Astronomy Answers
AstronomyAnswerBook: Seasons

This page answers questions about the seasons. The questions are:

• [525] Does Neptune have an autumnal equinox?
• [466] How do I calculate when the seasons begin?
• [296] How much earlier or later than on the traditional dates can the seasons begin?
• [290] Does the length of the day and of the night vary on the other planets (for example, Mars) like it does on Earth? How do I calculate those?
• [225] When exactly do the seasons begin?

[290]

1. Seasons

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.

[525]

There are two different possible causes for changes in the amount of sunlight per square meter ground (without taking clouds into account): the skewness 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 planet.

The influence of ε is approximated by

(Eq. 1) r(ε) = −0.3854 ln(ε + 7.495) + 1.774

if 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

(Eq. 2) re = ((1 + e)/(1 − e))²

De values of ε (in degrees), e, r(ε), 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 follows that 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 equal to 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.

[225]

2. The Beginning of the Seasons

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:

summer begins when

• it is the longest day of the year (with the Sun longest above the horizon).
• the Sun is highest in the sky for the year at noon.
• at sunrise, the Sun is furthest from the east point on the horizon, and at sunset it is furthest from the west point, in the direction of the nearest geographical pole.

The beginning of the (astronomical) summer is an solstice. In summer, daytime lasts longer than nighttime but gets shorter every day.

autumn begins when

• the night is equally long as the day for the first time since the beginning of spring.
• the Sun is due east at sunrise and due west at sunset for the first time since the beginning of spring.

The beginning of the (astronomical) autumn is an equinox. In autumn, daytime is shorter than nighttime and gets even shorter every day.

winter begins when

• it is the shortest day of the year (with the Sun shortest above the horizon).
• the Sun is lowest in the sky for the year at noon.
• at sunrise, the Sun is furthest from the east point on the horizon, and at sunset it is furthest from the west point, in the direction of the furthest geographical pole.

The beginning of the (astronomical) winter is a solstice. In winter, the daytime is shorter than the nighttime but is getting longer every day.

spring begins when

• the night is equally long as the day for the first time since the beginning of autumn.
• the Sun is due east at sunrise and due west at sunset for the first time since the beginning of autumn.

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.

[296]

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

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

1. a year of the seasons (a tropical year) is about a quarter of a day longer than 365 days, so the time of the beginning of a season gets about a quarter of a day later each year, until a leap day (almost every four years) moves it one day earlier again. Because of this, the time of the beginning can be up to about 5/4 of a day early or late compared to the average.
2. the gravity of the other planets and the Moon can slightly slow down or speed up the Earth in its orbit, so that the seasons begin slightly earlier or slightly later. This usually changes the times by at most about 9 minutes.
3. the beginning times of some seasons are slowly shifting. For example, the dates in June and September are getting earlier by about one day per 1000 years, and the date in December is getting later by about one day per 4000 years.

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):

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.

[466]

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:

= 

= 

= 

= 

For the year 2005 the average beginnings are at

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.

3. The Length of the Seasons

The next table shows how long the 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.

4. The Names of Solstices and Equinoxes

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.

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.

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Last updated: 2007-12-02