called the ascending node, and the node at which the object goes from north to south through the base plane is called the descending node.

nodes

Nor

Norma

Norma (Level) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Nor.

Normae

the northern solstice

The northern solstice is the solstice at which the Sun reaches the furthest north of the celestial equator. This is at midsummer's day at the start of summer in the northern hemisphere, and at midwinter's day at the start of winter in the southern hemisphere. In the Gregorian calendar, this is near 21 June.

Northern_Crown

northern_solstice

northern_solstices

Nubecula_Major

Nubecula_Minor

nuclear_force

nuclear_forces

nuclear_fusion

nuclear_reaction

nuclear_reactions

large numbers

Here is a list of British English and American English names of large numbers and small fractions, and the prefixes of units that go with them. In the title row, "om" stands for order of magnitude: this indicates how many zeros are involved. The prefix can be used with units: for example, a kilometer or km is equal to 1,000 meters, and a megameter or Mm is equal to 1,000,000 m or 1,000 kilometers.

Table 2: Very Large and Very Small Numbers

British American om prefix number
21 1.000.000.000.000.000.000.000
trillion 18 exa = E 1.000.000.000.000.000.000
quadrillion 15 peta = P 1.000.000.000.000.000
billion trillion 12 tera = T 1.000.000.000.000
billion 9 giga = G 1.000.000.000
million million 6 mega = M 1.000.000
thousand thousand 3 kilo = k 1.000
hundred hundred 2 hecto = h 100
ten ten 1 deca = da 10
tenth tenth −1 deci = d 1/10
hundredth hundredth −2 centi = c 1/100
thousandth thousandth −3 milli = m 1/1.000
millionth millionth −6 micro = µ 1/1.000.000
billionth −9 nano = n 1/1.000.000.000
billionth trillionth −12 pico = p 1/1.000.000.000.000
quadrillionth −15 femto = f 1/1.000.000.000.000.000
trillionth −18 atto = a 1/1.000.000.000.000.000.000
−21 1/1.000.000.000.000.000.000.000

numbers

NV

NVI

NVII

NVIII

Oo

<M O>

Oberon

A moon of 1522 km diameter at about 583,500 km from the planet Uranus. The gravity at its surface is about 0.0354 times as strong as on Earth. The moon goes once around its planet in about 13.5 days. The moon was discovered in 1787. Also called UIV (Uranus four).

Oct

Octans

Octans (Octant) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Oct.

Octant

Octantis

Oph

Ophelia

A moon of 42 km diameter at about 58,800 km from the planet Uranus. The gravity at its surface is about 0.0008 times as strong as on Earth. The moon goes once around its planet in about 10.3 hours. The moon was discovered in 1986. Also called UVII (Uranus seven). Its provisional designation was S/1986 U8.

Ophiuchi

Ophiuchus

Ophiuchus (Serpent Bearer) is an equatorial constellation, close to the ecliptic. The official abbreviation is Oph.

the opposition

oppositio = [Latin] opposite

Two celestial bodies are in opposition when they are in opposite directions in the sky (as seen from Earth). If only one celestial body is mentioned, then the second body is understood to be the Sun. Only planets or other celestial bodies that are further from the Sun than the Earth is can be in opposition (to the Sun). Around that time, they are above the horizon all night, and therefore the time of opposition is usually the best time to see the object. The opposite of an opposition is a conjunction.

oppositions

orbit-orbit_resonance

orbit-orbit_resonances

the orbital element

The orbits of things around much more massive celestial bodies (for example, of a planet around the Sun, or of a moon around a planet, or of a spacecraft around a moon, planet, or the Sun) are often very close to conic sections such as circles, ellipses, parabolas, hyperbolas or straight lines. Five numbers are needed to specify the size, shape, and orientation of such an orbit, and with a sixth number you can also fix the position of the thing in the orbit. These six numbers are the orbital elements.

The most common set of orbital elements consists of the length of the semimajor axis and the eccentricity for the size and shape of the orbit, and the inclination, the ecliptic longitude of the ascending node, and the argument of the perifocus (the three of them called the Euler angles) for the orientation of the orbit. For the sixth orbital element, either the mean anomaly at a certain time or a time at which the object goes through its perifocus is often used.

orbital_elements

the order of magnitude

An order of magnitude is a factor of about 10. Orders of magnitude are used a lot by astronomers and physicists, who study very small and very large things and so talk about numbers that may not be known very precisely but that have many zeros before or after the decimal point. If an astronomer says that some thing is three orders of magnitude larger than some other thing, then the astronomer means that the second thing is about 1000 times larger than the first thing. It could be 3000 times, or perhaps only 500 times, but not only 100 times or as much as 10,000 times.

For each extra order of magnitude, you must multiply by an extra factor of 10. Orders of magnitude are especially handy for very large numbers. It is easier to talk about 18 orders of magnitude than about the number one trillion (How many zeros did that have again in British English? And wasn't that different in American English?) or one million million million, or 1,000,000,000,000,000,000, or 1018. 18 orders of magnitude is about the ratio between the diameter of the Earth and the diameter of an atom, and also about the ratio of the diameter of the visible Universe and the diameter of the Earth.

orders_of_magnitude

Ori

Orion

Orion is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Ori.

Orionis

Orthosie

A moon of 2 km diameter at about 20,720,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 609.2 days. The moon was discovered in 2001. Also called JXXXV (Jupiter thirty-five). Its provisional designation was S/2001 J9.

Pp

<N P>

Paaliaq

A moon of 22 km diameter at about 15,200,000 km from the planet Saturn. The gravity at its surface is about 0.0005 times as strong as on Earth. The moon goes once around its planet in about 700.1 days. The moon was discovered in 2000. Also called SXX (Saturn twenty). Its provisional designation was S/2000 S2.

Painter's_Easel

Pallene

A moon of 8 km diameter at about 211,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 27.5 hours. The moon was discovered in 2004. Also called SXXXIII (Saturn thirty-three). Its provisional designation was S/2004 S2.

Pan

A moon of 25 km diameter at about 133,600 km from the planet Saturn. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 13.8 hours. The moon was discovered in 1990. Also called SXVIII (Saturn eighteen). Its provisional designation was S/1981 S13.

Pandora

A moon of 84 km diameter at about 141,700 km from the planet Saturn. The gravity at its surface is about 0.0006 times as strong as on Earth. The moon goes once around its planet in about 15.1 hours. The moon was discovered in 1980. Also called SXVII (Saturn seventeen). Its provisional designation was S/1980 S26.

the parallax

The parallax is a motion that an object seems to make because the observer has moved to a different location. The parallax of a star is the angle over which the star moves during a year because of the motion of the Earth around the Sun. If that parallax is equal to (p) arcseconds, then that star is at (1/p) parsec from the Earth.

the parsec

'par‧sec, plural parsecs; parallax + second

The parsec is approximately the distance at which the radius of the orbit of the Earth around the Sun covers an angle of 1 second of arc. A star at a distance of 1 parsec shows an annual parallax of 1 second of arc. A parsec corresponds to 648,000/π = about 206,264.8062 AU or 30,856,780,000,000 km or 19,173,560,000,000 miles or 3.085678 × 1013 m or 30.085678 Pm or 3.2616 lightyears. A parsec may be abbreviated to pc. In astronomy, distances occur that are large even when measured in parsecs. Therefore standard abbreviations with SI-prefixes are used, too, such as kpc for 1000 pc, Mpc for 1000 kpc, and even Gpc for 1000 Mpc.

parsecs

partial_solar_eclipse

Pasiphae

A moon of 60 km diameter at about 23,624,000 km from the planet Jupiter. The gravity at its surface is about 0.0023 times as strong as on Earth. The moon goes once around its planet in about 741.6 days. The moon was discovered in 1908. Also called JVIII (Jupiter eight).

