This page answers questions about the seasons. The questions are:
Seasons are period that return every year and that can be recognized by how the weather and plants and animals behave, or (more generally) how the amount of sunlight per square meter of ground changes. In some areas (especially outside of the tropics) there are warm seasons and cold seasons, and in other areas there are dry seasons and wet seasons.
There are two different possible causes for changes in the amount of
sunlight per square meter ground (without taking clouds into account):
ε of the rotation axis of the
planet, and the eccentricity
e of the orbit of the
planet around the Sun, which indicates the relative variation in the
distance to the Sun. The first one yields changes in the distribution
of time over days and nights, and the effects are in the opposite
direction in both hemispheres, so when the north has long days, then
the south has long nights, and the other way around. The second one
yields changes in the temperature but no changes in days and nights,
and works in the same direction in both hemispheres (so hotter
everywhere or colder everywhere).
These effects can occur also on the other planets, so those can have seasons, too. Only a planet that has its rotation axis perpendicular to its orbit around the Sun and that is always at the same distance from the Sun would have no seasons at all: On such a planet the Sun would be above the horizon equally long every day of the year, and the Sun would trace the exact same path through the sky every day.
We can compare the relative influence of these two effects (due to
ε and to
e) by the ratio of the
annual least and greatest amount of sunlight per square meter of
planetary surface per day, averaged over the surface of the whole
The influence of
ε is approximated by
r(ε) = −0.3854 ln(ε + 7.495) + 1.774
ε is measured in degrees. The error of this
approximation is at most 0.008.
r(ε) is the
planetary averaged ratio of the amount of sunlight per square meter
per day in winter and the same amount in summer.
The influence of
e is equal to
re = ((1 + e)/(1 − e))2
De values of
ε (in degrees),
re, and the product of
the latter two are listed in the following table for all planets and
for the Moon.
For example: the rotation axis of the Earth is tilted over 23°. It
r(ε) is 0.45, which means that on
a midwinter's day slightly less than half as much sunlight falls on a
square meter of earth's surface than on a midsummer's day (ignoring
the effects of the atmosphere). The orbit of the Earth has an
eccentricity of 0.017. It follows that
re is 0.94,
which means that at aphelion about 94% as much sunlight hits the
ground as at perihelion.
The last column shows approximately how noticeable the seasons are: the lower the number, the more pronounced are the seasons. Based on these calculations, the seasons are by far the most pronounced on Uranus and Pluto, and the seasons on Mars, Saturn, and Neptune are also more pronounced than those on Earth. The least pronounced seasons can be found on the Moon, Venus, and Jupiter.
The seasons on Mercury are strongly determined by the
e-effect, and the seasons on all other planets except
Venus, Jupiter, and the Moon are strongly determined by the
ε-effect. On Mercury, both hemispheres
therefore have the same season, on Venus, Jupiter, and the Moon not
much at all can be noticed of seasons, and on the other planets both
hemispheres have opposite seasons.
If you want to calculate the lengths of days and nights yourself, then
you can use the formulas of
the Solar Position Page,
and especially equation 37 on that page. The length of the day is
2*H/360° planet days (sols), and the length of
the night is equal to the rest of the planet day. If you want to
calculate the dates and times of solstices and equinoxes on the other
planets, then look at
the Seasons Calculation Page.
On Earth, the year is divided into four seasons that are tied to the motion of the Sun between the stars. According to the astronomical definitions:
The beginning of the (astronomical) summer is an solstice. In summer, daytime lasts longer than nighttime but gets shorter every day.
The beginning of the (astronomical) autumn is an equinox. In autumn, daytime is shorter than nighttime and gets even shorter every day.
The beginning of the (astronomical) winter is a solstice. In winter, the daytime is shorter than the nighttime but is getting longer every day.
The beginning of (astronomical) spring is an equinox. In spring, daytime is longer than nighttime and getting even longer every day.
The seasons in the one hemisphere of Earth are shifted by half a year or two seasons compared to the other hemisphere. When it is summer in the northern hemisphere (e.g., in Europe), then it is winter in the southern hemisphere (e.g., in New Zealand), and when it is spring in the south (e.g., in Argentina), then it is autumn in the north (e.g., in Siberia). So, the beginning of a certain season in a certain year does not always fall at about the same time everywhere on Earth.
