Astronomy Answers: AstronomyAnswerBook: Planets

Astronomy Answers
AstronomyAnswerBook: Planets


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1. Planets in General ... 1.1. What is a Planet? ... 1.2. Weather on Planets ... 1.3. Colors of Planets ... 1.4. Planetary Ingredients ... 1.5. Sunshine on Planets ... 1.6. Gravity on Other Planets ... 1.7. Jovian Planets ... 1.8. A Match to a Gas Planet ... 1.9. The Tenth Planet ... 1.10. Volcanoes on Other Planets ... 1.11. The Positions of the Planets ... 1.12. Hidden Planets ... 1.13. Planetary Conjunctions ... 1.14. The Best Visible Planet ... 1.15. The Hottest Planet ... 1.16. Planets are Hot on the Inside ... 1.17. Moons of Planets ... 1.18. The Orbits of Planets ... 1.19. The Law of Titius-Bode ... 1.20. Inclination of Planetary Orbits ... 1.21. Speeds of Planets ... 1.22. Life on Planets ... 1.23. Visiting Other Planets ... 1.24. Retrograde Motion of the Planets ... 1.25. Round Planets ... 1.26. Symbols of Planets ... 1.27. The Rotation Periods of Planets ... 1.28. Rotation Direction of the Planets ... 1.29. Repeating Positions of Planets ... 1.30. Distances from the Sun ... 1.31. Distances from Earth ... 1.32. When the Planets are Closest to the Earth ... 1.33. Size, Mass, and Density of Planets ... 1.34. Planetary Atmospheres ... 1.35. Does It Matter If You Shift a Planet? ... 2. Specific Planets ... 2.1. Venus ... 2.1.1. No Venus Moons ... 2.1.2. Venus Transits ... 2.2. The Earth ... 2.2.1. The Discovery that the Earth is Round ... 2.2.2. Measuring the Circumference of the Earth ... 2.2.3. The Discovery that the Earth Rotates Around Its Axis ... 2.2.4. The Determination of the Mass of the Earth ... 2.2.5. The Distance to the Sun ... 2.2.6. The Perihelion of the Earth ... 2.3. Mars ... 2.3.1. Water on Mars ... 2.3.2. Volcanoes on Mars ... 2.3.3. The Composition of the Planet Mars ... 2.3.4. Mars Closest to Earth ... 2.3.5. Discoloration of Vehicle Tracks on Mars ... 2.4. Jupiter ... 2.5. Saturn ... 2.6. Uranus ... 2.7. Pluto

This page answers questions about planets. The questions are:

The Solar System Page also contains information about the planets (that are part of the Solar System).

Specific planets:

Venus Earth Mars Jupiter Uranus Pluto

1. Planets in General

1.1. What is a Planet?

It is not easy to find a good definition of a planet. A planet is a clump of matter in space, but there are very many clumps of matter in space that we yet do not call planets, such as moons and asteroids and stars. There is no clear difference (regarding the measurements of the celestial body itself) between large planets and small stars, or between small planets and large asteroids or moons.

In 2006, the IAU adopted a definition of a planet, after much debate. A planet is now a celestial object that

  1. orbits directly around a star (and not around something else that itself orbits around a star)
  2. has a (nearly) round shape because of its own gravity
  3. has cleared the neighborhood of its orbit (i.e., has no similar objects near its orbit)

Rule 1 defines the difference between planets and moons. Some moons are larger than some planets, but because moons orbit around something else and not directly around the star (such as the Sun), they are not themselves planets.

Rule 2 defines the difference between planets and smaller bodies, which are not round but, for example, potato-shaped.

Rule 3 means that some round celestial bodies that orbit directly around the Sun are yet not planets. This holds also for Pluto, which from its discovery in 1930 until 2006 was called a planet, and for 1 Ceres (the largest of the asteroids) and for various celestial bodies that are about as large as Pluto and that move through the same region of space as Pluto.

If a celestial body satisfies rules 1 and 2 but not rule 3, then it is now called a dwarf planet.

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1.2. Weather on Planets

At certain latitudes, clouds and the weather tend to move to the east, but at other latitudes they tend to move to the west. Heat travels on average from hotter areas to colder areas. The equator is hotter than the poles, so, on average, hotter air travels from the equatorial region to the poles, and colder air travels from the poles to the equatorial region, so there is a north-south component to the prevailing winds.

Planets rotate around their axis, which generates Coriolis forces that make air traveling north or south deviate from its straight course. This causes the motion of the air (and the weather) to have an east-west component as well.

The direction of motion of the air and the weather varies with time, with latitude on the planet, and also with height. The exact details depend on properties of the atmosphere, on how fast the planet rotates, and on the amount of sunlight that the planet receives (i.e., on the distance from the Sun and on the seasons), and cannot be easily predicted without the use of a computer model.

If you want to know more, then please ask a meteorologist.

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1.3. Colors of Planets

Below, I show the average color of the planets, which I derived from a picture of that planet. You have to be a bit careful with such pictures, because their colors are often exaggerated to make certain details stand out better. I am not sure that all of the pictures that I used show the natural colors of the planets.

Planet Color Code Picture
Mercury #FFA7AE ALPO 2002-04-20
Venus #FFE49A APOD 2004-05-16
Mars #FFCFC2 APOD 2003-12-18
Earth #EFE9FF APOD 1999-01-31
Moon #FFF6E2 APOD 1999-12-22
Jupiter #FFFEF2 APOD 2000-10-11
Saturn #FFF5EB APOD 2003-08-17
Uranus #CEE9FF APOD 2001-08-26
Neptune #81FFFE HST 1994-06-28
Pluto #FFC49A APOD 2001-03-19

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1.4. Planetary Ingredients

The earth-like planets are made of rock and metal, which contains elements such as oxygen, silicon, iron, and nickel. The jupiter-like planets are made up mostly of hydrogen and helium gas. They may also have a rocky core far below the layers of gas, and that core is then probably made of similar elements as the earth-like planets. However, we don't know for sure yet if these Jovian planets even have a rocky core and what the composition of such a core is, because no space craft has ever gone below the uppermost cloud layers to take measurements there. See also question 281.

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Table 1 says what type each planet is.

Table 1: Planet Ingredients

Planet Inside Ice? Atmosphere?
Mercury rock no no
Venus rock no very thick
Earth rock poles tick
Mars rock poles very thin
Jupiter rock?; gas no very thick
Saturn rock?; gas no very thick
Uranus rock?; gas no very thick
Neptune rock?; gas no very thick
Pluto rock?; ice yes very thin?

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1.5. Sunshine on Planets

The amount of sunlight that a planet receives per unit area depends on the distance of the planet from the Sun. The next table shows the average distance \(r\) from the Sun (in AU), the amount \(L\) of sunlight that a planet receives per unit area, compared to the amount at a similar location on Earth, and the visual magnitude \(V\) of the Sun as seen from just outside that planet's atmosphere.

Table 2: Sunlight on Planets

Planet rLV
Mercury 0.387 6.67 −28.8
Venus 0.723 1.91 −27.4
Earth 1.000 1.00 −26.7
Mars 1.524 0.43 −25.8
Jupiter 5.203 0.037 −23.1
Saturn 9.539 0.011 −21.8
Uranus 19.181 0.0027 −20.3
Neptune 30.058 0.0011 −19.3
Pluto 39.44 0.00064 −18.7

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1.6. Gravity on Other Planets

Gravity on other planets works the same as gravity on Earth, but can be stronger or weaker than on Earth. If the gravity at the surface of a planet is stronger than the gravity at the surface of the Earth, then your weight is greater on that planet, and then you can jump less high on that planet than on Earth. If the gravity is less than on Earth, then your weight is less and you can jump higher than on Earth. See the answer to question 135.

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1.7. Jovian Planets

Jovian means "like Jupiter". A Jovian planet is a planet that is similar in composition and size to Jupiter. Such a planet is much larger than the Earth and is shrouded in gas layers that are thousands of kilometers or miles thick and that are made up mostly of hydrogen and helium (just like the Sun is). In our Solar System, Jupiter, Saturn, Uranus, and Neptune are Jovian planets.

A Jovian planet may have a rocky core far below the layers of gas, but perhaps it does not. You certainly cannot stand on the gas, and if there is a rocky core, then you would not survive if you stood on it, because of the temperature and pressure are enormous there.

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1.8. A Match to a Gas Planet

A gas planet (Jovian planet) won't explode if you hold a burning match to it, because the contents of a gas planet are not a combustible mixture.

An explosion that is caused by a burning match is a fast chemical reaction that releases a lot of energy. Not all chemical reactions release energy: many kinds of chemical reactions cost energy, and those won't happen all by themselves and cannot sustain themselves.

Chemical reactions that do release energy can occur spontaneously (if the right ingredients are brought together) and can keep going as long as there is enough of the ingredients left.

A gas planet is made up mostly from hydrogen gas, which does not do chemical reactions with itself. Gas planets also contain helium (only about a tenth as many helium atoms than hydrogen atoms), but helium is a noble gas and does not spontaneously react chemically with anything. A gas planet also contains other elements, but those make up only a tiny proportion (less than 1 percent). Even if you could ignite all of those other elements with a burning match (which you can't), even then only a very small fraction of the gas planet would burn.

On Earth, a match is dangerous because 20% of the Earth's atmosphere is oxygen, which readily starts chemical reactions with many types of fuel. Without the presence of that much oxygen, those fuels wouldn't burn. What's more, without oxygen even the wood of the match wouldn't continue burning.

So, if you want to make a gas planet explode then you need to inject about an equal amount of oxygen into it. You may then still also need a spark.

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1.9. The Tenth Planet

(Warning: Pluto was regarded as a planet from its discovery in 1930 until 2006, but is regarded as a dwarf planet since 2006. So, from 1930 until 2006 there were 9 planets in our Solar System, but since 2006 there are only 8. "The tenth planet" dates from the period between 1930 and 2006, when there were 9 planets known, and refers to "the next newly discovered planet".)

Until 2006, "possibly the tenth planet" was declared in the news once in a while when a new celestial body was discovered beyond the orbit of Neptune. Recent examples are 50000 Quaoar (also called 2002 LM60) and 90377 Sedna (2003 VB12). These two celestial objects are probably made of ice and dust and rock, just like comets and the dwarf planet Pluto. They are probably smaller than Pluto, but not very much smaller.

When such a "possibly new planet" is in the news, then usually not very much is yet known about the new object, except for its brightness in the sky, a nice name invented by its discoverers, and an estimate for its size, which is usually close to the size of Pluto. However, it is difficult to determine the size of such a small object at such a large distance. (See the Size Estimation Page for more about this.) The size that is presented is often based on the observed brightness and on an estimate for the albedo of the object. If you use a lower estimate for the albedo (so the surface of the object is darker), then you automatically get a larger estimate for the size, and so a better chance of getting your discovery into the newspaper. You should therefore take these reported sizes with a grain of salt, except if they have been determined in an independent manner (for example, through a measurement of the temperature of the object).

Celestial objects such as Quaoar and Sedna are presented as possible planets because they are presumably quite similar to Pluto: They have a similar size as Pluto, probably about the same composition, and are all beyond the orbit of Neptune. According to some people, these objects have as much right to be called planets as Pluto has, so either they should all be called planets, or none of them should. Since about 1992, hundreds of Kuiper Belt Objects have been discovered, of which Pluto, Quaoar, and Sedna are (so far) the biggest ones. It is quite possible that one may be discovered that is larger than Pluto.