Pasithee

A moon of 2 km diameter at about 23,004,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 712.6 days. The moon was discovered in 2001. Also called JXXXVIII (Jupiter thirty-eight). Its provisional designation was S/2001 J6.

Pav

Pavo

Pavo (Peacock) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Pav.

Pavonis

pc

Peacock

Peg

Pegasi

Pegasus

Pegasus is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Peg.

the penumbra

paene = [Latin] almost; umbra = [Latin] shadow

The penumbra is the outermost part of a sunspot, in which thread-like light and dark filaments mostly point away from the umbra. These structures are formed by strong magnetic field.

penumbrae

penumbras

Per

Perdita

A moon of 26 km diameter at about 76,420 km from the planet Uranus. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 15.3 hours. The moon was discovered in 1999. Also called UXXV (Uranus twenty-five). Its provisional designation was S/1986 U10.

periapsiden

the perifocus

peri- = [Greek] around; hapsis = [Greek] connection

The perifocus is the point in an orbit around a celestial body that is closest to the celestial body. The opposite point is the apofocus. For some celestial bodies more specific names are available that refer to just those bodies, for example perihelion for the Sun, perigee for the Earth, periselene for the Moon, perijove for Jupiter, or periastron for stars.

the perigee

peri- = [Greek] around; Gè = [Greek] Earth

The perigee is the point in an orbit around the Earth that is closest to the Earth. The opposite point is called apogee. The more general word, that can be used also for other bodies, is perifocus.

perigees

the perihelion

peri- = [Greek] around; Helios = [Greek] Sun

The perihelion is the point in an orbit around the Sun that is closest to the Sun. The opposite point is called aphelion. The more general word is perifocus.

perihelions

Persei

Perseus

Perseus is a northern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Per.

Phe

Phobos

A moon of 22 km diameter at about 9,380 km from the planet Mars. The gravity at its surface is about 0.0006 times as strong as on Earth. The moon goes once around its planet in about 7.7 hours. The moon was discovered in 1877. Also called MI (Mars one).

Phoebe

A moon of 219 km diameter at about 12,947,800 km from the planet Saturn. The gravity at its surface is about 0.0047 times as strong as on Earth. The moon goes once around its planet in about 550.4 days. The moon was discovered in 1898. Also called SIX (Saturn nine).

Phoenicis

Phoenix

Phoenix is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Phe.

the photon

A photon is a quantum of electromagnetic radiation. Electromagnetic radiation, such as light, behaves as if it is made up out of a stream of individual photons. All photons of a particular color (wavelength, frequency) have the same amount of energy, which is proportional to the frequency of the radiation.

photons

the photosphere

phootos = [Greek] light; sphairos = [Greek] ball

The photosphere is the deepest layer of the Sun that we can see. This layer reaches from the surface visible in the center of the solar disk to about 500 km above that height. The temperature in the photosphere reaches from about 6500 K at the bottom to about 4000 K at the top, and the density of material in the photosphere reaches from about 4000 to 200,000 times smaller than the density of air at sea level on Earth. Almost the whole photosphere is covered with granulation.

photospheres

photospheric

Photospheric means it has something to do with the photosphere of the Sun or a star.

photospherical

PI

Pic

Pictor

Pictor (Painter's Easel) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Pic.

Pictoris

Pisces

Pisces (Fishes) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Psc.

Piscis Austrinus

Piscis Austrinus (Southern Fish) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is PsA.

Piscis_Austrini

Piscium

the plage

plage = [French] beach

Plage is an area on the Sun that is brighter than its surroundings if one looks at it in the middle of a spectral line. The greater brightness means that relatively small magnetic flux tubes stick through the surface there. Plage is invisible in images taken in the continuum, except close to the edges of the solar disk, but even there the contrast between plage and its surroundings is quite small.

planet

'pla‧net, plural planets; planètès astèr = [Greek] wandering star

A planet is a large spherical object with a diameter between about 1000 and 300,000 km. A planet is massive enough that its own gravity keeps it round (in contrast to asteroids, comets, and small moons), but too small to generate energy in its core through nuclear fusion (in contrast to stars). A planet orbits directly around a star (in contrast to moons) and has no similar other objects near its orbit (in contrast to dwarf planets).

the planetary system

plural: planetary systems

A planetary system is a planet with all things that orbit around it, such as moons.

planetary_system

planetary_systems

planetocentric

planètès astèr = [Greek] wandering star; kentrum = [Greek] middle, center

Planetocentric means: with a planet at the center, or relative to the center of a planet. Planetocentric coordinate systems have the center of the planet for origin, and are almost always rectangular coordinate systems, but vary in the directions of the coordinate axes. Oft-used planetocentric coordinate systems are the planetocentric equatorial coordinate system and the planetocentric ecliptic coordinate system.

planetocentrical

planetocentrically

planets

Pluto

Pluto

Pluto is the ninth planet of the Solar System, counting from the Sun. Pluto is a small icy planet with one moon, Charon. Pluto has a diameter of 2274 km and is number 17 on the list of largest objects in our Solar System.

pluto

the polar day

plural: polar days

A polar day is a day during which the Sun does not get below the horizon for at least 24 hours. Similarly, a polar night is a night during which the Sun does not get above the horizon for at least 24 hours. Polar days and polar nights occur only above the polar circles, near the geographical poles.

polar_day

polar_days

polar_night

polar_nights

the pole

A pole of a celestial body is one of the two intersections of the surface of the body and the rotation axis of the body. Traditionally, the pole from above where the body appears to rotate counterclockwise is called the north pole, and the pole from above where the body appears to rotate clockwise is called the south pole. Seen from the north pole, the Sun goes through the sky from left to right. Seen from the south pole, the Sun goes from right to left.

poles

Polydeuces

A moon of 8 km diameter at about 377,420 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 65.7 hours. The moon was discovered in 2004. Also called SXXXIV (Saturn thirty-four). Its provisional designation was S/2004 S5.

the pore

poros = [Latin] opening

A pore is like a sunspot but has no penumbra.

pores

Portia

A moon of 135 km diameter at about 66,100 km from the planet Uranus. The gravity at its surface is about 0.0025 times as strong as on Earth. The moon goes once around its planet in about 12.3 hours. The moon was discovered in 1986. Also called UXII (Uranus twelve). Its provisional designation was S/1986 U1.

pp-reaction

pp-reactions

Praxidike

A moon of 6 km diameter at about 20,907,000 km from the planet Jupiter. The gravity at its surface is about 0.0003 times as strong as on Earth. The moon goes once around its planet in about 617.4 days. The moon was discovered in 2000. Also called JXXVII (Jupiter twenty-seven). Its provisional designation was S/2000 J7.

the precession

praecessio = [Latin] going first

The rotation axis of the Earth is slowly spinning around the poles of the ecliptic because of the gravitational attraction between the equatorial bulges of the Earth and the Moon and Sun. It takes about 26,000 years before the axis has spun around once. This spinning motion is called precession.

