Online Astronomy eText: The Sky / The Planets
Seasons on the Other Planets
(work in progress; also see The Seasons and Climatic Zones)

      This page will discuss how the seasons on the other planets compare to the seasons on the Earth. Planets, such as Mercury, Venus and Jupiter, which have a tilt (inclination of the axis of rotation to the orbital motion) near zero or 180 degrees, so that their axis of rotation is perpendicular to their orbit, have no seasons. Planets, such as Mars, Saturn and Neptune, which have a tilt similar to that of the Earth, have seasons similar to the Earth (although all three of those planets, being further from the Sun, have longer seasons, and colder ones, than the Earth). Planets, such as Uranus and Pluto, which have a tilt close to 90 degrees, so that they are rotating on their "side", and can have one pole or the other face the Sun for long periods of time, have extreme seasons, in which even the planet's Equator experiences considerable variations in temperature (although both of these are so far from the Sun that it is cold everywhere, all the time). (11/28/06: more detailed summary added below illustration of tilts)

Review of Seasons on the Earth
      (BRIEF review of how our tilt causes our seasons, and if we were not tilted, we would have no seasons)

The Tilts of the Other Planets
     Note about measuring tilts: In the diagram below, three planets (Venus, Uranus and Pluto) are shown with two values for their rotational tilt -- a positive one, which is larger than 90 degrees, and a negative one, which is the supplement of the postive value (180 degrees minus the positive value). Traditionally, the positive value was the tilt -- the angle between the rotational pole of the planet's orbit, and the rotational pole of the planet, defining each according to the "right-hand" rule: if you turn the fingers of your right hand in the direction of the rotational motion, your thumb, held at right angles to that direction, points to the North Pole of the rotation. However, in 1982 the International Astronomical Union decided to define the North Pole of each planet as the pole that was on the north side of the Earth's orbit, which reversed the naming of the poles for the three planets with tilts greater than 90 degrees. This is the reason for the negative value, which is the angle between the orbital pole and the North Pole, using the new definition of the North Pole. When using this definition, the planet's rotation is considered to be "backwards" (stars circle around the North Celestial Pole of the planet in a clockwise, instead of counter-clockwise motion), rather than "upside-down". (For calculation purposes (see Rotation Period and Day Length), the direction of rotation must be treated as a negative number when the tilt is greater than 90 degrees, whichever definition you use for the "north" pole of the planet.)

Diagram of Planetary Tilts

Summary of relationship of tilts to seasons (detailed discussion below):
(1) If the planet has no tilt (Mercury, Venus, Jupiter), it has no seasons
(2) If the planet has a tilt similar to ours (Mars, Saturn, Neptune), it has seasons similar to ours
(3) If the planet has a tilt close to 90 degrees (Uranus, Pluto), it has extreme seasons

The Seasons on the Other Planets
     (summary, in lieu of finished discussion)
     (1) NO TILT: If a planet has no tilt, it has no seasons. On every part of the planet, the Sun is up for half a day, then down for half a day, and day and night are equal length. At the Poles, the Sun circles around the horizon forever. At the Equator, it rises vertically at the East point on the horizon, passes directly overhead, then sets vertically at the West point on the horizon. At mid-latitudes, it rises at the East point on the horizon, but at an angle to the vertical which is equal to your latitude; passes North or South of overhead by an angle equal to your South or North latitude; then sets at the West point on the horizon, at the same angle that it rose at the East point.
     Since the Sun follows the same path every day, any given place on the planet receives the same amount of solar heating every day.. This does not mean, however, that there are no temperature variations on the planet. There are still day/night variations, climatic zone variations, and daily or weekly variations in the weather due to, e.g., changes in cloud cover (presuming there are clouds, of course). For example, on Mercury, it can be as much as 800 degrees above zero Fahrenheit at the Equator in the early afternoon (about 7 Earth weeks after sunrise), but close to zero Fahrenheit near the Pole. If you were standing in the sunlight near the Pole, you'd get very hot -- almost as hot as at the Equator -- but in the shade, where only the light reflected by surrounding rocks heated you, it would be relatively cold (in fact, some craters have bottoms in permanent shade, and temperatures there are close to absolute zero).
     (2) TILT LIKE OURS: Seasons would be similar to ours, other than differences due to their distance from the Sun. Thus Mars, with a tilt similar to ours, has the same seasonal changes as the Earth, but since Mars is further from the Sun, all its seasons are colder than ours, and since its larger orbit entails a longer orbital period, all its seasons are longer than ours.
     (3) EXTREME TILT: On the Earth, there are extreme seasons at the Poles, where the Sun is up for half a year, then down for half a year, but it never gets very warm (i.e., it is always arctic), because the Sun is always low in the sky. At the Equator, there are hardly any seasons, and it is always warm (i.e., it is always tropical), because the Sun is up exactly half a day every day of the year, and passes fairly close to overhead every day of the year, so that it shines down on the ground about the same every day of the year, as well.
     For planets with an extreme tilt, however, as the Sun moves further and further North, it gets higher and higher, and on Uranus, can be nearly overhead for years on end (since the half "year" that the Sun is up is 42 Earth years), making it substantially warmer than the Equator ever gets (where the Sun is only nearly overhead for an hour or two). Meanwhile, at the Equator, the Sun, being close to the celestial North Pole, which is at the North point on the horizon (at the Equator on ANY planet), is very low in the sky, providing less than 1/10th the heat per hour that it is doing at the Pole at the same time (particularly during the half-day when it is down!).
     SUMMARY: So on planets with no tilt, there are no seasonal variations -- just day/night and climatic zone variations. On planets with tilts similar to ours, the seasons may be warmer and shorter if the planet were closer to the Sun (but none are), or colder and longer if the planet were further from the Sun (as all are), but seasons are similar to ours -- namely, no seasons worth speaking of at the Equator, extreme seasons but always arctic at the Poles, and middling seasons, such as we are familiar with, at mid-latitudes. And on planets with extreme tilts, the Poles can become warmer than the Equator, and the Equator can become colder than the Poles. Of course, the fact that the only planets with such tilts never get warmer than 300 degrees below zero Fahrenheit sort of spoils the concept of "warmer" and "colder", but the variations are real, nonetheless.

(reminders for the author...)
(review of all motions of stars and Sun, to this point? in any event, introduction to the next topic, which is how the various factors discussed so far, and others not discussed, such as day and night, and climatic zones, affect planetary temperatures, ? with specific examples ?)
(think about whether night and day and climatic zones should come before seasons, or afterwards... good arguments for both ways of doing it)