In August 2006, the IAU invented a definition of planets that means that Pluto and similar celestial objects are not planets, but dwarf planets. Since that time, Pluto is no longer a planet, but a dwarf planet.

Pluto has been assigned a number (134340), just like the other Kuiper Belt Objects (KBOs) and asteroids, so that databases with information about KBOs don't have to make an exception for the only KBO without a number (i.e., Pluto).

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1.10. Volcanoes on Other Planets

There are active volcanoes on at least the Earth and the moon Io that orbits Jupiter. There are also volcanoes on Mars (such as Olympus Mons, the largest volcano now known in the Solar System), but they have been quiet for about the last hundred million years. Radar satellites have found structures on Venus that might be volcanic in origin, but we don't yet know for sure. The so-called seas on the Moon are volcanic in origin, but they are thousands of millions of years old, and the lava probably came from cracks in the ground rather than from volcanoes. At least, I haven't heard of any volcanoes on the Moon. No volcanoes are known to exist on Mercury, and Pluto is too small and too cold to have any. The other planets (Jupiter, Saturn, Uranus, and Neptune) are giant gas planets of which we can only see the thick layers of gas on the outside, and you can't build volcanoes out of gas.

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1.11. The Positions of the Planets

If you want to know the positions of the planets in the sky with great precision, then you can have them calculated by JPL or by a planetarium program, or you can look them up in an astronomical almanac (such as the "Astronomical Ephemerides" or [in Dutch] the Sterrengids), or you can calculate them yourself using formulas from an appropriate book (such as the book "Astronomical Algorithms" by Jean Meeus). If you are satisfied with reasonable accuracy, then you can use the formulas from the Sky Positions Page.

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1.12. Hidden Planets

Undiscovered planets could only be hiding between the other planets if they have no measurable influence on the other planets. That means that they'd have to either have very little mass (so you'd sooner call them asteroids or comets than planets) or be very far from the Sun (so they'd move along their orbits very slowly).

I don't think there could be an "anti-Earth" on the other side of the Sun in the same orbit as the Earth, because, firstly, it is very unlikely that such a planet would be formed exactly on the other side of the Sun in exactly the same orbit as the Earth, and, secondly, it is very unlikely that such a planets would always remain at the exact opposite side of the Sun from the Earth.

The orbit of the Earth changes slowly under the influence of the gravity of the other planets, and if, for example, Mars is closest to the Earth so it can change Earth's orbit the most, then Mars is furthest from the anti-Earth and so changes its orbit the least. The history of the gravity of the other planets that is felt would be different for the anti-Earth from what it would be for the Earth, so the anti-Earth would not always remain on the opposite side of the Sun.

A minor difference in the size of the semimajor axis (roughly equal to the average distance from the Sun) of the orbits of the Earth and the anti-Earth would already be sufficient. If, for example, the anti-Earth were one kilometer (one part in a hundred fifty million) closer to the Sun than the Earth, then the orbital period (the year) of the anti-Earth would be 0.3 seconds less than that of Earth. After one year, the anti-Earth would lead the Earth by 0.3 seconds, after two years it would be 0.6 seconds, and so on. After ten million years the anti-Earth would already lead the Earth by 37 days and would be about 17 degrees from the Sun in the Earth's sky, and the Earth is already much older than ten million years.

The influence of the gravity of the anti-Earth on the other planets and on space probes we send that way would most likely be noticeable. After all, the planet Neptune was also discovered from its gravitational tugging on the other planets, even though those planets remain so far from Neptune that the gravitational acceleration that Neptune causes in these other planets is less than the gravitational acceleration that the anti-Earth would cause in the terrestrial planets.

It is possible for several celestial bodies to be in the same orbit around the Sun for a long time, but only if the smaller bodies (say, the asteroids) have much less mass than the larger body (say, the planet) in the same orbit, and only if the smaller bodies are about 60 degrees away from the larger one, as seen from the Sun. As seen from the larger body, the smaller ones are then about 60 degrees from the Sun, and as seen from the smaller bodies the larger one is about 60 degrees from the Sun, too. This is the case, for example, with the so-called Trojan and Greek asteroids that go around the Sun in the same orbit as Jupiter.

There are also examples of celestial bodies that go around in nearly the same orbits, for example the so-called shepherd moons of Saturn. They have orbits that differ in size by only a few miles, so they regularly get close to one another, and then they swap orbits.

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1.13. Planetary Conjunctions

Once in a while, reports start circulating about special conjunctions of planets that are supposed to influence the Earth or to herald special events. Astrology is filled with these kinds of ideas, but people also start looking at the skies when special calendar dates are reached in our calendar or in someone else's calendar.

For example, there was some commotion in 1996 because of the presumed 6000th anniversary of the traditional creation date of the Earth as derived from the Bible, and again in May 2000 when some of the planets were relatively close together in the sky. On 21 December 2012, a new large cycle will start in the calendar of the Maya, and some people expect that special phenomena in the sky ought to accompany such an occasion, because they believe that the Maya had very accurate knowledge of the motion of the planets and incorporated that knowledge into their calendar.

However, conjunctions of the planets have no influence on Earth and are not important, except that they provide opportunities to take nice pictures of them. It is sheer coincidence if the positions of the planets at a nice round date in an arbitrary or specific calendar are more interesting than usual, not least because the positions of more than two planets don't have any nice, round periods, and as far as I know no planet's period is used in any popular calendar, except for the periods of the Earth and the Moon.

As far as December 2012 is concerned, we can predict the positions of planets for that period with great accuracy. A couple of those predictions are displayed on the Planetary Phenomena Page for 2012. You can get positions of planets calculated for dates far into the past or the future by the Horizons-system of NASA's JPL at //ssd.jpl.nasa.gov/horizons.html. I don't think there is anything special about the positions of the planets in December 2012. What I mean by that is the following:

If you write the positions of the planets for a number of randomly selected dates and also for 21 December 2012 each on a different sheet, without writing the dates themselves on those sheets, and if you then ask someone who doesn't already know the positions of the planets on those dates to pick the sheet with the most special positions of the planets, then I predict that the sheet for 21 December 2012 won't be much more favorite than the sheets for the other dates.

I think that the return to the beginning of the Long Count of the Maya in 2012 is just as unimportant as the 6000th Biblical birthday of the world (celebrated in 1996) and the planetary conjunction in May 2000 and all kinds of other dates that have been predicted in the past for the end of the world. Someone who didn't hear about it beforehand didn't notice anything special on any of those dates, and I predict that the same will happen in December 2012.

Science sees no reason why there should be an instant ice age or any other sudden disasters in the year 2012. I see the many wild reports and predictions about 21 December 2012, but none of them provide sound evidence for the claims that are made. Some people say that the end of the world will come, but others say that we'll enter a new age of peace and harmony. These can't both be true. I think that neither is true, and that 21 December 2012 will pass pretty much like any other day, except that there'll be lots of nervous people who look at the sky a lot.

The date of 21 December 2012 is a nice round date in the calendar of a civilization that collapsed many centuries ago. It is not at all clear why we should pay more attention to the calendar of this civilization than to the calendars of countless other civilizations from the past and from today, where 21 December 2012 is not a nice round date. Also, it is not clear that that civilization itself expected the world to change on that date, or that anything world-changing has happened on earlier nice round dates in their calendar.

There have been many predictions of the end of the world in the past, and none of them have come true (see //www.bible.ca/pre-date-setters.htm). Those predictions weren't backed by science, and neither are the wild predictions about 21 December 2012.

There is some chance that natural disasters will happen in 2012, or even precisely on 21 December 2012, but that chance is not much different in 2012 than in other (recent) years.

There is nothing special about 21 December 2012, except that it happens to be a nice round date in some calendar. Any calendar has nice round dates for some days, but that doesn't mean the end of the world.

Some people make money claiming that the end of the world is coming (e.g., to sell their books). You should be suspicious of any claims of momentous changes without obvious reasons.

If you want to read more about this, then you could take a look at //en.wikipedia.org/wiki/Mayanism#December_21.2C_2012.

I am not at all worried about 21 December 2012, except perhaps that some people will do stupid things because they believe the world will end so they can do bad things with impunity because there won't be time to hold them accountable.

You can read more about the calendars of the Maya on the Historical Calendars Page, more about planetary conjunctions on the Planetary Conjunctions Page, and more in general on the Page about 21 December 2012.

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1.14. The Best Visible Planet

I'd say Jupiter is the planet you can see most often without a telescope. Jupiter is less than 15 degrees from the Sun in the sky (i.e., hard or impossible to see) about 10 percent of the time, and Venus is less than 15 degrees from the Sun about 25 percent of the time. Venus is often brighter than Jupiter, so it can be a bit closer to the Sun than Jupiter and yet be visible, but the difference is not so great that Venus is yet visible more often than Jupiter is.

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1.15. The Hottest Planet

Which planet is the hottest depends on where you measure the temperature. If you look at the average heat (infrared radiation) coming from the planets, then Mercury is the hottest, mostly because it is closest to the Sun. If you measure the temperature at the bottom of the atmosphere (if the planet has an atmosphere), then Venus is certainly hotter than Mercury (we've measured this). Jupiter, Saturn, Uranus, and Neptune are probably much hotter still, but we don't know exactly how hot these last four planets are below their clouds, because we've not been able to measure anything so far below their clouds yet, which are probably thousands of miles thick.

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1.16. Planets are Hot on the Inside

The question why planets are hot on the inside is really three questions:

  1. why is the center of a planet hotter than its surface?
  2. where does the heat inside a planet come from?
  3. why haven't planets cooled down to the temperature of space yet?

On the first point: Even if a planet starts out with the same temperature everywhere, it will soon be colder at the surface than in the center. This is because a planet can only lose heat at its surface, so it is easy for heat at the surface to escape, but hard for heat in the center to escape. A planet is always hotter the deeper you go under its surface, except perhaps if the planet is heated from the outside.

On the second point: I know of three possible sources of the heat inside a planet:

  1. gravity. Planets formed from a big cloud of gas and dust. When such a cloud collapses and forms a planet, then the parts of the cloud collide with each other and lose part of their speed. The parts that come from the greatest distance tend to go the fastest (just like you tend to go faster at the bottom of a taller rollercoaster). The kinetic energy (energy of speed) that they lose is turned into heat, so the newly formed planet is very hot. The larger the planet grows, the greater its gravity becomes, and the more heat it derives from additional gas and dust that crashes into it, so the bigger planets tend to end up the hottest inside after their formation.

    Large gas planets may continue to settle, shrink ("collapse") very slowly, for billions of years and generate some heat in that way. Jupiter and Saturn are known to have their own sources of heat, and gravity is the likely source.

  2. radioactivity. Some atoms can spontaneously fall apart into several pieces (usually one very big piece and one or more very small pieces), and this generates a little bit of heat. If there is enough radioactive material present in a planet, then this may contribute a significant amount of heat.
  3. solar radiation. The Sun shines on the planet and heats it from the outside. This prevents the planet from cooling down below the average surface temperature that fits with the amount of sunlight that is absorbed by the surface.

On the third point: In general, the bigger a thing is, the longer it takes for it to cool down. A small thing like a pebble quickly loses heat until it has the same temperature as its surroundings, but a big thing like a planet takes billions of years to cool down.

So, a planet is hot inside, and hotter inside than outside because (a) it turned gravitational and radioactive energy into heat, (b) heat escapes easily from the surface but not from the interior, and (c) the planet hasn't had enough time to cool down to its equilibrium temperature yet.