One result of the precession is that the vernal equinox slowly moves between the stars in the sky, at a speed of one degree each 71.6 years. This motion is called the precession of the equinoxes. The places between the stars in the sky where the Sun is at the beginning of the seasons are tied to the vernal equinox, and so are also influenced by the precession. Since about the year −68, the vernal equinox moves through the modern constellation of Pisces (the Fishes), and it will move into the constellation of Aquarius (the Waterman) around the year 2597.

Another result of the precession is that the poles of the sky also move between the stars. At the moment, the celestial north pole is quite close to the bright star alpha Ursae Minoris (alpha UMi, also called Polaris or the North Star).

precession_of_the_equinoxes

the prime meridian

plural: prime meridians

The prime meridian is the meridian of which the longitude is zero. It is a free choice which meridian should be the prime meridian. In the past, each country used to use a prime meridian for their maps that ran through their own capital city. Nowadays almost everyone uses the prime meridian that runs through Greenwich in England.

proleptic

Proleptic means that it is applied to cases from before it was invented. This is used most in calendars: you can apply the rules of a new calendar to years from before that new calendar was invented.

proleptical

proleptic_calendar

proleptic_calendars

Prometheus

A moon of 100 km diameter at about 139,400 km from the planet Saturn. The gravity at its surface is about 0.0005 times as strong as on Earth. The moon goes once around its planet in about 14.8 hours. The moon was discovered in 1980. Also called SXVI (Saturn sixteen). Its provisional designation was S/1980 S27.

the prominence

'pro‧mi‧nence, plural prominences; prominere = [Latin] to stick out, be prominent

A prominence is a bright cloud of solar gas that sticks out beyond the limb of the Sun. If such a cloud is seen against the even brighter solar disk, then it appears dark and is called a filament.

prominences

Prospero

A moon of 30 km diameter at about 16,256,000 km from the planet Uranus. The gravity at its surface is about 0.0006 times as strong as on Earth. The moon goes once around its planet in about 5.4 years. The moon was discovered in 1999. Also called UXVIII (Uranus eighteen). Its provisional designation was S/1999 U3.

Proteus

A moon of 419 km diameter at about 117,650 km from the planet Neptune. The gravity at its surface is about 0.0078 times as strong as on Earth. The moon goes once around its planet in about 26.9 hours. The moon was discovered in 1989. Also called NVIII (Neptune eight). Its provisional designation was S/1989 N1.

proton-proton_reaction

proton-proton_reactions

PsA

Psc

Ptolemy

Puck

A moon of 162 km diameter at about 86,000 km from the planet Uranus. The gravity at its surface is about 0.0030 times as strong as on Earth. The moon goes once around its planet in about 18.3 hours. The moon was discovered in 1985. Also called UXV (Uranus fifteen). Its provisional designation was S/1985 U1.

Pup

Puppis

Puppis (Stern) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Pup.

Pyx

Pyxidis

Pyxis

Pyxis (Mariner's Compass) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Pyx.

Qq

<O Q>

the quadrature

Two celestial bodies are in quadrature when they are 90° (i.e., a quarter of the circumference of the sky) apart in the sky.

quadratures

Quaoar

the quasar

quasar, plural quasars; [English] quasi-stellar radio source

A quasar is a celestial body that at first sight looks like a star (i.e., like a small point of light), but that after closer investigation often turns out to be in the center of a galaxy. A quasar is far brighter than the rest of the galaxy, and can therefore be seen at much greater distances than the galaxy itself. Corrected for the distance, quasars are about 100 times brighter than the brightest galaxies.

Astronomers think that quasars are really large black holes that capture great amounts of material from the surrounding galaxy, and that that material emits much energy as it falls into the black hole, not just as visible light, but also as, for example, radio waves, which is where quasars were first discovered.

All quasars are far away, which means they existed long ago only. It seems that many galaxies were quasars for a while in their youth.

quasars

Rr

<P R>
radio

the radio wave

radius = [Latin] stick, radius

Radio waves are electromagnetic waves with wavelengths greater than those of microwaves.

radioactivity

radio_waves

Ram

the red giant

A red giant or red giant star is a star that is much bigger and cooler and redder than the Sun is. Red giants have diameters up to a few hundred times as large as that of the Sun. In about 5 thousand million years, the Sun will turn into a red giant, when the hydrogen in the center of the Sun starts to run out. The diameter of the Sun will then grow to about 40 times the size it has today.

the redshift

A redshift is a doppler shift, of characteristics (such as spectral lines) in the frequency spectrum of light, to smaller frequencies, so that yellow light gets more reddish. The opposite of redshift is blueshift. The ratio of the frequencies of redshifted characteristics and the frequencies of the unshifted characteristics is also called redshift, usually denoted by a variable called (z).

Systematic redshift is found in the spectra of all but the closest of galaxies, which is seen as evidence for the expansion of the Universe.

redshifts

red_giant

red_giants

red_shift

red_shifts

relativistic

Relativistic means that the speed or escape velocity becomes so great (namely comparable to the speed of light) that the classical theories of force and motion cannot make sufficiently accurate predictions anymore. To be able to make accurate predictions for relativistic circumstances, you need the full theories of relativity.

relativity_theories

relativity_theory

resolution

the resonance

In astronomy, a resonance is a case where small effects that can usually be ignored now do have a measurable impact because they happen exactly in step with something else. An orbit-orbit resonance occurs, for example, when a multiple of the orbital period of a moon is equal to the orbital period of another moon, or a multiple of it. A spin-orbit resonance occurs, for example, when a multiple of the spin period of a moon (around its own axis) is equal to the orbital period of the moon, or a multiple of it.

resonances

Ret

Reticuli

Reticulum

Reticulum (Net) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Ret.

retrograde_motion

Rhea

A moon of 1529 km diameter at about 527,070 km from the planet Saturn. The gravity at its surface is about 0.0269 times as strong as on Earth. The moon goes once around its planet in about 4.5 days. The moon was discovered in 1672. Also called SV (Saturn five).

the right ascension

The right ascension is the coordinate from the equatorial coordinate system in the sky that has the same role as the longitude in other coordinate systems. The right ascension is measured from the vernal equinox. The right ascension is usually measured not in degrees as the other longitudes are, but rather in units of time, such that 360 degrees correspond to 24 hours of right ascension, and 15 degrees to 1 hour of right ascension. Just like for real time, an hour (symbol: h) of right ascension is divided into 60 minutes (symbol: m), and one minute into 60 seconds (symbol: s). Here is an example of a right ascension: 5h23m12s, or 5 hours, 23 minutes, and 12 seconds.

right_angle

right_ascensions

the ring

plural: rings

In general, a ring is something flat and round that has an open center. In astronomy, rings occur around planets and then consist of small rocks, dust particles, and ice particles that orbit around the planet in the planet's equatorial plane.

rings

River_Eridanus

Rosalind

A moon of 72 km diameter at about 69,900 km from the planet Uranus. The gravity at its surface is about 0.0013 times as strong as on Earth. The moon goes once around its planet in about 13.4 hours. The moon was discovered in 1986. Also called UXIII (Uranus thirteen). Its provisional designation was S/1986 U4.

