Why do planets have seasons?

The chance that a planet will rotate in exactly the same plane as its orbit is pretty remote.  So is the chance that its orbit will be exactly circular. (I have a planet in the sequel to Homecoming that is seasonless because its equator is in its orbital plane, but that’s fiction.)

The direction of the axis of rotation, the planet’s pole, plays a large role in its weather, and especially in the planet’s seasons.  Seasons on Earth are controlled primarily by the tilt of the earth’s equator relative to the plane of its orbit.  This tilt for the earth is 23 degrees and 27 minutes.

An increase in the tilt would enhance the contrast between the seasons, but reduce the difference in the total annual solar heating between the equator and the poles.  (If the tilt is greater than about 60 degrees, the poles actually get more annual average incoming energy than the equator, as is the case on Uranus.)

In contrast, if the equator were closer to being in the same plane as the planet’s orbit, there would be less difference in the seasons and more in the difference of the average annual energy budget between the equator and the poles.

In general the oceans, which hold heat and release it slowly, will be most influenced by the average annual energy budget, while the land areas are more influenced by the seasonal energy input.  Thus the progression of the seasons will be strongly influenced both by the distribution of land and water on the planet and by the inclination of its axis.

Water, by the way, is not a rare substance in the universe.  It is the simplest possible chemical compound of the most abundant element in the universe, hydrogen, and the third most abundant, oxygen.  (The second most abundant, helium, does not usually form chemical compounds and can be ignored for this purpose.)

Water is going to be lacking on a real planet only if it has been removed at some point in the planet’s history.  In our system, the near lack of water on Mercury and its scarcity on Venus is probably due to the fact that water, being relatively volatile, was blown out of the inner Solar system by the tantrums of the early sun.  Mars also seems short of free water, but that is likely due to its small size and correspondingly weak gravitational field and ice is still a possibility.

Getting water in liquid state is largely a function of planetary temperature and atmospheric pressure. Making the planet the right size (gravity) and putting the planet at the right distance from its sun are probably the critical factors here.

Solstices are defined as the dates when the planet’s axis points most nearly directly at the planet’s sun. In one hemisphere the sun will be at its highest in the sky and the days will be at their longest. In the opposite hemisphere the sun will be at its lowest and the days will be shortest. Close to the poles, the sun may appear to be above or below the horizon for 24 hours at a time. Summer and winter are defined as starting at the solstices.

Equinoxes are defined as the dates when the planet’s axis is at right angles to the line between the planet and its sun. At this time the difference in the radiation received at the pole and at the equator is at its maximum, but there is no difference between the northern and southern hemispheres. Spring and fall are defined as starting at the equinoxes.

There is another way of getting seasons on a planet, minor but perceptible on Earth, but possibly major on a planet whose equator is close to the plane of its orbit.  This is the ellipticity, or elongatedness, of its orbit.  The ellipticity of the Earth’s orbit is only about 0.016 at the present, though it has been as high as 0.06 in the past.  But ellipticity influences both the energy received from a planet’s sun, with maximum energy coming at perihelion (planet closest to sun) and the length of the seasons, since the planet moves fastest in its orbit when it is closest to its sun.  Seasons controlled by the orbital eccentricity differ from those due to axial tilt in several ways.

Firstly, pure tilt-controlled seasons are opposite in the two hemispheres, while pure perihelion-controlled seasons are the same in the northern and southern hemispheres.

Secondly, pure tilt-controlled seasons are equal in length–the summer half-year is the same length as the winter half-year, counting the equinoxes (when the sun is 90 degrees from its midsummer position relative to the stars) as the dividing points.  Perihelion-controlled seasons have a long winter and a short summer.

Thirdly, the length of time the sun is above the horizon each day varies markedly over a tilt-controlled seasonal cycle, as does the direction of sunrise and sunset at medium to high latitudes.  Tilt-controlled seasons at the equator have a double cycle, with the most incoming energy at the equinoxes.  In pure perihelion-controlled seasons, there is no change in the apparent path of the sun in the sky, and the equatorial seasons follow those nearer the poles.

Fourthly, in pure tilt-controlled seasons, there is no change in the apparent size of the sun in the sky.  In perihelion-controlled seasons, the sun appears larger in summer and smaller in winter.

Most real planets have both kinds of seasonality at once, with one dominating.  On Earth, tilt dominates, but summer is slightly longer than winter because Earth is at perihelion around January 4.  (This is why the number of days between the spring and fall equinoxes is slightly more than the number between the fall and spring equinoxes—or more simply, why February is the shortest month, while July and August are the only two adjacent months with 31 days.)

Since tilt, perihelion date and eccentricity are the variables that can change in a multi-body system (and if fact do change slightly over a few thousand to hundreds of thousands of years on Earth, just due to the other planets in our system) the seasons in a multiple star system are likely to be rather variable and not really conducive to earthly life.  Even on Earth, the slight variations in orbital elements produced by the other planets are thought to be enough to account for the alternation between ice ages and interglacials.