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1.17. Moons of Planets

Mercury and Venus have no moons. All other planets in our Solar System have at least one moon. You can read more about the moons in the Solar System on the Moon Page from the Universe Family Tree.

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1.18. The Orbits of Planets

The ancient Greek philosophers already debated about the structure of the orbits of the planets. The most widespread opinion in ancient writings about this subject until the 16th century was that all of the planets and the Sun and Moon orbit around the Earth.

However, it is said (for example by the Roman writer Chalcidius who lived around AD 300) that Herakleides of Pontus, who lived in Greece in the 4th century BC, claimed that Mercury and Venus revolve around the Sun while the Sun orbits around the Earth [Dreyer, p. 126]. No writings of Herakleides himself have survived into modern times, so we cannot read it in his own words.

Martianus Capella included the idea that Mercury and Venus orbit around the Earth in an encyclopedia in the 5th century AD [Dreyer, p. 127].

The first person to propose that all planets orbit around the Sun was Nicholas Copernicus, whose book describing this idea was printed just before his death in 1543 [Dreyer, Chapter XIII] [Pannekoek, Chapter 18] [Crowe, Chapter 6].

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The distances between the planets change all the time, because each planet goes along its own orbit at its own speed. You can compare it to different runners running their laps at different speeds each in their own lane. Sometimes the runners are half a lap away from each other, and sometimes they are close together for a while. Likewise, two planets are sometimes closer together, and sometimes further away from each other. See also question 54.

Fig. 1: Orbits of Inner Planets
Fig. 1: Orbits of Inner Planets

Figure 1 shows the orbits of the planets (from the inside out) Mercury, Venus, the Earth, and Mars. The solid lines trace the orbits as seen from high above the north pole of the Solar System. The small square in the center shows the location of the Sun. The dashed lines show how far the planet can get above or below the ecliptic: If the dashed curve is somewhere above the solid orbit (close to the beginning of this page), then the planet is at that location that far above the ecliptic, and if the dashed curve is below the solid orbit, then the planet is at that location that far below the ecliptic. Where the solid curve and the corresponding dashed curve intersect, the orbit has a node. In each orbit, the small square shows the location of the planet on 1 January 2005, and there's a plus sign for each 10 days. In this picture, the planets move counterclockwise. The units along the horizontal and vertical axes are Astronomical Units of about 150 million kilometers or 93 million miles.

Fig. 2: Orbits of Outer Planets
Fig. 2: Orbits of Outer Planets

Figure 2 shows the orbits of (from the inside out) Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, in the same way as the previous picture. The small square in the middle is the Sun. In each orbit, the small square indicates the position of the planet at 1 January 2005, and the pluses mark the position every 2 years. The planets move counterclockwise in this picture. The units along the horizontal and vertical axes are Astronomical Units of about 150 million km or 93 million miles. The orbit of Pluto is not shown completely. The vertical lines connect positions of Pluto on the solid orbit with the corresponding positions on the dashed orbit. The length of each vertical line shows how much Pluto is above or below the ecliptic there.

Fig. 3: Orbits of Outer Planets (2)
Fig. 3: Orbits of Outer Planets (2)

Figure 3 is the same as Figure 2, but shown at a smaller scale so the orbit of Pluto is shown completely.

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If you look down on the Solar System from very high above the North Pole of the Sun, then you'll see all planets travel around the Sun in the same direction, namely counterclockwise, which is also the direction in which the Sun rotates around its own axis.

The planets are kept in their orbits because they and the Sun attract each other through gravity. The Sun attracts a planet just as hard as the planet attracts the Sun, but the Sun is very much more massive than the planets so it is much harder to move, and that's why the planets have wide orbits while the Sun hardly moves at all.

It is also important that space is very empty, so that the planets have no friction in their orbits. If space were full of some gas, then the friction of the gas on the planet would slow the planet down and then it would not stay in its orbit but gradually get closer to the Sun and finally fall into the Sun. Something like this does happen to satellites that are not high enough above the Earth: they are slowed down gradually by the very small amount of friction they get from the very dilute outermost layers of the Earth's atmosphere, and eventually they fall back into the atmosphere of the Earth and burn up.

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The orbits of all planets are ellipses. An ellipse is a circle that has been squashed in one direction, so it looks a bit like an oval. The orbits of the planets have been squashed by only a tiny bit, so if you draw them as circles, then that is usually good enough.

You can see the difference between an elliptical orbit and a circular orbit much better by where the Sun is in that orbit. In a circular orbit, the Sun is exactly in the middle of the circle, but in an elliptical orbit the Sun is some distance away from the center. By how far the Sun is from the center of the orbit, compared with the size of the semimajor axis (something like the radius) of the orbit, is called the eccentricity. The eccentricity \(e\) and flattening \(1 - b/a\) of the orbits of the planets is shown in the following table.

Table 3: Planet Orbit Shapes

e1 − b/a
Mercury 0.206 0.021
Venus 0.007 0.000023
Earth 0.017 0.00014
Mars 0.093 0.0044
Jupiter 0.048 0.0012
Saturn 0.056 0.0015
Uranus 0.046 0.0011
Neptune 0.009 0.000040
Pluto 0.249 0.031

For example, the orbit of Mercury is a fraction 0.021 or 2.1% less "wide" than it is "long", but the Sun is shifted a fraction 0.206 or 20.6% or just over one fifth from the center to the farthest edge of the orbit.

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1.19. The Law of Titius-Bode

There is a pattern to the distances between the Sun and some of the planets, which is called the Law of Titius-Bode. It says that the distance \(a_n\) of each planet from the Sun is equal to about

\begin{equation} a_n = 0.4 + 0.3 × 2^n \end{equation}

measured in AU, where \(n\) begins at minus infinity, then jumps to 0, and after that keeps increasing by 1.

There is another formula that ties the orbital period of a planet to its distance from the Sun, which is a form of Kepler's Harmonic Law. It says that the period measured in years is equal to the distance measured in AU to the power 3/2 = 1.5.

If we combine the Law of Titius-Bode and Kepler's Harmonic Law, then we get that the period \(P_n\) of a planet, measured in years, is about equal to

\begin{equation} P_n = (0.4 + 0.3 × 2^n)^{3/2} \end{equation}

Kepler's Harmonic Law is quite accurate, but the Law of Titius-Bode is only approximate, so the combination of the two is also only approximate. The next table shows some numbers. \(n\) is the number to put into the formula. \(a_n\) is the distance according to the Law of Titius-Bode. The "(real)" column next to that shows what the real average distance of the planet is (the semimajor axis). \(P_n\) shows the orbital period according to the formula. The "(real)" column next to that shows what the real orbital period is. The "(n)" column shows what number you have to put into the formula to get the real orbital period.

Table 4: Titius-Bode

\({n}\)\({a_n}\)(real) \({P_n}\) (real) (n)
Mercury \({-\infty}\) 0.4 0.39 0.25 0.24
−1 0.55 0.41
Venus 0 0.7 0.72 0.59 0.62 0.11
Earth 1 1.0 1.00 1.00 1.00 1.00
Mars 2 1.6 1.52 2.02 1.88 1.91
3 2.8 4.69
Jupiter 4 5.2 5.20 11.86 11.86 4.00
Saturn 5 10.0 9.54 31.62 29.46 4.93
Uranus 6 19.6 19.18 86.77 84.02 5.97
Neptune 30.06 164.77 6.63
Pluto 7 38.8 39.44 241.68 248.4 7.03
8 77.2 678.31
9 154.0 1911.1

So, the pattern works reasonably well for Venus through Uranus and for Pluto, but it has no room for Neptune, and Mercury doesn't really fit (there are an infinite number of negative numbers \(n\) between Mercury and Venus), and there is a gap (\(n\) = 3) between Mars and Jupiter.

The Titius-Bode Law was discovered around 1770, when only the same planets were known that the ancient astronomers from Babylon and Greece already knew (Mercury through Saturn). Some people thought that the gap for \(n\) = 3 indicated that there was an as yet undiscovered planet there. In 1781 a new planet was discovered which was called Uranus. Uranus turned out to fit the Titius-Bode Law quite well (for n = 6), so people started looking for the mysterious \(n\) = 3 planet more carefully. In 1801 Ceres, the first of the asteroids, was discovered at a distance from the Sun that fit the \(n\) = 3 gap reasonably well. Since then, many thousands of asteroids have been discovered, mostly between the orbits of Mars and Jupiter. Neptune was discovered in 1846, and it didn't fit the pattern at all. Pluto was discovered in 1930 and does seem to fit the pattern again, except that there is then no room in the sequence for Neptune.

Even though most of the planets seem to follow the Law of Titius-Bode quite well, there is probably quite a lot of chance to the distances between the planets and the Sun. When astronomers put a model of a big cloud of gas and dust in their computers and calculate how it might clump together into a solar system, then they find that many different arrangements of planets are possible, of which most don't follow a Titius-Bode-like law. In other words, if there is some guiding principle that makes most planets follow a Titius-Bode-like law, then astronomers haven't found it yet.

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1.20. Inclination of Planetary Orbits

The orbits of the planets are almost but not quite in the same plane. The inclination of a planetary orbit is the angle between the planet's orbit and the ecliptic. The values of these inclinations can be found on the Calculation Page about Positions in the Sky. The orbits of the planets are displayed above.

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1.21. Speeds of Planets

Planets orbit around the Sun. Their least, average, and greatest speed in their orbit around the Sun are listed in the following table, measured in kilometers per second. Multiply by 0.621 to get miles per second. Multiply by 2237 to get miles per hour. The average speed is the speed that the planets have when their distance from the Sun is equal to the length of the semimajor axis of their orbit.

Table 5: Planet Orbital Velocities

Name Minimum Average Maximum
Mercury 38.86 47.87 58.98
Venus 34.78 35.02 35.26
Earth 29.29 29.78 30.29
Mars 21.97 24.13 26.50
Jupiter 12.44 13.06 13.71
Saturn 9.11 9.64 10.19
Uranus 6.49 6.79 7.12
Neptune 5.38 5.43 5.48
Pluto 3.67 4.74 6.11

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1.22. Life on Planets

We don't even know exactly how life arose on Earth, so we can't be very sure about how life may arise on other planets. If we assume that only things with bodies can be alive, and that only the fundamental forces now known are important (which excludes "energy beings" and similar things), then you need fairly complicated molecules to have life. For life as we know it on Earth, you'd need at least the following things:

A biosphere is a region where beings live without support from outside (and are not just visiting, like astronauts in a space station or on the Moon). Indigenous life has not been detected outside of the Earth, so the only biosphere we currently know is on Earth.

Where else in the Solar System could there be life? I've seen an estimate that planets at distances between 0.84 and 1.7 AU could sustain life. That range includes the Earth (1.0 AU) and Mars (1.5 AU) but not Venus (0.7 AU). Venus today has such a giant greenhouse effect that its surface temperature is 470 degrees Centigrade ― a bit on the high side for life. Mars is now very cold (on average −48 degrees Centigrade) but there has been flowing water on it in the past.

The deeper you go under the surface of a large celestial body, the warmer it gets. You can therefore have flowing water (and hence perhaps life) even beyond 1.7 AU from the Sun, if you go deep enough under the surface. The moon Europa of Jupiter seems to be a good candidate for an ocean deep under the ice that is at its surface, and that ocean could harbor life.