Ss

<Q S>
S/1975_J1_=_S/2000_J1

S/1978_P1

S/1979_J1

S/1979_J2

S/1979_J3

S/1980_S1

S/1980_S13

S/1980_S25

S/1980_S26

S/1980_S27

S/1980_S28

S/1980_S3

S/1980_S6

S/1981_S13

S/1985_U1

S/1986_U1

S/1986_U10

S/1986_U2

S/1986_U3

S/1986_U4

S/1986_U5

S/1986_U6

S/1986_U7

S/1986_U8

S/1986_U9

S/1989_N1

S/1989_N2

S/1989_N3

S/1989_N4

S/1989_N5

S/1989_N6

S/1997_U1

S/1997_U2

S/1999_J1

S/1999_U1

S/1999_U2

S/1999_U3

S/2000 J11

A moon of 4 km diameter at about 12,560,000 km from the planet Jupiter. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 287.5 days. The moon was discovered in 2000.

S/2000_J10

S/2000_J2

S/2000_J3

S/2000_J4

S/2000_J5

S/2000_J6

S/2000_J7

S/2000_J8

S/2000_J9

S/2000_S1

S/2000_S10

S/2000_S11

S/2000_S12

S/2000_S2

S/2000_S3

S/2000_S4

S/2000_S5

S/2000_S6

S/2000_S7

S/2000_S8

S/2000_S9

S/2001_J1

S/2001_J10

S/2001_J11

S/2001_J2

S/2001_J3

S/2001_J4

S/2001_J5

S/2001_J6

S/2001_J7

S/2001_J8

S/2001_J9

S/2001_U1

S/2001_U2

S/2001_U3

S/2002 N1

A moon of 48 km diameter at about 15,728,000 km from the planet Neptune. The gravity at its surface is about 0.0011 times as strong as on Earth. The moon goes once around its planet in about 4.7 years. The moon was discovered in 2002.

S/2002 N2

A moon of 48 km diameter at about 22,422,000 km from the planet Neptune. The gravity at its surface is about 0.0011 times as strong as on Earth. The moon goes once around its planet in about 8.1 years. The moon was discovered in 2002.

S/2002 N3

A moon of 48 km diameter at about 23,571,000 km from the planet Neptune. The gravity at its surface is about 0.0011 times as strong as on Earth. The moon goes once around its planet in about 8.7 years. The moon was discovered in 2002.

S/2002 N4

A moon of 60 km diameter at about 48,387,000 km from the planet Neptune. The gravity at its surface is about 0.0012 times as strong as on Earth. The moon goes once around its planet in about 25.6 years. The moon was discovered in 2002.

S/2002_J1

S/2003 J10

A moon of 2 km diameter at about 23,042,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 714.4 days. The moon was discovered in 2003.

S/2003 J12

A moon of 1 km diameter at about 15,912,000 km from the planet Jupiter. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 410.0 days. The moon was discovered in 2003.

S/2003 J14

A moon of 2 km diameter at about 23,614,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 741.2 days. The moon was discovered in 2003.

S/2003 J15

A moon of 2 km diameter at about 22,627,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 695.2 days. The moon was discovered in 2003.

S/2003 J16

A moon of 2 km diameter at about 20,963,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 619.9 days. The moon was discovered in 2003.

S/2003 J17

A moon of 2 km diameter at about 23,001,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 712.5 days. The moon was discovered in 2003.

S/2003 J18

A moon of 2 km diameter at about 20,514,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 600.1 days. The moon was discovered in 2003.

S/2003 J19

A moon of 2 km diameter at about 23,533,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 737.4 days. The moon was discovered in 2003.

S/2003 J2

A moon of 2 km diameter at about 29,541,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 2.8 years. The moon was discovered in 2003.

S/2003 J23

A moon of 2 km diameter at about 23,563,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 738.8 days. The moon was discovered in 2003.

S/2003 J3

A moon of 2 km diameter at about 20,221,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 587.3 days. The moon was discovered in 2003.

S/2003 J4

A moon of 2 km diameter at about 23,930,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 756.1 days. The moon was discovered in 2003.

S/2003 J5

A moon of 4 km diameter at about 23,495,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 735.6 days. The moon was discovered in 2003.

S/2003 J9

A moon of 1 km diameter at about 23,384,000 km from the planet Jupiter. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 730.4 days. The moon was discovered in 2003.

S/2003 N1

A moon of 28 km diameter at about 46,695,000 km from the planet Neptune. The gravity at its surface is about 0.0005 times as strong as on Earth. The moon goes once around its planet in about 24.3 years. The moon was discovered in 2003.

S/2003_J1

S/2003_J11

S/2003_J13

S/2003_J20

S/2003_J21

S/2003_J22

S/2003_J6

S/2003_J7

S/2003_J8

S/2003_S1

S/2003_U1

S/2003_U2

S/2003_U3

S/2004 S10

A moon of 6 km diameter at about 20,735,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 3.1 years. The moon was discovered in 2005.

S/2004 S11

A moon of 6 km diameter at about 17,119,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 836.7 days. The moon was discovered in 2005.

S/2004 S12

A moon of 5 km diameter at about 19,878,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 2.9 years. The moon was discovered in 2005.

S/2004 S13

A moon of 6 km diameter at about 18,403,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 932.6 days. The moon was discovered in 2005.

S/2004 S14

A moon of 6 km diameter at about 19,856,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 2.9 years. The moon was discovered in 2005.

S/2004 S15

A moon of 6 km diameter at about 19,338,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 2.8 years. The moon was discovered in 2005.

S/2004 S16

A moon of 4 km diameter at about 22,453,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 3.4 years. The moon was discovered in 2005.

S/2004 S17

A moon of 4 km diameter at about 19,447,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 2.8 years. The moon was discovered in 2005.

S/2004 S18

A moon of 7 km diameter at about 20,129,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 2.9 years. The moon was discovered in 2005.

S/2004 S7

A moon of 6 km diameter at about 20,999,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 3.1 years. The moon was discovered in 2005.

S/2004 S8

A moon of 6 km diameter at about 25,108,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 4.1 years. The moon was discovered in 2005.

S/2004 S9

A moon of 5 km diameter at about 20,390,000 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 3.0 years. The moon was discovered in 2005.

S/2004_S1

S/2004_S2

S/2004_S5

S/2005 S1

A moon of 6 km diameter at about 136,500 km from the planet Saturn. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 14.3 hours. The moon was discovered in 2005.

S/2005_P1

S/2005_P2

Sagitta

Sagitta (Arrow) is an equatorial constellation, close to the Milky Way. The official abbreviation is Sge.

Sagittae

Sagittarii

Sagittarius

Sagittarius (Archer) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Sgr.