The position of the Sun between the stars is practically the same, as seen from any place on Earth. The geocentric ecliptic longitude λ, geocentric right ascension α, geocentric declination δ of the Sun (all measured relative to the equinox of the date) and the average date (in the Gregorian calendar) at the beginning of each season in each hemisphere are
|0 hours||0°||0°||spring||autumn||20 March|
|6 hours||most N||90°||summer||winter||21 June|
|12 hours||0°||180°||autumn||spring||23 September|
|18 hours||most S||270°||winter||summer||21 December|
The listed dates in the Gregorian calendar are averages around the year 2000, measured in Universal Time (UTC): The actual dates can be a day earlier or later. The exact time of the beginning of the seasons in the Gregorian calendar varies from year to year because
Between the years 1900 and 2100, the earliest and latest begin times of the seasons are as follows (in TDT, which is roughly equal to UTC):
|March||19 14:06||21 19:14||20 16:34|
|June||20 06:34||22 15:04||21 10:45|
|September||21 22:58||24 05:42||23 02:19|
|December||20 20:50||23 00:19||21 22:33|
The first number is the date, and after that the time in hours and minutes is given. For example, the season that starts in December does so on 20 December at 20:50 hours at the earliest, on 23 December at 00:19 hours at the latest, and on 21 December at 22:33 hours on average.
A simple method (and therefore of limited accuracy) to predict the beginning of the seasons is to add the following time periods for each calendar year:
|365 days||5 hours||49 minutes||2 seconds|
|365 days||5 hours||47 minutes||57 seconds|
|365 days||5 hours||48 minutes||31 seconds|
|365 days||5 hours||49 minutes||33 seconds|
For the year 2005 the average beginnings are at
|spring||autumn||20 March||11:33:19 UTC|
|summer||winter||21 June||06:39:11 UTC|
|autumn||spring||22 September||22:16:34 UTC|
|winter||summer||21 December||18:34:51 UTC|
With these timestamps for 2005 and these season lengths you get the on average most accurate timestamps for other years, for the period between the years 1900 and 2100.
So, the northern spring of 2006 begins (approximately) 365 days, 5 hours, 49 minutes, and 2 seconds later than 20 March 2005 12:33:19 UTC, and the northern spring of 2007 again 365 days, 5 hours, 49 minutes, and 2 seconds later.
The listed quantities are the best ones between the years 1900 and 2100, but the true timestamps (between 1900 and 2100) deviate by up to 19 minutes (standard deviation 6 minutes) from the results of the described calculations. It is therefore not useful to present the results of the calculations a precision greater than 1 minute (but you should keep the seconds in intermediate results, to prevent round-off errors). To get more accurate results, the calculations get far more difficult. For this, I can recommend [Meeus].
The longest day is the day that the summer begins. Around that time the length of the daytime period (when the Sun is above the horizon) varies only very slowly. The difference between the longest day and the next longest day is at most 6 seconds in the Netherlands and Belgium. For over a week around the beginning of summer, the length of the day is within one minute of the length of the longest day (assuming equal conditions of the atmosphere). There is a similarly small difference between the length of the shortest day and the length of the next shortest day.
One can also define the seasons in different ways than having them start at an equinox or solstice. Some alternatives are:
The next table shows how long the (astronomical) seasons are on
average around the year 2000, and how much those lengths change on
average each year. The
λ is the geocentric ecliptic
longitude of the Sun at the beginning of the season.
A solstice or equinox marks the beginning of a season. There are two equinoxes per solar year, so we need to have a way of indicating which one we mean, and the same for the two solstices each year. We can mention the season of which the solstice or equinox marks the beginning (for example, the summer solstice or the autumn equinox), but those are different on each hemisphere of Earth. We can also mention the month in which the solstice or equinox falls (for example, the March equinox or the December solstice), but those are not the same in all calendars (not all calendars in the world have months of March and December), and they need not be the same every year for a given calendar (if that calendar does not follow the seasons closely enough). The name "vernal equinox" is used for the equinox that marks the beginning of spring in the northern hemisphere, and the name "autumnal equinox" is sometimes used for the equinox that is not the vernal equinox, but in the southern hemisphere that autumnal equinox actually marks the beginning of spring! That's why it is useful if there are also names for equinoxes and solstices that do not depend on the season or on the calendar. The independent names that I propose are listed in the following table, together with the other names.