The Moon is at the same distance from the Sun as the Earth is but is yet as dead as a doornail. That is because the Moon is much less massive than the Earth so (1) it has not enough gravity to keep an atmosphere and water from evaporating into space, (2) it has solidified to great depth and so has no geological activity at its surface (if it ever had any).

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1.23. Visiting Other Planets

No human has ever gone beyond the Moon, and Mars and the other planets are always more than 100 times further away from Earth than the Moon is, so nobody has visited any other planet yet. After the Moon, Mars is the only candidate for a visit from Earth. I expect that no humans will visit Mars before 2015, and perhaps even a lot later. A trip to Mars would be much more dangerous and difficult than a trip to the Moon, and trips to the Moon were already quite dangerous and difficult. See the page about Space Travel for more information.

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1.24. Retrograde Motion of the Planets

A superior planet (one that is further away from the Sun than the Earth is) usually moves towards the east between the stars, but around its opposition it temporarily moves to the west and then makes a kind of loop between the stars, before continuing with its usual eastward motion. The temporary westward motion is called retrograde motion.

The retrograde motion of a planet shows up if you look at its motion relative to the stars in the sky. It does not matter where those stars are in the sky, as long as they are above the horizon, so people noticed this retrograde motion already thousands of years ago.

Usually a planet moves eastward along the ecliptic, but when the Earth overtakes the planet on an inside curve, then the planet seems to move westward along the ecliptic for a while.

If you want to show retrograde motion with a planetarium program then you should find settings such that the stars don't seem to move, or such that the planet doesn't seem to move. In the first case, you'll see the planet trace a loop between the stars, and in the second case, you'll see all stars trace the same loop relative to the planet. Many planetarium programs enable you to tie the viewing direction to a particular celestial body such as a planet or a star. It is also convenient to select the ecliptic or celestial equator as the base plane so the image does not seem to wobble in the course of the seasons, and it is a good idea to make the ground (horizon) transparent, because otherwise the planet or star will disappear below the horizon at some date.

If tying the viewing direction to a planet or star is not possible with your planetarium program, then you can change the time in steps of 23 hours and 56 minutes. Then the stars (almost) don't move. In that case, too, it is convenient to make the ground transparent.

That the stars return to their old spots after 23 hours and 56 minutes whereas the Sun takes on average 24 hours to do this is because we orbit once around the Sun in a year, so the stars seem to rotate 366 times around the Earth while the Sun makes 365 loops. A sidereal day is equal to 365/366 days, or about 4 minutes shorter than a solar day.

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1.25. Round Planets

The planets are round because they have so much mass. Because of gravity, all mass tries to get as close together as possible. For a planet that does not rotate around its own axis, the ideal distribution has all points at the surface at the same distance from the center, and this means a round planet without any mountains or valleys. If the planet does rotate around its own axis, then the ideal shape is a sphere that is slightly flattened, so that the diameter from pole to pole is a bit less than the diameter at the equator.

To assume the ideal shape, the material in the planet must be able to flow down freely to fill in any valleys. The outside of moons and Earth-like planets (Mercury, Venus, Earth, Mars) are not fluid, so there can be small deviations from the ideal shape there, in the form of mountains and valleys, but if a mountain gets too high then it will be (slowly) crushed by its own weight, or it will slowly sink into the ground until it is no longer too high.

In practice, a planet, moon, star, or asteroid is round if its diameter is at least about 1000 km. For solid things that are much smaller than 1000 km, the internal forces can withstand gravity, so those things need not be round.

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1.26. Symbols of Planets

The symbols that are used to denote the planets are shown below. The symbols for the first six planets date back to the Middle Ages (if not earlier). The symbols for the other planets were of course only invented after those planets were discovered. The second column shows the symbols as pictures, and the third column as characters. Many web browsers cannot yet show those characters properly, so they may appear strange or not at all in your web browser. It is also possible that the picture and the character aren't exactly the same because for some planets more than one symbol has been used in the past.

Table 6: Planets: Symbols

Planet Symbol
Picture Sign
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto

The symbol for Venus is also used as the symbol for a woman or something female. The symbol for Mars is also the symbol for a man or something male. The symbol for Neptune is the top part of a trident: images of the Roman god of the sea (Neptune) often have him carry such a thing at the top end of a long stick. The symbol for Pluto is a combination of the first letter of the first and last names of the discoverer of Pluto, Percival Lowell.

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1.27. The Rotation Periods of Planets

All planets orbit around the Sun and turn around their own axis. The associated periods can be measured relative to the stars (sidereal) or relative to the Sun, as seen from the planet or from Earth (synodical). The most important periods are:

sidereal orbital period

the period in which the planet goes once around the Sun, measured relative to the stars. This period is also called the planet year. As seen from the planet, the Sun returns to (about) the same place between the stars after this much time.

synodical orbital period

the period in which the planet repeats its phenomena relative to the Sun (such as opposition or conjunction), as seen from Earth. This is also called just the synodical period of the planet.

sidereal rotation period

the period in which the planet turns once around its axis, measured relative to the stars. The rotation period is also called the period of revolution. As seen from the planet, the stars are back in the same positions again after this much time.

synodical rotation period

the period in which the planet turns once around its axis, measured relative to the Sun. This is also called the synodical period of revolution or the planet day or sol (which means "Sun" in Latin). As seen from the planet, the Sun goes through the sky in one planet day.

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The following table shows the most important periods of the planets, and also the length of the semimajor axis of their orbit (roughly their average distance from the Sun), and the speed of their equator (rotation speed) and the average speed with which they orbit around the Sun (orbital speed). The sidereal rotation period and synodical rotation period (planet day, sol) is given in Earth days of 86400 seconds each. The sidereal orbital period (planet year) is given in Earth years of 365.25 days (i.e., Julian years) and in planet days (sols). The synodical (orbital) period of the planet is given in Earth days. The rotation periods are measured at the equator.

Table 7: Planets: Periods

Planet Rotation Period Distance Orbital Period Speed
Sidereal Synodical Sidereal Synodical Rotation Orbital
Days AU years Days sol Days m/s km/h km/s
Mercury 58.646 175.94 0.387 0.2408 87.969 0.500 115.9 3.0 10.9 47.9
Venus 243.01 116.75 0.723 0.6152 224.71 1.925 583.9 1.8 6.5 35.0
Earth 0.997270 1.000000 1.000 1.0000 365.26 365.26 465 1670 29.8
Mars 1.025956 1.027491 1.524 1.8808 686.98 668.60 779.3 241 867 24.1
Jupiter 0.41007 0.41011 5.203 11.862 4332.6 10564 398.9 12700 45600 13.1
Saturn 0.4264 0.4264 9.539 29.457 10759 25232 378.1 10300 37000 9.6
Uranus 0.6125 0.6125 19.181 84.02 30688 50103 369.9 3040 10900 6.8
Neptune 0.7667 0.7667 30.058 164.77 60182 78494 367.5 2350 8460 5.4
Pluto 6.3867 6.3863 39.44 248.4 90728 14207 366.7 13 47 4.7

For example: As seen from Mercury, the Sun returns to the same spot in the sky after 175.94 days (1 planet day or sol), the stars return to the same spots in the sky after 58.646 days (0.5 sols), and the Sun returns to the same spot between the stars after 0.2408 years (87.95 days, 1 planet year). As seen from Earth, Mercury repeats its phenomena such as conjunctions and greatest elongations after 115.9 days.

For Venus, Uranus, and Pluto, the synodical rotation period (planet day) is shorter than the sidereal rotation period, while for the other planets the synodical rotation period is longer than the sidereal rotation period. The difference is so small for some planets that you can't tell from the values in the table. The difference between the two groups of planets is in the orientation of their axis of rotation. Venus, Uranus, and Pluto are more upside down than right side up.

On Mercury, a day lasts longer than a year, and in fact exactly twice as long. The rotation period and orbital period of Mercury are caught in a resonance such that 3 sidereal rotation periods are exactly equal to 2 sidereal orbital periods.

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The rotation period that a planet has today depends on the rotation that the planet got when it formed and on all changes that have occurred since then.

It is likely that a planet in general is formed with a rotation period (day) that is much less than the orbital period (year), because material in orbit around the Sun has a "natural" rotation period that is comparable to the orbital period, but this material was swept up from a very large area into a much smaller planet, and something that rotates and shrinks (gets closer to the rotation axis on average) rotates faster (like a figure skater who pulls in her arms while she's rotating). However, planets are (it is thought) formed by the collision and sticking together of every larger proto-planets, so the manner in which the last few collisions happened to happen has great influence on the rotation of the newly formed planet.

The rotation of a planet can change after it was formed, by subtle gravitational influence from its neighbors. It seems likely that the rotation of Mercury was slowed down (in part) because of tidal forces from the Sun, which is close to that planet. At least it seems that Mercury is now locked in a 2:3-resonance with its orbital period. The rotation of the Earth is now slowing down because of tidal influence from the Moon. The very slow rotation of Venus and the strange orientation of the rotation axis of Uranus might mean that they have collided in the past (probably toward the end of their formation) with an object of similar size.

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If a planet is further from the Sun, then it takes longer for that planet to orbit once around the Sun. A far-away planet takes longer to orbit the Sun because that planet has a greater distance to travel around the Sun, and also because that planet travels slower along its orbit. A far-away planet travels slower than a closer-by planet because the force of gravity between a planet and the Sun gets weaker when the planet and the Sun are further apart.

If planet A is \(y\) times further from the Sun than planet B is, then planet A takes \(y\sqrt{y}\) longer to orbit the Sun than planet B does. The factor \(y\) is because the orbit of A is so much longer than that of B, and the factor \(\sqrt{y}\) is because planet A travels that much slower along its orbit than planet B does.

For example, Jupiter is about 5.2 times further from the Sun than the Earth is, so Jupiter takes about \(5.2\sqrt{5.2} = 12\) times as long to orbit the Sun as the Earth takes, so Jupiter takes about 12 years to do that.

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1.28. Rotation Direction of the Planets

Whether a planet rotates clockwise or counterclockwise around its own axis depends on which pole you look at. If you look at the planet from above one pole, then it rotates clockwise, and if you look at the planet from above the other pole, then it rotates counterclockwise.

The following table provides information about the rotation direction of the planets. The columns "North" and "South" show whether the planet turns clockwise (cw) or counterclockwise (ccw) if you look at the planet from above that pole. The columns "East" and "West" show whether the Sun rises or sets in about that direction. For example, the Earth rotates clockwise if you look at it from above the south pole, and as seen from Earth the Sun rises in about the east and sets in about the west.

Table 8: Planetary Rotation Directions

north south east west
Mercury ccw cw rise set
Venus cw ccw set rise
Earth ccw cw rise set
Mars ccw cw rise set
Jupiter ccw cw rise set
Saturn ccw cw rise set
Uranus cw ccw set rise
Neptune ccw cw rise set
Pluto cw ccw set rise

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1.29. Repeating Positions of Planets

For each pair of planets, you can calculate how long it takes for the one planet to catch up with the other one in its orbit around the Sun. That period is called the synodical period of both planets, and after that period both planets are again in the same relative position. The average length of the synodical period of each planet (compared to the Earth) is listed in table 7.