Sails

saros

sarosses

saros_number

saros_numbers

saros_series

the satellite

satellite, plural satellites

A satellite is another name for a moon.

satellites

Saturn

Saturn; Saturnus = [Latin] father of Jupiter

Saturn is the sixth planet of the Solar System, counting from the Sun. Saturn is a jovian planet with a very dense and thick atmosphere (made mostly of hydrogen and helium), many moons, and wide rings. Saturn has an equatorial diameter of 120,536 km and is number 3 on the list of largest objects in our Solar System.

Scales

Schwarzschild

schwarzschild

schwarzschild_diameter

schwarzschild_diameters

schwarzschild_radii

schwarzschild_radius

schwarzschild_singularities

schwarzschild_singularity

the science

science, plural sciences

A science is a collection of knowledge and methods to check that knowledge for correctness and to get more knowledge. There are many sciences, such as physics, astronomy, mathematics, chemistry, geography, history, economics, and biology. A scientist is someone who does science for his work. A very important part of science is to check knowledge for correctness so that ideas that do not fit reality can be abandoned and only good ideas remain. Pseudo-sciences such as astrology and alchemy lack this self-correction.

sciences

scientific

scientifical

scientist

scientists

Scl

Sco

Scorpii

Scorpion

Scorpius

Scorpius (Scorpion) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Sco.

Sct

Sculptor

Sculptor is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Scl.

Sculptoris

Scuti

Scutum

Scutum (Shield) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Sct.

season

seasons

Sea_Goat

the second

second, plural seconds; hora minutus secundus = [Latin] second diminished hour

A second is a unit of time. A second is the 60th part of a minute.

seconds

seconds_of_arc

second_of_arc

Sedna

selenographic

Selènè = [Greek] Moon; graphia = [Greek] description

Selenographic means it describes the Moon. The selenographic coordinate system has the lunar equator for base plane, and uses the longitude and latitude for coordinates.

selenographical

semimajor_axes

semimajor_axis

september_equinox

Ser

Serpens

Serpens (Serpent) is an equatorial constellation, close to the ecliptic. The official abbreviation is Ser.

Serpent

Serpentis

Serpent_Bearer

Setebos

A moon of 30 km diameter at about 17,418,000 km from the planet Uranus. The gravity at its surface is about 0.0006 times as strong as on Earth. The moon goes once around its planet in about 6.0 years. The moon was discovered in 1999. Also called UXIX (Uranus nineteen). Its provisional designation was S/1999 U1.

Sex

Sextans

Sextans (Sextant) is an equatorial constellation, close to the ecliptic. The official abbreviation is Sex.

Sextant

Sextantis

Sge

Sgr

Shield

a shooting star

A shooting star is the same as a meteor.

shooting_star

shooting_stars

SI

Siarnaq

A moon of 40 km diameter at about 17,531,000 km from the planet Saturn. The gravity at its surface is about 0.0007 times as strong as on Earth. The moon goes once around its planet in about 867.1 days. The moon was discovered in 2000. Also called SXXIX (Saturn twenty-nine). Its provisional designation was S/2000 S3.

sidereal

siderus = [Latin] star

Sidereal means it has something to do with the stars. A sidereal period of time (such as the sidereal month and the sidereal year) is a period of time after which a celestial body reaches the same position relative to the stars.

the sidereal time

The sidereal time is equal to the right ascension that passes through the celestial meridian at that moment. If a given sidereal time (between 0 and 24 hours) returns, then the stars are again in the same places in the sky (seen from the same place on Earth). The sidereal time runs a little faster than ordinary time: 24 hours of sidereal time correspond to 23 hours 56 minutes 4 seconds of ordinary time. Near the equinox of september, the sidereal time and solar time are almost equal.

sidereal_time

sidereal_year

sidereal_years

SII

SIII

sin

sinh

Sinope

A moon of 38 km diameter at about 23,939,000 km from the planet Jupiter. The gravity at its surface is about 0.0014 times as strong as on Earth. The moon goes once around its planet in about 756.5 days. The moon was discovered in 1914. Also called JIX (Jupiter nine).

SIV

SIX

Skathi

A moon of 8 km diameter at about 15,541,000 km from the planet Saturn. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 723.7 days. The moon was discovered in 2000. Also called SXXVII (Saturn twenty-seven). Its provisional designation was S/2000 S8.

the Small Magellanic Cloud

The Small Magellanic Cloud is a small, irregular galaxy that is very close to our own Milky Way Galaxy, at about 210,000 lightyears from us. The Small Magellanic Cloud is in the constellation Tucana and can (sometimes) be seen with the unaided eye from places south of 30 degrees north latitude.

The Small Magellanic Cloud (abbreviation SMC) is called "Nubecula Minor" in Latin, and is also called NGC 292.

SMC

software

solar

'so‧lar; solaire = [French] of the Sun, from [Latin] Sol = Sun

Solar means it has something to do with the Sun.

the solar eclipse

A solar eclipse is the covering of the disk of the Sun by the Moon. If the Sun is wholly covered by the Moon then it is a total solar eclipse. If only part of the Sun is covered, then it is a partial solar eclipse.

the solar flare

Solar flares are explosions near the solar surface in which the local magnetic field is relaxed. This releases a lot of energy which heats up the solar gas and launches it into space. Solar flares usually occur near active regions.

the solar physicist

physicus = [Latin] physicist

A solar physicist is a physicist or astronomer that studies the Sun using the tools of physics. The science that studies the Sun is called solar physics.

solar physics

physica = [Latin] physics

Solar physics is the science that studies the Sun. It is part of astronomy. People who do solar physics are called solar physicists.

the Solar System

solar system, plural solar systems

The Solar System is the star system of the Sun and consists of the Sun itself and everything that orbits around the Sun, such as planets (e.g., the Earth), comets, and asteroids. Because the Sun is a star, star systems of other stars are also sometimes called solar systems.

the solar time

Solar time is the time measured according to the position of the Sun in the sky. It is 12 o'clock solar time (noon) when the Sun is highest in the sky. North of the tropics, the Sun is then due south, and south of the tropics it is due north then. You can measure the solar time with a well-adjusted sundial. Solar time is usually not equal to the official clock time. In the Netherlands and Belgium, solar time is about 40 minutes behind official clock time in winter, and about one hour and 40 minutes behind in summer.

solar_activity_cycle

solar_cycle

solar_cycles

solar_eclipse

solar_eclipses

solar_flares

solar_maximum

solar_maximums

solar_minimum

solar_minimums

solar_physicists

solar_system

solar_systems

solar_time

the solid angle

The surface area of a constellation is a solid angle, for example measured in square degrees or in steradians. The length, width, and circumference of a constellation are (plane) angles, for example measured in degrees or radians. A small part of the sky with a length and width of 1 degree has a solid angle (surface area) of about 1 square degree. All directions put together (both above and below the horizon) cover 129600/π = (about) 41253 square degrees, or 4π = (about) 12.566 steradians.

the solstice

solstice, plural solstices

A solstice is a moment when the declination of the Sun changes from increasing to decreasing, or from decreasing to increasing. There are two solstices each tropical year: the northern solstice and the southern solstice. The solstices mark midsummer's day and midwinter's day, and the beginning of the astronomical seasons of summer and winter.

solstices

the southern solstice

The southern solstice is the solstice at which the Sun reaches the furthest south of the celestial equator. This is at midsummer's day at the start of summer in the southern hemisphere, and at midwinter's day at the start of winter in the northern hemisphere. In the Gregorian calendar, this is near 21 December.