|0°||spring equinox||autumn equinox||March equinox||ascending equinox|
|90°||summer solstice||winter solstice||June solstice||northern solstice|
|180°||autumn equinox||spring equinox||September equinox||descending equinox|
|270°||winter solstice||summer solstice||December solstice||southern solstice|
For example: the equinox that happens when the Sun has a geocentric ecliptic longitude of 0° can be called the spring equinox in the northern hemisphere and the autumn equinox in the southern hemisphere, the March equinox in the Gregorian calendar, and the ascending equinox independently from hemisphere or calendar. The "ascending" part of the ascending equinox is the same as the "ascending" part of the ascending node: The ascending node of an orbit is the place in the orbit where the planet passes from south to north of the ecliptic, and in the same way the ascending equinox is the equinox through which the Sun passes from south to north of the celestial equator.
It is quite difficult to detect the moment of an equinox. About 2000 years ago people used instruments such as an equatorial ring (http://en.wikipedia.org/wiki/Equatorial_ring) for that. A disadvantage of such an instrument is that it must be aligned very accurately, otherwise it yields false results. And to do the alignment properly you need to know accurately what your latitude is and where the north point is. Without something like an equatorial ring, you can determine the time of the equinox no better than to about the nearest day.
The seasons on one side of the equator (for example in Egypt) are the opposite from what they are on the other side of the equator (for example in Australia). When it is summer in Egypt then it is winter in Australia.
You can see for yourself how this works if you have a world globe and a flashlight. A typical world globe (for an example, see http://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/GEO_Globe.jpg/220px-GEO_Globe.jpg) does not have the north and south poles at exactly the top and bottom, but a bit off to the side. This represents that the rotation axis of the Earth does not make a right angle with the orbit of the Earth around the Sun.
Put the globe on a small but stable table in the middle of the room. The rotation axis of the Earth always points in the same direction relative to the stars (actually, it moves but so slowly that we can ignore it here), so you must not change the orientation of the globe once you have put it on the table. For example, you can have the north pole of the globe point toward the door of the room. You can turn the globe around its axis (representing the rotation of the Earth around its axis) but you must not rotate the foot of the globe (you must keep the north pole pointing toward the door).
Now stand a few meters away from the globe in the direction where the north pole of the globe points and then shine a flashlight towards the globe. Ideally, the flashlight should be at the same height from the floor as the globe. This represents the Sun illuminating the Earth. If someone else turns the globe around its axis (careful not to move or rotate the foot of the globe) then that represents the Earth rotating around its axis. The parts of the globe that receive light from the flashlight experience daytime, and the rest experiences nighttime.
Because the flashlight is in the direction where the north pole of the globe points, the north pole of the globe is always in the light of the flashlight, even when the globe rotates all the way around its axis. This represents midsummer's day in the northern part of the world. A small area near the north pole also remains in the light 24 hours a day, because the north pole is tilted toward the flashlight (Sun). The boundary of that area is the arctic circle at 67 degrees latitude.
On the other side, the south pole and an area around it (above 67 degrees south latitude) remain in darkness 24 hours a day, because the south pole is tilted away from the Sun then. It is midwinter's day over there.
If it is daytime on the north pole, then it is nighttime on the south pole, because they are on opposite sides of the planet. Northern areas then get more sunlight than usual, and southern areas then get less sunlight than usual. This explains why the seasons are opposite on opposite sides of the equator.
If you take the flashlight and walk around the table that has the globe on it, keeping the light pointing toward the globe, then that represents the Sun moving around the Earth relative to the stars (actually, the Earth moves around the Sun, but for this experiment it is easier to move the Sun around the Earth, and the results are the same).
If you have walked a quarter of the way around the table, then that represents an equinox (the beginning of spring or autumn). From the location of the flashlight, you can now see both poles of the Earth. All places on Earth then have 12 hours of daylight and 12 hours of nighttime. If you walk another quarter of the way around the table, then that represents a solstice (the beginning of winter or summer). Now the south pole of the Earth is in the light 24 hours a day, and the north pole is always in darkness. If you complete your walk around the table then that represents a full year.
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Last updated: 2013-04-21