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You can calculate a synodical period from a sidereal period as follows: If \(P_\text{sid1}\) is the sidereal period of the fast object (e.g., a moon in its orbit around Jupiter) and \(P_\text{sid2}\) is the sidereal period of the slow object (e.g., Jupiter in its orbit around the Sun), measured in the same units, and both orbit in the same direction (e.g., Jupiter counterclockwise around the Sun, and the moon counterclockwise around Jupiter), then the synodical period \(P_\text{syn}\), in the same units, is determined by

\begin{equation} \frac{1}{P_\text{syn}} = \frac{1}{P_\text{sid1}} - \frac{1}{P_\text{sid2}} \end{equation}

or

\begin{equation} P_\text{syn} = \dfrac{1}{\dfrac{1}{P_\text{sid1}} - \dfrac{1}{P_\text{sid2}}} \end{equation}

If they orbit in opposite directions, then you should replace the minus sign by a plus sign, but for planets and for moons in the Solar System the minus sign is almost always correct.

For exaple, if \(P_\text{sid1}\) is equal to 27.3 days (the sidereal period of the Moon, as seen from Earth), and \(P_\text{sid2}\) is equal to 365 days (the sidereal period of the Sun, as seen from Earth), then \(1/P_\text{syn} = (1/27.3) - (1/365)\) = about 1/29.5, so then the synodical period is equal to about 29.5 days. This is the average time between two successive Full Moons.

The conjunctions and other relative phenomena of the fast and slow objects repeat after on average one synodical period; for example the conjunctions of the Sun and the Moon as seen from Earth, such as Full Moon and New Moon.

Three celestial objects are involved for a synodical period: the object where the observer is, and the two other ones that move through the observer's sky. The same synodical period also holds if the observer moves to one of the other two objects. So, as seen from Earth, the Sun and the Moon are closest together in the sky about once every 29.5 days, and as seen from the Moon, the Earth and the Sun are closest together about once every 29.5 days, and as seen from the Sun, the Earth and the Moon are closest together about once every 29.5 days.

Now we apply this to the moons of Jupiter. If \(P_\text{moon}\) is the sidereal period of such a moon (in its orbit around Jupiter), and \(P_\text{J}\) is the sidereal period of Jupiter (in its orbit around the Sun), then the synodical period \(P_\text{syn}\) of the moon is equal to

\begin{equation} P_\text{syn} = \dfrac{1}{\dfrac{1}{P_\text{moon}} - \dfrac{1}{P_\text{J}}} \end{equation}

After on average one such synodical period, Jupiter and the moon are in about the same relative positions, so the time between successive transits or occultations of the moon will be close to multiples of the synodical period.

As soon as you look at more than two planets at the same time, there is no fixed period anymore after which those planets return to particular relative positions. More than two planets never return to exactly the same relative positions that they had before. That is because the ratios of the orbital periods of the planets are not exact fractions.

If, for example, Mars took exactly 2 years to go around the Sun, and Jupiter exactly 12 years, then after every 12 years the Earth would have gone around the Sun exactly 12 times, Mars exactly 6 times, and Jupiter exactly 1 time, so then all three of them would be in the same relative positions again.

There can be a common period even if the orbital periods are not whole numbers. The orbital periods from the table for the Earth, Mars, and Jupiter are, rounded to the nearest tenth of a year, equal to 1 year, 19/10 years, and 119/10 years. Those periods all fit into a single common period of 2261 years, namely 2261 times for the Earth, 1190 times for Mars, and 190 times for Jupiter, so if the orbital periods of those planets were exactly equal to the rounded values, then they'd return to the same positions after every 2261 years. That's almost two hundred times longer than before when we rounded to the nearest full year.

For all planets together there'd then be a common period of 816.821.286.456 years (about 817 thousand million years), and that is many times more than the age of the Universe (which is currently about 14 thousand million years), so even if the orbital periods were exactly equal to the values rounded to a tenth of a year, the common period would not be useful in practice, because it is so very long.

But the orbital periods are not exactly equal to their rounded values, and that makes a lot of difference. If, for example, we were to use values that were rounded to the nearest hundredth of a year, so 1 year for Earth, 188/100 years for Mars, and 1187/100 years for Jupiter, then the smallest common period would be 55,789 years (55789 times for the Earth, 29675 times for Mars, and 4700 times for Jupiter), and that is more than ten times greater than last time. For all planets together the period would then be 7.865.034.998.354.446.554 years, which is enormously longer than last time.

If you take ever more accurate approximations for the orbital periods of the planets, then the common period after which the planets return to the same relative positions gets on the whole longer and longer. If you could use the real values for the orbital periods, with infinite precision, then the common period would also be infinitely long.

You can read more about the repetition of planetary positions on the Conjunctions Page. Do you want to calculate the common periods that I mention above for yourself? Then see the Calculation Page for Common Periods.

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1.30. Distances from the Sun

The following table shows the distances of the planets from the Sun, measured in astronomical units (AU), millions of kilometers (Gm), millions of miles ("Mmi"), and in lightseconds (ls). The number of lightseconds is how many seconds it takes sunlight to reach that planet. The Astronomical Unit is almost exactly equal to the mean average distance between the Sun and the Earth.

Table 9: Planets: Distances from the Sun

Planet Least Average Greatest
AU Gm Mmi ls AU Gm Mmi ls AU Gm Mmi ls
Mercury 0.306 46 28 153 0.387 58 36 193 0.467 70 43 232
Venus 0.718 106 66 358 0.723 108 67 360 0.728 109 68 363
Earth 0.983 147 91 490 1.000 150 93 499 1.017 152 95 507
Mars 1.381 207 128 689 1.524 228 142 760 1.666 249 155 831
Jupiter 4.951 741 460 2470 5.203 778 484 2596 5.455 816 507 2722
Saturn 9.008 1348 837 4503 9.539 1427 887 4767 10.069 1506 936 5032
Uranus 18.275 2734 1699 9146 19.181 2869 1783 9590 20.088 3005 1867 10034
Neptune 29.800 4458 2770 14890 30.058 4497 2794 15025 30.316 4535 2818 15160
Pluto 29.58 4425 2750 14818 39.44 5900 3666 19732 49.19 7359 4573 24645

The planet Mercury is the closest to the Sun of all planets, at about 0.39 AU (or about 58,000,000 kilometers). There are asteroids or planetoids ("minor planets") that can get closer to the Sun. A number of them are: Phaeton (which gets down to 0.14 AU), Hephaistos (0.36 AU), Icarus (0.19 AU), and Talos (0,19 AU). Some comets even fall into the Sun, and are never seen again. Of all space probes that have been launched from Earh, the one that got closest to the Sun, as far as I know, was the German "Helios 2" probe that got to about 45,000,000 kilometers (0.30 AU) from the Sun, in 1976.

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1.31. Distances from Earth

The planets and the Earth all orbit around the Sun, each at its own speed, so the distance of a planet from the Earth is not always the same. The least, average, and greatest distances from Earth (over 20 synodical periods starting at 1 January 2000, except for Pluto) are listed in the following table, measured in Astronomical Units (AU), millions of kilometers (Gm) and millions of miles ("Mmi"). A lightyear is equal to about 63,178 AU, so to convert the distances to lightyears you need merely divide them by 63,178. For example, the average distance of Jupiter to the Earth is 5.28/63178 = 0,000082 lightyears.

Table 10: Planets: Distances from the Earth

Planet Least Average Greatest
AU Gm Mmi AU Gm Mmi AU Gm Mmi
Mercury 0.55 82 51 1.04 156 97 1.45 217 135
Venus 0.27 40 25 1.14 171 106 1.74 260 162
Mars 0.37 55 34 1.70 254 158 2.68 401 249
Jupiter 3.95 591 367 5.28 790 491 6.45 965 600
Saturn 8.05 1204 748 9.62 1439 894 11.05 1653 1027
Uranus 18.83 2817 1750 20.02 2995 1861 21.09 3155 1960
Neptune 28.93 4328 2689 30.01 4489 2790 31.10 4653 2891
Pluto 28.56 4273 2655 39.44 5900 3666 50.21 7511 4667

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1.32. When the Planets are Closest to the Earth

Each planet is closest to the Earth, in its perigee, once during each synodical period (see above). For the inferior planets, this is approximately when they are in their inferior conjunction, and for the superior planets this is approximately when they are in opposition. You can find the dates at which each planet is closest to the Earth (and their distance at that time) in the Planetary Phenomena Pages. Look for dates for which the column with title "*" contains a "p" and the column with title "AU" contains a number. For example: on 2003-10-30 (30 October 2005) Mars is again in its perigee, closest to the Earth. Its distance is then 0.4641 AU, which corresponds to 69 million kilometers or 43 million miles.

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1.33. Size, Mass, and Density of Planets

The sizes, mass, and density of the Sun and the planets are listed in the following table. Shown are: the radius from the center to the equator, the radius from the center to the pole (both in kilometers), the oblateness (by what fraction the polar radius is less than the equatorial radius), the surface area (in millions of square kilometers), the volume (in thousands of millions of cubic kilometers), the mass (compared to that of the Earth), and the density (compared to that of water).

Table 11: Planets: Size, Mass, Density

Name Radius Oblateness Area Volume Mass Density
Equator Pole
km km Mm2 Mm3 Earth = 1 water=1
Sun 695990 695990 0 6087176 1412204556 332946 1.40
Mercury 2439 2439 0 75 61 0.055 5.41
Venus 6051 6051 0 460 928 0.815 5.25
Earth 6378 6357 0.003353 509 1083 1 5.50
Mars 3396 3379 0.005 144 163 0.011 3.91
Jupiter 70850 66530 0.061 59230 1398000 318 1.24
Saturn 60330 54900 0.09 41620 837000 95 0.62
Uranus 25400 24600 0.03 7860 66600 14.5 1.24
Neptune 24300 23600 0.03 7200 58300 17.1 1.61
Pluto 1150 1150 0 17 6 0.0021 2.06

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1.34. Planetary Atmospheres

An atmosphere is a layer of gases around a planet. If you look carefully enough, then you can find some gas molecules or atoms around any planet, but if you start measuring that accurately, then you can't call space a vacuum anymore, either. We'll adopt the practical definition that an atmosphere must be detectable from great distance (for example through spectral lines associated with the molecules and atoms of the atmosphere, or through observations of the occultation of stars by the planet) and must yield friction to a landing spacecraft or a meteorite. With that definition, all planets except Mercury and (probably) Pluto have an atmosphere, and the moon Titan of Saturn has one, too. The below table shows the pressure at the bottom of the atmosphere of all planets and of the moons Titan (of Saturn) and Triton (of Neptune), measured in units of 1 bar (which is roughly equal to the air pressure at the surface of the Earth). The atmosphere of the giant gas planets are so thick that we don't know exactly where they end or what the pressure is there. Estimates for Jupiter and Saturn are around 2 million bar. In the table I've written "≫ 100" which means "much larger than 100".

Table 12: Atmospheric Pressure on Planets and Moons

Name Pressure
Mercury 0
Venus 92
Earth 1
Mars 0.007
Jupiter ≫ 100
Saturn ≫ 100
Titan 1.5
Uranus ≫ 100
Neptune ≫ 100
Triton 0.00016
Pluto 0.00001

Though many planets have an atmosphere, there is only one that has an atmosphere in which we can live, and that is the Earth. The atmospheres of the other planets that have one contain far too little oxygen or none at all. Some people have made plans to transform the atmospheres of Venus and Mars into air in which we can live, but I doubt that that will happen any time soon.

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A long time ago, most people thought that the Earth stood still and that the starry sky rotated around the Earth. It has to be like that, those people thought, because the natural condition of things is to slow down and then stand still, in the same way that a ball always stops after a while when you made it roll, so if the Earth rotated around its axis, the atmosphere would naturally still want to stand still, so then the surface of the Earth would rub along the atmosphere at great speed, so then there would have to be a great storm everywhere all the time. And if you threw something up into the air, then it would naturally not follow the rotation of the Earth, so then it would always go quickly sideways in the same direction, compared to the ground. There is no such storm, and things that you throw into the air just come down again, so the Earth does not rotate.