Southern_Cross

Southern_Crown

Southern_Fish

southern_solstice

southern_solstices

Southern_Triangle

special_theory_of_relativity

the spectral line

spectum = [Latin] to watch

A spectral line is a very narrow range of wavelengths (colors) at which a bright object such as the Sun shines more brightly or less brightly than at neighboring wavelengths. A spectral line in which an object shines less brightly is called an absorption line, and a spectral line in which an object shines more brightly is called an emission line.

Every substance has its own set of spectral lines, so spectral lines give a sort of fingerprint of the associated substance. The strength of a spectral line (how much absorption or emission it has) depends on many things, including the temperature and gas pressure of the material, and sometimes also of the strength of the magnetic field at that location. The exact wavelength or frequency of a spectral line also depends (through the doppler effect) on the speed of the material relative to the observer. Astronomers often use filters to look at one spectral line in turn, to measure the things that the spectral line is sensitive to.

spectral_lines

the speed of light

The speed of light in a vacuum is a constant of nature, by definition equal to 299,792,458 m/s. The speed of light in a medium, such as water or glass, is less than the speed of light in a vacuum by a factor that is equal to the index of refraction of the medium. If no medium is explicitly mentioned, then you can assume that the speed of light in a vacuum is meant.

speed_of_light

spin-orbit_resonance

spin-orbit_resonances

the spiral galaxy

'spi‧ral 'ga‧lax‧y

A spiral galaxy is a galaxy with a flat disk in which (when seen from above) bright "arms" appear to come from the center. A spiral galaxy also contains a number of globular clusters, of which most are outside of the disk. In the center of the disk there may be a bar-shaped structure: then the galaxy is a barred spiral galaxy.

spiral_arm

spiral_arms

spiral_system

spiral_systems

Sponde

A moon of 2 km diameter at about 23,487,000 km from the planet Jupiter. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 735.2 days. The moon was discovered in 2001. Also called JXXXVI (Jupiter thirty-six). Its provisional designation was S/2001 J5.

spring

spring_equinox

standard_time

standstill

standstills

the star

astèr = [Greek] star

A star is a ball of (mostly) hydrogen gas that is so massive that it

  1. is held together by its own gravitation.
  2. has such a high gas pressure and temperature in its center that nuclear fusion of hydrogen to helium occurs there, which provides the energy that makes the star shine brightly.

stars

star_cluster

star_clusters

star_system

star_systems

stellar_system

stellar_systems

stellar_time

Stephano

A moon of 20 km diameter at about 8,004,000 km from the planet Uranus. The gravity at its surface is about 0.0004 times as strong as on Earth. The moon goes once around its planet in about 684.2 days. The moon was discovered in 1999. Also called UXX (Uranus twenty). Its provisional designation was S/1999 U2.

Stern

stice

stices

strong_nuclear_force

strong_nuclear_forces

summer

summer_solstice

the Sun

sun, plural suns

The Sun is the star in the middel of our Solar System, which provides us here on Earth with light and warmth. Because the Sun is a star, stars are sometimes referred to as suns. The Sun is by far the largest object in our Solar System, with a diameter of 1,391,980 km.

sun

sunrise

suns

sunset

the sunspot

Sunspots are regions on the solar surface where the magnetic field is very strong, up to 3000 gauss in strength. Sunspots look a lot darker than their surroundings in almost all kinds of observations, because they are a few thousand degrees cooler than their surroundings and because they are also large. Sunspots range in diameter from about 2500 to over 50,000 km. A sunspot is often roughly circular in shape, though some have a very irregular appearance. Sunspots have two very distinct parts: the dark umbra in the middle, and the less-dark penumbra around the umbra. The difference between a sunspot and a pore is that a pore has no penumbra.

the solar cycle

The number of sunspots on the Sun varies fairly regularly with a period of about 11 years which is called the sunspot cycle. When the number of sunspots is smallest (zero or close to it) then the Sun is in the sunspot minimum. When the number of sunspots is greatest, then the Sun is in the sunspot maximum. The strength of other phenomenons changes with the number of sunspots, for example the number of solar flares and the number of coronal mass ejections. The sunspot cycle is therefore also called the solar activity cycle.

sunspots

sunspot_cycles

sunspot_maximum

sunspot_maximums

sunspot_minimum

sunspot_minimums

supercluster

superclusters

the superior planet

A superior planet is a planet that is further from the Sun than the Earth is. Only superior planets can be in opposition, and their elongation can have any value up to 180 degrees. The superior planets are: Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. The opposite of a superior planet is an inferior planet.

superior_conjunction

superior_planets

supernova

supernovae

supernovas

the surface brightness

The surface brightness of a thing is the amount of light that you receive from it per unit solid angle. In astronomy, surface brightness is often measured in magnitudes per square degree, which is the magnitude of a star of which you'd receive as much light as from a square degree of the surface of the thing. The surface brightness does not change with distance or (if you have perfect optics) with the magnification of the telescope.

The surface brightness of the Sun (and hence also of most other things with a surface temperature of 5700 K) is about 10.9 magnitudes per square second of arc. The surface brightness of a typical galaxy is about 20 magnitudes per square second of arc.

Suttungr

A moon of 7 km diameter at about 19,459,000 km from the planet Saturn. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 2.8 years. The moon was discovered in 2000. Also called SXXIII (Saturn twenty-three). Its provisional designation was S/2000 S12.

SV

SVI

SVII

SVIII

Swan

Swordfish

SX

SXI

SXII

SXIII

SXIV

SXIX

SXV

SXVI

SXVII

SXVIII

SXX

SXXI

SXXII

SXXIII

SXXIV

SXXIX

SXXV

SXXVI

SXXVII

SXXVIII

SXXX

SXXXI

SXXXII

SXXXIII

SXXXIV

Sycorax

A moon of 190 km diameter at about 12,179,000 km from the planet Uranus. The gravity at its surface is about 0.0041 times as strong as on Earth. The moon goes once around its planet in about 3.5 years. The moon was discovered in 1997. Also called UXVII (Uranus seventeen). Its provisional designation was S/1997 U2.

synodic

synodical

sunodos = [Greek] meeting; from sun = [Greek] together, and hodos = [Greek] road

A synodical period is the period after which the phases of a celestial body as seen from another celestial body repeat themselves. If only a single body is mentioned, then the synodical period as seen from Earth is usually meant.

After a synodical month, the Moon has reached the same phase again (for example: full moon). The time between two oppositions or (similar) conjunctions of a planet is the synodical period of the planet. After that time, the Earth has caught up again with the planet (or the planet with the Earth) in its orbit around the Sun. The time after which a certain sunspot returns to the same location on the solar disk is the synodical period of the Sun (but that is not the same everywhere on the Sun, because of differential rotation).