However, those people of long ago were wrong. The starry sky stands still, and the Earth rotates around its axis. The natural order of things is not that they slow down and stand still, but (according to the First Law of Newton) that they move along a straight line at constant speed. If you want to change their direction or speed, then you have to apply a force to them. The force of gravity keeps the atmosphere and other things as close as possible to the ground, and friction with the Earth causes the atmosphere to rotate with the Earth on average, so the atmosphere on average stands still relative to the ground. When things move across a rotating planet, then they notice the rotation in the form of Coriolis forces. Coriolis forces cause flows of air in the atmosphere or water in the oceans far from the equator to not go in straight lines but with very large eddies.

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1.35. Does It Matter If You Shift a Planet?

Where things in the Universe move to is determined mostly by the force of gravity. All things that have mass generate gravity, but things with more mass generate stronger gravity than things with less mass, and gravity decreases with increasing distance. The things with the most mass have the most influence on other things in the Universe, and mostly on things in their neighborhood.

A planet has far less mass than a star. The Sun, for example, has about 1000 times more mass than all planets of the Solar System put together. Because of this, a planet only dominates the part of space that is close to it. The gravity of Earth, for example, only dominates out to about 850,000 km from Earth. Further away than that, orbits of moons and other things are disturbed so strongly by the gravity of the Sun that they escape from the gravity of the Earth. 850,000 km may look impressive, but is only 1/180th part of the distance to the Sun.

If a planet were suddenly transported to another part of the Solar System, or even removed completely, then that would only have great effect in the part of space where the planet dominated before, and in the part of space where the planet ends up. If one of the planets of the Solar System (other than Earth) were removed, then the other planets would not notice this very much, and their orbits woud remain practically the same.

2. Specific Planets

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2.1. Venus

We don't yet know exactly why the atmosphere of Venus evolved so differently than that of the Earth, because we don't yet have all information necessary to choose between various possibilities. How an atmosphere evolves depends on which gases are in it, how much gravity the planet has, what kind of rocks or minerals are at the bottom of the atmosphere, how far the planet is from its star, how much light the star emits, the surface temperature of the star, whether there is a suitable liquid (such as water) on the planet, and whether the planet is geologically and biologically active (with volcanos and plants and such). We don't yet know much about the lower layers of the atmosphere of Venus or about the composition of the rocks at its surface, because conditions are so terrible there that space probes don't last very long.

It is likely that the atmospheres of Venus and the Earth started out similar, but today the atmosphere of Venus is very different from that of Earth: It has a humongous greenhouse effect, surface temperatures of about 450 or 800 higher than on Earth and atmospheric pressure at the surface that is about 90 times as great as on Earth. Certainly one important cause of this difference is that Venus is 30% closer to the Sun than the Earth is. This means that even without any greenhouse effect the average surface temperature on Venus would be about 50 ℃ or 90 ℉ higher than on Earth (if all other things were equal), which means that any water on Venus would likely have evaporated quickly, which is one way of getting a greenhouse effect.

The chilling conclusion is that apparently fairly modest changes in initial conditions can produce huge differences in the final outcome. If the Earth had been just a little closer to the Sun, then it, too, might have gone the way of Venus, and then we would not have been here.

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2.1.1. No Venus Moons

How many and what kind of moons a planet gets depends a lot on chance, so we cannot say for sure why Venus has no Moons. Moons

  1. can be formed together with the planet around which they orbit.
  2. can form elsewhere and be captured by the planet.
  3. can form from the debris of a collision with or near the planet

The large moons of the jovian planets are probably examples of type 1. Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2. Such moons are often the furthest from the planet. The moons of Mars are probably examples of type 2 as well. Our Moon is probably an example of type 3.

I think that a planet has a greater chance of having moons if the planet has more mass (i.e., is larger) and if the planet is further away from the Sun. Venus does not have a very large amount of mass and is also rather close to the Sun. The Earth does not have a lot more mass than Venus and is only about 40 % further from the Sun than Venus is, so perhaps it is stranger that the Earth has a large moon than that Venus has no moons.

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2.1.2. Venus Transits

A transit of Venus across the Sun means that Venus passes in front of the Sun so it looks like a black spot against the bright solar disk. To get a transit of Venus, Venus must be sufficiently close to the ecliptic (the path of the Sun between the stars as seen from Earth), so Venus must then be close to one of the two nodes of its orbit. In addition, the Sun must then be in the same direction as Venus, as seen from Earth, so Venus must then be near an inferior conjunction with the Sun, and the Earth must then be near the same node of the orbit of Venus as Venus itself.

You get a transit of Venus if

  1. there is an inferior conjunction of Venus ("c"on the Planet Phenomena Pages) sufficiently close to a passage of Venus through a node of its orbit ("x" on the Planet Phenomena Pages).
  2. the passage of Venus through a node of its orbit is sufficiently close to the passage of the Earth through that same node (as measured by the ecliptic longitude). The Earth is now near the ecliptic longitude of a node of the orbit of Venus around 7 June and 8 December (in the Gregorian calendar), so there is a chance for a transit of Venus if Venus passes through a node of its orbit near those dates. These dates get later by about one day each 360 years.

You can in principle calculate how close is "sufficiently close". See the Transits Calculation Page for this. Then, you can use the methods described on the Saros Calculation Page and the Period Ratio Calculation Page to set up a prediction series for transits of Venus, just like you can use the Saros to predict eclipses of the Sun and the Moon. It is quite a lot of work to calculate all necessary parameters with sufficient accuracy.

However, given a list of transit years, you can fairly easily derive a formula that produces those years. Based on all transits of Venus that I found between the years 1000 and 3000, here are such formulas, in which \( j \) is the year:

\begin{align} 395 × j \bmod 243 & = 129 \text{ or } 130 \\ [395.0003 × j] \bmod 243 & = 53 \text{ or } 54 \end{align}

where \(\bmod\) represents the remainder after division, and \([x]\) the whole number nearest \(x\). The first formula is valid for transits of Venus in June, and the second one for transits in December. The first formula predicts one transit too many in the considered period, and the second formula five too many. The first formula says that there is another transit of Venus every 243 years after 1032 and 1040. The second formula cannot easily be translated into such a recipe.

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2.2. The Earth

The Sun rises in the east so the Earth rotates around its own axis towards the east. As seen from the North Pole this is counterclockwise, but as seen from the South Pole this is clockwise.

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The Earth's circumference at the equator is about 40000 km or 24900 miles and the Earth rotates once per 24 hours, so the rotation speed at the equator is 40000/24 = 1670 km/h or 24900/24 = 1038 mph.

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The polar diameter of the Earth (from one pole through the center to the other pole) is 12713.51 km or 7899.83 mi. The equatorial diameter (from one spot on the equator through the center to the opposite spot on the equator) is 12756.28 km or 7926.41 mi. The equatorial diameter is 42.77 km or 26.58 mi greater than the polar diameter.

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You don't feel that the Earth rotates because the Earth rotates at a (nearly) constant rate. The whole world has adjusted to that a long time ago, so you don't notice it.

If a motor cyclist goes through a curve fast then he leans towards the center of the circle of which the curve is a part, because only then are all forces including the centrifugal force because of the curve) balanced and does he not fall over. If you go through a sharp curve on your bicycle, then you lean into the curve, too, for the very same reason: otherwise, you'll fall over. And if you're in a merry-go-round that goes very fast, then you'll lean towards the center so you won't fall over. You'll then still feel a force pushing you towards the edge, just as if you're standing on an incline, but if the bottom of the merry-go-round were not horizontal but suitably inclined itself, then it would feel as if you were just standing up straight. Because the merry-go-round goes around so fast, the direction that feels like "down" has changed a bit there.

If you stir a drink quickly, then the surface of the fluid becomes curved (lower in the middle, and higher near the edges where the speed is greatest). The fluid then goes through the curve fast, so it feels a centrifugal force. Some of the fluid then moves around until there is an equilibrium again, and that is when the surface is curved. When the fluid gets to rest again (because of friction with the cup) then some of the fluid flows back to the middle again, and then there is a balance once more.

The Earth has done something similar. The Earth rotates and so has a centrifugal force, but the magma (on the inside) and water (on the outside) could adjust by redistributing themselves slightly (with a bit more at the equator and a bit less at the poles), so you don't notice it.

We can measure the influence of the rotation, however. Because of the rotation of the Earth, the equator is about 20 km (15 mi) further from the center of the Earth than the poles are, and your weight is about half a percent less at the equator than at the poles.

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2.2.1. The Discovery that the Earth is Round

The first people of which it is claimed that they said that the Earth was round were a number of Greek philosophers that lived between about 600 and 450 BC in what are now southern Italy and western Turkey, but were part of the Greek world then. Unfortunately, the books of these philosophers have been lost a long time ago, so we have to make do with comments about their thoughts in books of authors of much later times, who often copied those comments from yet others (and perhaps even copied them wrong). Moreover, there is a big difference between claiming something and proving something. The philosophers that are nominated for the honor are: Thales of Milete (624 - 547 BC; but his nomination is questionable), Pythagoras of Samos (580 - about 500 BC), Alcmaeon of Croton and Parmenides of Elea (about 504 - 450 BC).

As far as I know, Aristotle of Stagira (384 - 322 BC) was the first person of whom we can read in (copies of) his own books that he thought the Earth was round. He even quoted good proof of that, such as the fact that the shadow of the Earth that the Moon passes through during a lunar eclipse is always round.

It is quite possible that others had discovered earlier that the Earth was round, but that they didn't write that down or that their books and thoughts have been lost completely. One of the consequences of the curvature of the Earth is that a ship that is far away appears to sink behind the horizon, and a sharp sailor may have noticed that long before 600 BC, because there was already a lively trade across the Mediterranean Sea around 1500 BC.

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2.2.2. Measuring the Circumference of the Earth

The diameter and circumference of the Earth can be estimated, for example, if you know the north-south distance along the surface that corresponds to a difference of one degree in latitude. The circumference corresponds to 360 degrees, so it is 360 times as large as the distance that corresponds to one degree. The latitude can be determined, for example, from the height of the Pole Star above the horizon. See question 167.

The classical estimate of the circumference of the Earth by Eratosthenes around 240 BC was based on this idea, except that it looked at noon shadows instead of the height of the Pole Star to determine latitude differences. See //en.wikipedia.org/wiki/Eratosthenes. The diameter of the Earth is approximately equal to the circumference divided by the number pi (π). Approximately, because the Earth is not a perfect sphere.

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2.2.3. The Discovery that the Earth Rotates Around Its Axis

We don't know for sure who first discovered that the Earth rotates around its axis, because that happened in a time that is so long ago that most of the people and books from that time have been forgotten. I can tell you who the first people were of whom we know that they wrote that the Earth rotates around its axis.

The apparent motion of the Sun, Moon, planets, and stars in the sky can be explained in two ways: (1) the Sun, Moon, planets, and stars orbit around the Earth once a day, or (2) the Earth rotates around its axis once a day.