The synodical periods of the planets are, rounded to whole days: Mercury 116, Venus 584, Mars 780, Jupiter 399, Saturn 378, Uranus 370, Neptune 367, Pluto (at present) 367. Calculate it yourself.

Periods of motion measured relative to the stars are called sidereal periods.

synodical_month

synodical_months

Tt

<R T>
Table_Mountain

tan

tanh

Tarvos

A moon of 15 km diameter at about 17,983,000 km from the planet Saturn. The gravity at its surface is about 0.0003 times as strong as on Earth. The moon goes once around its planet in about 900.9 days. The moon was discovered in 2000. Also called SXXI (Saturn twenty-one). Its provisional designation was S/2000 S4.

Tau

Tauri

Taurus

Taurus (Bull) is an equatorial constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Tau.

Taygete

A moon of 5 km diameter at about 23,280,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 725.5 days. The moon was discovered in 2000. Also called JXX (Jupiter twenty). Its provisional designation was S/2000 J9.

Tel

Telescope

Telescopii

Telescopium

Telescopium (Telescope) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Tel.

Telesto

A moon of 22 km diameter at about 294,710 km from the planet Saturn. The gravity at its surface is about 0.0004 times as strong as on Earth. The moon goes once around its planet in about 45.4 hours. The moon was discovered in 1980. Also called SXIII (Saturn thirteen). Its provisional designation was S/1980 S13.

the temperature

The temperature is a measure for how hot something is. There are several units in use for temperature. In Europe, the most common unit is the degree Celsius (written ). In the USA, the degree Fahrenheit is used (). In science, the kelvin (no capital letters; also written K ― without °) is used a lot. The freezing point of water at sea level on Earth is at 0℃, 32℉, and 273 K. The boiling point of water is then at 100℃, 212℉, and 373 K.

temperatures

Tethys

A moon of 1059 km diameter at about 294,670 km from the planet Saturn. The gravity at its surface is about 0.0150 times as strong as on Earth. The moon goes once around its planet in about 45.4 hours. The moon was discovered in 1684. Also called SIII (Saturn three).

Thalassa

A moon of 82 km diameter at about 50,080 km from the planet Neptune. The gravity at its surface is about 0.0015 times as strong as on Earth. The moon goes once around its planet in about 7.5 hours. The moon was discovered in 1989. Also called NIV (Neptune four). Its provisional designation was S/1989 N5.

Thebe

A moon of 99 km diameter at about 221,900 km from the planet Jupiter. The gravity at its surface is about 0.0041 times as strong as on Earth. The moon goes once around its planet in about 16.2 hours. The moon was discovered in 1979. Also called JXIV (Jupiter fourteen). Its provisional designation was S/1979 J2.

Thelxinoe

A moon of 2 km diameter at about 21,162,000 km from the planet Jupiter. The gravity at its surface is about 0.0000 times as strong as on Earth. The moon goes once around its planet in about 628.8 days. The moon was discovered in 2004. Also called JXLII (Jupiter forty-two). Its provisional designation was S/2003 J22.

Themisto

A moon of 8 km diameter at about 7,284,000 km from the planet Jupiter. The gravity at its surface is about 0.0003 times as strong as on Earth. The moon goes once around its planet in about 127.0 days. The moon was discovered in 2000. Also called JXVIII (Jupiter eighteen). Its provisional designation was S/1975 J1 = S/2000 J1.

theories_of_relativity

the theory of relativity

plural: theories of relativity

A theory of relativity is a theory in which there is no privileged position or time. More specifically, a Theory of Relativity is one of the two theories of that name that were invented by Albert Einstein.

theory_of_relativity

Thrymr

A moon of 7 km diameter at about 20,474,000 km from the planet Saturn. The gravity at its surface is about 0.0001 times as strong as on Earth. The moon goes once around its planet in about 3.0 years. The moon was discovered in 2000. Also called SXXX (Saturn thirty). Its provisional designation was S/2000 S7.

Thyone

A moon of 4 km diameter at about 20,939,000 km from the planet Jupiter. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 618.9 days. The moon was discovered in 2001. Also called JXXIX (Jupiter twenty-nine). Its provisional designation was S/2001 J2.

the tidal force

plural: tidal forces

Tidal forces are forces due to a change in gravity from place to place. On Earth, tidal forces of the Moon cause the tides, and also cause the slow deceleration of the rotation of the Earth (and the lengthening of the day). Tidal forces cause moons to eventually show always the same side to their planet. The vulcanism on Io is probably kept going by tidal forces of Jupiter acting on Io's interior.

tidal_boundary

tidal_force

tidal_forces

the tide

plural: tides

On Earth, the tides are the regular rising and falling of the sea level because of the tidal forces of the Moon.

tides

time

timezone

timezones

time_measurement

time_travel

time_zone

time_zones

Titan

A moon of 5151 km diameter at about 1,221,870 km from the planet Saturn. The gravity at its surface is about 0.1381 times as strong as on Earth. The moon goes once around its planet in about 16.0 days. The moon was discovered in 1655. Also called SVI (Saturn six).

Titania

A moon of 1577 km diameter at about 436,300 km from the planet Uranus. The gravity at its surface is about 0.0386 times as strong as on Earth. The moon goes once around its planet in about 8.7 days. The moon was discovered in 1787. Also called UIII (Uranus three).

total_solar_eclipse

Toucan

TrA

the transit

transit, plural transits

A transit is

  1. the moment when a celestial body crosses the meridian (sense 2).
  2. a passage of an apparently smaller body in front of an apparently larger body. More particularly: the crossing of Mercury or Venus in front of the Sun, as seen from Earth.

transits

Tri

Triangle

Trianguli

Trianguli_Australis

Triangulum

Triangulum (Triangle) is a northern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Tri.

Triangulum Australe

Triangulum Australe (Southern Triangle) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is TrA.

Trinculo

A moon of 10 km diameter at about 8,504,000 km from the planet Uranus. The gravity at its surface is about 0.0002 times as strong as on Earth. The moon goes once around its planet in about 749.3 days. The moon was discovered in 2001. Also called UXXI (Uranus twenty-one). Its provisional designation was S/2001 U1.

Triton

A moon of 2706 km diameter at about 354,800 km from the planet Neptune. The gravity at its surface is about 0.0795 times as strong as on Earth. The moon goes once around its planet in about 5.9 days. The moon was discovered in 1846. Also called NI (Neptune one).

tropical

In astronomy, a tropical period of time is a period of time measured relative to the passage through the vernal equinox, and so is linked to the seasons. The tropical year is the period after which the Sun returns to the vernal equinox, and the tropical month is the period after which the Moon returns to the vernal equinox.

tropical_year

tropical_years

true_anomaly

true_conjunction

true_conjunctions

Tuc

Tucana

Tucana (Toucan) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Tuc.

Tucanae

Twins

twin_paradox

Tycho

Tycho_Brahe

Uu

<S U>
UI

UII

UIII

UIV

UIX

ultraviolet

ultra = [Latin] further

Ultraviolet radiation is a form of electromagnetic radiation with wavelengths just smaller than those of visible light, i.e., just beyond those of violet light.