In ancient times, most people thought that the Earth is fixed and the stars rotate around the Earth. There were only a few who said that the Earth rotates. The first ones that we know about are Hiketas of Syracuse (a city on the island of Sicily) and Herakleides of Pontus (a region that is now in Turkey), who both studied in the school founded by Pythagoras (582 - 496 BC). We don't know much about them, not even exactly when they lived, but it was probably somewhere between about 530 BC and 350 BC. There may have been others before them, but in that case their history and books have gotten lost. The next one after them was Aristarchus of Samos (310 BC - about 230 BC). The most famous (at least in the middle ages) of the Greek astronomers was Ptolemy, who wrote the Almagest around AD 150, which was the most important astronomical book for about 1500 years, but Ptolemy thought that the Earth did not rotate, either around its axis or around the Sun, and so did almost everyone else for about 1500 years, until the Renaissance.

Nicolaus de Cusa (1401 - 1464) thought that the Earth rotated, but also thought that the stars rotated around the center of the Earth at the same time, which is incorrect. Celio Calcagnini (1479 - 1541) wrote that the Earth rotated, but he tried to also explain other things as a result of the rotation of the Earth, such as the seasons, that are not caused by that rotation at all, so it seems that he didn't really understand what he was writing about. In 1543, Nicolaus Copernicus (1473 - 1543) published a book in which he swept away Ptolemy's ideas and said that the Earth rotates around its axis, the stars are fixed, and the Earth orbits around the Sun as well. After this, more and more people started to agree with Copernicus that the Earth rotates.

You can demonstrate the rotation of the Earth using a Foucault Pendulum, as invented by Jean Foucault (1819 - 1868) and exhibited for the first time in 1851. The pendulum swings to and fro, but the line along which it swings slowly rotates, because of the rotation of the Earth.

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2.2.4. The Determination of the Mass of the Earth

Newton's Law of Gravity shows the connection between the gravity between two things, the mass of the things, and the distance between the things. The Law also contains a constant of nature which is usually written as \(G\) and which is called the (universal) gravitational constant. If you know four of these five quantities, then you can calculate the missing fifth one. If you want to use the Law of Gravity to calculate the mass of the Earth, then you need to know the four other quantities from the Law of Gravity in a specific case, and you especially need to know the gravitational constant \(G\).

To calculate the gravitational constant, we need to know the gravity between two objects, the masses of the objects, and the distance between the objects. The difficulty is to measure the gravity, because here on Earth the gravity of the Earth is very much greater than the gravity between any two things that we can handle in a laboratory.

You can render the influence of the gravity of the Earth unimportant by measuring the gravity horizontally between two objects, and you can measure that gravity by balancing it with a force that you know.

Cavendish was the first one to do this experiment. He attached two small metal balls to the end of a light wooden bar that was suspended from the ceiling by a long metal thread tied to the middle of the bar. He then put two heavy metal balls next to the light balls. The gravity between the light and heavy balls made the light balls pull slightly closer to the heavy balls, so the bar rotated a little and the thread was wound up. The torsion of the thread produces an opposing force that depends on the angle by which the thread is wound up and on the characteristics of the thread. If the thread is wound up sufficiently, then its opposing force exactly balances the force of gravity between the balls. You can then calculate the gravitational constant from the angle over which the bar has rotated and from the distances and characteristics of the balls, bar, and thread.

Once you know the gravitational constant \(G\), you can calculate the mass of the Earth if you know the distance and orbital period of a (light) thing that orbits around the Earth, such as the Moon or an artificial satellite. The mass of the Earth turns out to be about 6 × 1024 kg.

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2.2.5. The Distance to the Sun

The distance to the Sun was originally estimated using trigonometry. One method is to see what the distance is in the sky between the Sun and the Moon when the Moon is in First or Last Quarter. If the Sun is infinitely far away, then that angular distance is 90 degrees. If the distance to the Sun is finite, then the angular distance is (slightly) less than 90 degrees. However, in practice the difference from 90 degrees is only 0.0026 degrees, which was far too small for the Ancient Greeks to measure accurately, so even though the method was sound, it could not be used in practice.

The distance to the Sun remained essentially unknown (except that it was known to be "large") until 1761 when a transit of Venus could be used to estimate it. For the basic details of the technique, see //spiff.rit.edu/classes/phys445/gettys/venus_ex/venus_ex.html. For a description of the history, see //sunearth.gsfc.nasa.gov/sunearthday/2004/vt_edu2004_venus_back_his.htm.

Nowadays, we can use radar to determine the distance to our closest neighbors (the Moon, Venus) directly. Radar waves travel at the speed of light, which is accurately known, so the time it takes before reflected waves arrive yields the distance if it is multiplied by the speed of light.

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2.2.6. The Perihelion of the Earth

The Earth is closest to the Sun (i.e., in its perihelion) on average around 14:00 UTC on 3 January each year (around the year 2000). This calendar date gets later by about one day per one hundred years. The precise times when the Earth passes through its perihelion oscillate through about 2.5 days around the average because of the gravitational attraction by the Moon, and through about 1 day because of leap years. The Earth and the Moon orbit around a common center of mass and that center of mass follows a smooth elliptical orbit around the Sun with a perihelion at an ecliptic heliocentric longitude of 102.9 degrees (the \(Π\) of the Earth, as noted in a table on the Calculation Page about Positions in the Sky). Because the Earth orbits around the common center of mass, the perihelion of the Earth is sometimes a bit earlier than average, and sometimes a bit later. These oscillations are tied to the phase of the Moon. Roughly speaking, the perihelion passage of the Earth will be around the average time when it is New Moon or Full Moon, will be about 30 hours early (i.e., usually on 2 January) if it is near Last Quarter, and about 30 hours late (i.e., usually on 4 January) if it is near First Quarter. In a leap year, there is extra delay of about 9 hours, and in a year directly following a leap year the perihelion is 9 hours earlier (and in the following years it is about 3 hours early and 3 hours late, respectively).

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2.3. Mars

The planet Mars has always stirred the imagination because the conditions at the surface of Mars are, of all the planets, most like those on Earth. On Mars, you can find, just like on Earth, an atmosphere, wind, clouds, large volcanoes, ice caps at the poles, color differences that are linked to the seasons, and evidence for flowing water.

With all of these correspondences, Mars is yet not friendly to earthlings. The atmosphere of Mars is as thin as it is about 22 kilometers above the Earth, and a human on Mars without a space suit and oxygen supply would suffocate immediately. There is no ozone layer around Mars, so harmful ultraviolet rays can reach the surface of Mars without any problem and these would cause severe sunburn of the skin of a person without a space suit. As far as the atmosphere and harmful solar radiation are concerned, Mars is nearly as bad as outer space.

At the equator of Mars, it can heat up to about 20 during the day, but at night it gets bitterly cold even at the equator, and outside the tropics of Mars it is always very cold. Viking 1 and 2 landed at 22° and 48° north latitude, respectively, in the middle of martian summer, but yet the daily highest temperatures were only −40℃ and −50℃. At the poles, the temperature can drop to −140℃. The average temperature over the whole planet over one Mars year is about −60℃.

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2.3.1. Water on Mars

There appears to have been flowing water on Mars in the past, because some pictures of the surface show branching flow channels. Those flow channels are dry now, and so far no signs of seas or large lakes have been found. It may be that the dry beds did not carry steady flows, but only intermittent or single floods, such as dry streams (washes) in deserts that carry water temporarily after a rainstorm. Conditions today are not good for open water. Only near the equator does the temperature sometimes get above freezing, but the atmosphere is so dry there than any open water would evaporate very quickly. Away from the equator, the temperature is always below freezing, so water can exist there only as ice. The poles of Mars contain some water ice but are made up mostly of carbon dioxide ice (dry ice).

It is currently just too cold on Mars for open water (like lakes and seas) to exist. Mars is a desert.

The water on Mars is now frozen in the polar ice caps (which also contain carbon dioxide ice, at least in winter), or hidden below the ground. A very tiny amount of water is in the air, in the form of invisible water vapor.

In most places on Mars the temperature never gets above the freezing point of water, so the water below the ground is likely in the form of ice as well (in so-called permafrost, which also occurs on Earth in areas close to the poles, such as in northern Siberia and northern Canada).

It seems that it has been wetter on Mars in the past. It must have been warmer then, at least at the surface. What could have caused such warming?

The most likely explanation is the greenhouse effect. A greenhouse is a house made mostly of glass which allows sunlight to enter and heat up the inside but which makes it difficult for heat to escape. In a greenhouse you can grow plants for which it would be too cold outside of the greenhouse. Certain gases have the same effect if they are in the atmosphere: They allow sunlight to pass to the surface, but make it more difficult for heat to escape into space. With the greenhouse effect, it can be a lot warmer at the surface, even when the amount of sunlight remains the same.

There is not much of a greenhouse effect on Mars today, because there are not enough greenhouse gases in its atmosphere, which is very thin anyway. Most of the gas in the atmosphere of Mars is carbon dioxide gas, which is a greenhouse gas, but a far greater amount of carbon dioxide is frozen at the poles, because it is so very cold at the poles (about −140 degrees Celsius or Centigrade) that even carbon dioxide freezes. The poles of the Earth are not nearly as cold as that, and carbon dioxide does not freeze at the poles of the Earth.

If all of the carbon dioxide ice at the poles of Mars were to thaw and turn into gas, then there would be a much stronger greenhouse effect, and then the temperature at the surface would rise.

This sounds like a catch-22 situation, or a chicken-and-egg problem (which came first?). To thaw the carbon dioxide ice you need higher temperatures, but to get higher temperatures you need the carbon dioxide ice to thaw.

One solution to this problem is that greenhouse gases may come from elsewhere, too, for example from volcanoes. There are some really big volcanoes on Mars, but they have not been active for the last few hundred million years. The gases that they emitted when they were active may have caused a greenhouse effect then, and if those volcanoes become active again in the future, then they can release greenhouse gases again and raise the temperature.

Another solution is to change how strong the seasons are on Mars. If there is a large difference between summer and winter, then the water ice and carbon dioxide ice can thaw at the pole where it is summer, and turn into water vapor and carbon dioxide gas, which cause a greenhouse effect. If there is no difference between summer and winter, then the Sun does not ever get high enough above the horizon at the poles to thaw the ice.

At the moment the seasons on Mars are about as strong as the seasons on Earth, but the summer is not strong enough to thaw all of the ice at the pole where it is summer. The strength of the summer on Mars changes slowly (over hundreds of thousands of years) and can get at least twice as strong as it is now. (The strength of the seasons on Earth cannot change that much, because the gravity of the Moon prevents it.) When Mars is in a time period when the summer is much stronger, then much more of the ice at the pole thaws. Perhaps then there is enough carbon dioxide in the atmosphere to cause enough of a greenhouse effect so that the temperature at the pole does not get low enough even in winter to make the carbon dioxide freeze again. Then it may even get warm enough, and the atmosphere dense enough, for water to flow on Mars once again.

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2.3.2. Volcanoes on Mars

The big volcanoes on Mars are very large and probably took thousands of millions of years to form. It is even possible that they aren't quite dead yet and may erupt again in the future.

You can estimate the age of a volcano on Mars by counting how many meteorite craters of various sizes there are on the slopes of the volcano. If there are very many meteorite craters, then the volcano must be vey old. If there are hardly any meteorite craters, then the volcano is quite young. If some parts have many craters and others have few craters, then parts of the volcano are old and parts are young, so then the volcano must have been active during a long time (but perhaps only once in a while rather than all the time).

Olympus Mons and Arsia Mons, two of the big volcanoes on Mars, have parts that have so many meteorite craters that their age is estimated at about 2500 million years, but also have parts that have so few meteorite craters that their age is estimated to be only about 100 million years. Those volcanoes are therefore very old but have been active relatively recently.

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2.3.3. The Composition of the Planet Mars

We don't know for sure what the planet Mars is made of, because we have not been able to probe the interior of Mars. However, most of Mars probably consists of the same elements as most of the Earth does, namely iron, bound oxygen, silicon, and sulfur, though Mars likely contains more iron oxide and iron sulfide, and less metallic iron than the Earth does (given that Mars has a lower average density than the Earth does).

At the surface of Mars, the Viking landers detected significant amounts of silicon, iron, magnesium, calcium, sulfur, and aluminum, and lesser amounts of others elements. Compared to the surface of the Earth, the surface of Mars contains much more iron and sulfur, and much less calcium and aluminum.

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2.3.4. Mars Closest to Earth

The Earth and Mars both orbit around the Sun. Mars is on average about half again as far away from the Sun than the Earth is and therefore goes slower in its orbit than the Earth does. Once every 26 months the Earth overtakes Mars, and then Mars is in opposition (opposite the Sun in the sky) and in its perigee (closest to the Earth).

The orbits of Mars and the Earth are not perfect circles but rather like circles that are a little squashed, and the orbits are also shifted a little so that the Sun is not quite in the center of the orbits. This so-called eccentricity of the orbits (and especially of the orbit of Mars) causes the smallest distance whenever the Earth overtakes Mars to be different every time.

If the Earth overtakes Mars in a stretch where their orbits are closer together, then the smallest distance between the planets will be smaller than average, as was the case in 2003. Such an extra-small distance happens about every eighth opposition, so about once every 17 years.

The mutual distance is at its very, very smallest if the Earth doesn't just overtake Mars in the inside curve (as happens at every perigee of Mars) but if that happens exactly when Mars is closest to the Sun (in its perihelion). Those two things seldomly coincide, and you can always do a little better. The smallest distance of Mars from the Earth in 2003 was apparently the smallest since many thousands of years, but wasn't that much smaller than the next-smallest time that you can tell the difference in practice.

The point in the orbit of Mars that is closest to the Sun is in the region of space that the Earth is closest to every year on 29 August, so Mars is the very closest to Earth if it is in opposition on 29 August, as it was in 2003.

The average smallest distance between Mars and the Earth is 78 million kilometers (and the average greatest distance is 378 million kilometers), but that smallest distance can vary, because of the eccentricity, between about 56 million km and about 101 million km: that is almost twice as much. In 2003 it was 56 million km.

When Mars gets closer than average, then it looks brighter than all stars and (in a big telescope) also larger than usual. However, the distance is not the only thing that counts. If Mars is in an unfavorable part of the zodiac (for someone far outside the tropics), then it does not get high above the horizon, it loses more of its brightness through the atmosphere, and it doesn't remain above the horizon for very long. Other oppositions may be much better in those regards, even if Mars is not so close to Earth then.

Even (marginally) smaller distances of Mars will occur more frequently in the future, because the orbit of Mars is slowly stretching out so it can get closer to the Sun and so also closer to Earth.

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2.3.5. Discoloration of Vehicle Tracks on Mars

Pictures sent back to Earth by NASA's "Spirit" and "Opportunity" rovers show tracks that were made by the rovers or by their landing craft. Those tracks have a different shade of color (often darker) than the undisturbed ground around it. One way that you can make ground look darker on Earth is to wet it: wet ground looks darker than dry ground. However, I think it is highly unlikely that the darker color of the tracks on Mars has anything to do with water, because the air on Mars is extremely dry and any water at the surface would have evaporated a long time ago. Any dampness of the top layer of the ground would have disappeared a long time ago, too.

One can imagine other reasons why the tracks can have a different shade of color:

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2.4. Jupiter

Jupiter (and the other planets) do not have much influence on the orbit of the Earth around the Sun from year to year. The VSOP model of the motion of the planets (from which the positions of the planets can be calculated quite accurately for thousands of years) indicates that the distance between the Sun and the Earth can get up to about 2400 km (about 2/5 of the diameter of the Earth) larger or smaller because of the influence of Jupiter.

That the influence of Jupiter on the orbit of the Earth around the Sun is so much smaller than the influence of Jupiter on the wobble of the Sun (see question 456) is because Jupiter affects the Earth and the Sun almost exactly the same. If both the Sun and the Earth are shifted over the same distance in the same direction, then the distance between them remains the same.

A difference of 2400 km in the distance of the Earth from the Sun corresponds to a difference of about one part in thirty thousand in the amount of sunlight per unit area that reaches the Earth. This is far smaller than the observed variation, which is more like one part in a thousand, and which is mostly tied to the sunspot cycle which is on average about 11 years long and not related to Jupiter. See //en.wikipedia.org/wiki/Solar_variation. The effect of the 11-year sunspot cycle on the weather seems to be very small (at least, I haven't seen it mentioned as an important consideration for weather or climate forecasting), so the effect of Jupiter on the weather should be completely negligible.

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2.5. Saturn

The rings of Saturn aren't solid, like wedding bands, but are made of many small pieces of ice and rock and each go around Saturn by themselves, just as if they are all joining in a giant game of musical chairs (except that there are no chairs and no music near Saturn).

If you look at the rings from a very large distance, then the small rocks in them are much too small to see and then it looks like the rings are made out of one piece, just like a meadow looks like a big green piece of cloth if you see it from far away, but the meadow is really made of very many single blades of grass.

The rings of Saturn appear very large, but they are very flat and thin, and if you could see them from very close up, then you would see that there is still a lot of room between the little rocks in the rings. If you could take a large broom and sweep all of the little rocks from the rings into a big pile, then that pile would be 1000 times smaller than the rings are now.

Astronomers haven't discovered yet where the rings of Saturn come from. They do have some ideas about it, but they don't know yet which of those ideas is correct. One of those ideas is that a very long time ago a small moon came so close to Saturn that the gravity of Saturn pulled it apart into a million little pieces that then started to go around Saturn and formed the rings of Saturn.

Another idea is that a long time ago a small moon went around Saturn and then some other small moon or big rock crashed into that moon so hard that they broke into a million little pieces that then started to go around Saturn and formed the rings of Saturn.

Something like that could happen also near the Earth or near another planet, so perhaps the Earth will someday (very many years from now) have some pretty rings like Saturn.

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2.6. Uranus

Uranus is the seventh planet, counting from the Sun. Its diameter is almost exactly four times greater than that of the Earth, and its mass is about 14.5 times greater than that of the Earth. Uranus is on average about 19 times further from the Sun than the Earth is, and takes about 84 years to complete an orbit around the Sun. Uranus looks blue and its dense atmosphere is made of gas, mostly hydrogen and helium. The gas layers of Uranus are probably thousands of kilometers thick. The outside of Uranus has a temperature of −210 degrees Centigrade (−350 degrees Fahrenheit). So far, at least 27 moons have been found orbiting Uranus, of which at least 21 have received an official name. The largest moons are Titania and Oberon, which both have a diameter between 760 and 790 km (i.e., less than a quarter of the diameter of our Moon). Uranus was discovered in 1781 by William Hershel.

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We have so far not detected any life on Uranus. However, we've only been able to see the cloud tops of Uranus, which are so very cold (−210) that it seems hard to believe that there could be any life there. If there is life on Uranus, then I'd expect it to reside far below the cloud tops where it is warmer, but not as far down as the rocky surface. If Uranus has a rocky surface at all, then it is under thousands of miles of cloud layers, and at intolerably high temperature and pressure, so I would not expect any life there, either. Also see the answer to question 151.

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2.7. Pluto

Pluto is merely a relatively large example of a large class of celestial bodies in the Kuiper Belt beyond the orbit of Neptune, that were left over from the formation of the Solar System. Because Pluto has a low mass density (2.0 times that of water) astronomers assume that it is composed of ice and rock, just like certain moons of other planets, and just like comets. If such an object is disturbed and comes to the inner regions of the Solar System, then we see it as a comet. There are probably thousands or even millions more of such objects, but most of the are likely much smaller than Pluto.

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The smallest details that we have been able to see on Pluto so far are a few hundred kilometers (or miles) in size, which is much larger than a mountain, so we do not know if there are mountains on Pluto. On the one hand there is probably a lot of ice in the rocks on Pluto, and if you make a tall mountain out of that, then it sags easily, but on the other hand the gravity on Pluto is low so tall mountains aren't pulled down strongly on Pluto. All in all, I do not know if mountains on Pluto would be high or low.

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If we send a spaceship to Pluto, then that spaceship will not have people in it. Pluto is so far away from the Sun that it is so horribly cold there that air would freeze and fall to the ground as snow, if there even was any air there. It would be very hard to keep people alive on Pluto, so there is no good reason for people to go there. See //pluto.jhuapl.edu/mission.htm for a proposed unmanned mission to Pluto.

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Not just Pluto but all objects in the Solar System are remains from the formation of the Solar System. For every object, and also for Pluto, one can wonder if it at one time was part of a larger object.

Celestial objects can grow by clumping together (the Earth captures many tons of space dust every day), and can fall apart ―or at least get smaller┄ as a result of collisions. That there have been many and large collisions in the past can still be seen today from the many large impact craters on, for example, Mercury and the Moon, and on other moons and asteroids of which we have detailed pictures.

The clumping together to form a larger object is also a form of collision, so a collision can lead to growth or to shrinkage, depending on the circumstances.

I don't think that it is likely that all matter in the Solar System beyond Neptune was once part of a few planets that later fell apart through collisions and formed the millions of relatively small objects in the Kuiper Belt, because then there would have had to be a long period before those collisions when there was no destruction but only growth, so that all matter could end up in those few large planets. What would explain that there were no destroying collisions during all that time? And what would explain that after the planets fell apart the bits and pieces wouldn't clump together again to form new planets?

The current models of the formation of the Solar System seem to imply that there is today too little mass in the Kuiper Belt to explain the formation of bodies as large as Pluto. Two possible explanations for Pluto and its siblings are (1) the Kuiper Belt objects formed closer to the Sun and later drifted away; (2) there was once much more matter in the Kuiper Belt which has since disappeared (escaped from the Solar System?). See //en.wikipedia.org/wiki/Kuiper_belt#Mass_and_size_distribution.

Maybe one day we'll know more about this. If mass clumps together into an object of about 1000 km or more, then gravity and the temperature inside the object get high enough to separate the various minerals and elements: the heavier ones fall to the center, and the lighter ones float on top. If such an object later falls apart, then you expect that the pieces that once formed the crust are made of lighter materials, and the pieces that come from the center are made of heavier materials. By determining the composition of a small object you can get clues about whether that object was once part of a larger object. That is what people do for meteorites: there are stone meteorites and iron meteorites, and of iron meteorites people think that they once were in the center of larger asteroids that fell apart because of a collision (see //en.wikipedia.org/wiki/Iron_meteorite).

If all Kuiper Belt objects have the same composition, then it is unlikely that they once formed a large object together, because the total mass of Kuiper Belt objects is estimated at 1/30th to 1/10th of the Earth's (see //en.wikipedia.org/wiki/Kuiper_belt) and an object with that much mass seems big enough to have separation of elements in its interior. For comparison: the Moon has separation of elements in its interior, and has a mass of only 1/81st that of the Earth.

We don't have very much information yet about the objects in the Kuiper Belt, because they are very small and ver far away. The New Horizons mission to Pluto may shed some light on this. See //en.wikipedia.org/wiki/New_Horizons.



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