UMa

the umbra

umbra = [Latin] shadow, plural umbrae, umbras

The umbra is the middle part of a sunspot, which is (relatively) very dark because it is a lot cooler than the rest of the solar surface. The magnetic field in the umbra is very strong and points almost straight up.

umbrae

umbras

Umbriel

A moon of 1169 km diameter at about 266,000 km from the planet Uranus. The gravity at its surface is about 0.0233 times as strong as on Earth. The moon goes once around its planet in about 99.5 hours. The moon was discovered in 1851. Also called UII (Uranus two).

UMi

Unicorn

universal_time

the Universe

the 'U‧ni‧verse, plural universes

The Universe is all space put together, with everything that's in it. Because of the finite age of the Universe and the finite speed with which light and other information carriers move, we can have knowledge of even in principle only a finite part of the Universe. That part is also often called the Universe (or the Visible Universe). For more information about the Universe, see the Universe Page of the Universe Family Tree.

universe

universes

Uranus

Uranus

Uranus is the seventh planet of the Solar System, counting from the Sun. Uranus is a jovian planet with a very dense and thick atmosphere (made mostly of hydrogen and helium), many moons, and narrow rings. Uranus has a diameter of 51,118 km and is number 4 on the list of largest objects in our Solar System.

Ursa Major

Ursa Major (Great Bear) is a northern constellation. The official abbreviation is UMa.

Ursa Minor

Ursa Minor (Little Bear) is a northern constellation. The official abbreviation is UMi.

Ursae_Majoris

Ursae_Minoris

UTC

UTC means "Coordinated Universal Time" and is the time that belongs to the meridian of 0° on Earth. Each UTC second lasts exactly as long as all other ones (namely exactly as long as the second from the International System of Units). UTC is kept synchronized with the somewhat irregular rotation of the Earth by occasionally inserting a leap second as needed.

UV

uv

UVI

UVII

UVIII

UX

UXI

UXII

UXIII

UXIV

UXIX

UXV

UXVI

UXVII

UXVIII

UXX

UXXI

UXXII

UXXIII

UXXIV

UXXV

UXXVI

UXXVII

Vv

<T V>
Vel

Vela

Vela (Sails) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Vel.

Velorum

Venus

Venus = [Latin] goddess of love

Venus is the second planet of the Solar System, counting from the Sun. Venus is a terrestrial planet, with a thick atmosphere of mostly carbon dioxide but no moons or rings. Venus has a diameter of 12,104 km and is number 7 on the list of largest objects in our Solar System.

the vernal equinox

The vernal equinox is the intersection of the equator of the sky and the ecliptic, through which the Sun passes during the March equinox, at the beginning of spring in the northern hemisphere, and autumn in the southern hemisphere. The vernal equinox is the zero point of longitude and latitude of the equatorial and ecliptic coordinate systems. The period between two successive passages of the Sun through the vernal equinox is the tropical year.

vernal_equinox

Vir

Virgin

Virginis

Virgo

Virgo (Virgin) is an equatorial constellation, close to the ecliptic. The official abbreviation is Vir.

visible light

Visible light is a form of electromagnetic radiation with wavelengths between about 400 and 800 nm. It seems silly to talk about "visible" light; after all, you can always see light, can't you? Yet, there are good reasons:

  1. some kinds of electromagnetic radiation that we cannot directly see are yet called light, for example infrared and ultraviolet "light".
  2. the boundary between colors we see well and "colors" we cannot see at all is not sharp but gradual, so we cannot pinpoint an exact border between visible and invisible light.
  3. not everybody is equally sensitive to any given color. A color that is just visible to some may be invisible to some others. Should such a color be considered to be light or not?
  4. visible light does not differ fundamentally from invisible light (such as infrared and ultraviolet light) that is just beyond the visible colors.

Scientists therefore use the phrase "visible light" for the wavelength range of electromagnetic radiation that the average person can see at least a little bit, with vague boundaries on the low and high sides.

visible_universe

a void

plural: voids

In general, a void is an empty space. In astronomy, a void is a giant bubble-shaped region of the Universe where there are no or hardly any galaxies. There are filaments at the edges of such voids.

voids

Vol

Volans

Volans (Flying Fish) is a southern constellation, close to the ecliptic, close to the Milky Way. The official abbreviation is Vol.

Volantis

the VSOP model

From the VSOP model (Variations Séculaires des Orbites Planétaires) of P. Bretagnon and G. Francou, the positions of the planets can be calculated very accurately for a few thousand years into the past and into the future. This model is available for free. See //adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1988A%26A...202..309B&db_key=AST&data_type=HTML.

Vul

Vulpecula

Vulpecula (Fox) is an equatorial constellation, close to the Milky Way. The official abbreviation is Vul.

Vulpeculae

Ww

<U W>

the waning moon

The waning Moon is the Moon during the period in which the area of the lunar disk that is illuminated decreases. This period begins at Full Moon and runs until New Moon.

waning_moons

Water_Carrier

Water_Snake

the waxing moon

The waxing Moon is the Moon during the period in which ever more of the disk of the Moon becomes illuminated. This period begins at New Moon and ends at Full Moon.

waxing_moons

weak_nuclear_force

weak_nuclear_forces

week_number

week_numbers

the weight

You feel weight when a force acts on the surface of your body. If you stand quietly on the ground, then the ground pushes up against your feet but not against your head, and gravity pulls down all parts of your body equally strongly, so then it feels as if your head and your feet are pushed together, and that gives the feeling of weight.

If you're loose from the ground (for example if you just jumped up), then there is no force pushing against your feet, so then your feet and head are no longer pushed together, and then you feel weightless.

If you sit in an accelerating car, facing forward, then the chair pushes harder against your back, and you feel that as extra weight.

If you sit or stand quietly, then your weight depends on your mass and on the strength of gravity. On the Moon, gravity is less strong than on Earth, so your weight is less on the Moon than on Earth.

Even though weight is a measure for physical force, it is usually described in units of mass, such as the kilogram or pound. A kilogram of weight should be interpreted as "the amount of force of gravity that would act on a mass of one kilogram on Earth". Even on Earth your weight depends on where you are. At the equator you weigh about half a percent (one part in 200) less than at the poles.

weightless

weightlessness

Whale

winter

winter_solstice

Wolf

Xx

<V X>

the X-rays

X-rays are a form of electromagnetic radiation that has a relatively large amount of energy per photon and is therefore dangerous. X-rays are emitted naturally by material that is very hot, such as the gases in a solar flare that can have temperatures of hundreds of thousands or even millions of degrees, and by certain radioactive materials.

x_ray

x_rays

Yy

<W Y>

the year

The year is a period of time that is related to the motion of the Earth around the Sun. There are quite a few different kinds of years in use:

years

Ymir

A moon of 18 km diameter at about 23,040,000 km from the planet Saturn. The gravity at its surface is about 0.0004 times as strong as on Earth. The moon goes once around its planet in about 3.6 years. The moon was discovered in 2000. Also called SXIX (Saturn nineteen). Its provisional designation was S/2000 S1.

Zz

<X Z>

the zenith

samt ar-ras = [Arab] the way of the head

The zenith is the point in the sky that is straight above your head. The altitude of the zenith is 90°. The opposite point is the nadir, and the horizon is midway between the two.



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Last updated: 2017-04-24