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November 25

Planet axis configuration and seasons

I can't remember if I have asked this before or not, but I was curious anyway.

We all know the change of seasons on the Earth happens because the axis is tilted in relation to the orbit. So I thought of three alternate configurations for a planet:

Axis is exactly perpendicular to the orbit, pointing "up" and "down" on the orbit plane: No annual change in seasons. Temperature depends only on latitude and time of day.
Axis is parallel to the orbit: No seasons. Temperature depends only on time of day.
Axis points towards and away from the sun: No seasons. One side experiences permanent sunlight and the other experiences permanent darkness.

I know there might be local weather changes that might affect the temperature and sunlight but I'm ignoring these here for the sake of simplicity.

Have I understood these scenarios correctly, and is it possible they might actually occur (not on our Earth but on some other planet)? JIP | Talk 10:54, 25 November 2019 (UTC)[reply]

2 and 3 do not work like that due to conservation of angular momentum: The axis of rotation is fixed in space and therefore changes its direction with respect to the sun as the planet orbits around it. In the solar system, Uranus has its axis closely within the orbital plane and it does experience seasons much more extreme than ours. Scenario 1 works; if you want to remove diurnal variation, too, you need tidal locking. --Wrongfilter (talk) 11:29, 25 November 2019 (UTC)[reply]

Some seasonal effects also result from orbits being not exactly circular, but rather elliptical as is nearly always the case. The degree of effect is proportional to the degree of ellipticality, or eccentricity, of the orbit in question.

Earth's orbit is only modestly eccentric (with aphelion (furthest distance from the Sun) being in July and perihelion in January), so the resultant effects are mostly swamped by the greater ones of its axial inclination (they tend to ameliorate the Northern and intensify the Southern hemisphere's seasons, but only slightly). They are however readily observable on rocky planets like Mars and Pluto which have greater orbital eccentricities. Similar effects are detectable, though less obvious, on the gas and ice giant planets of our Solar system.

For an entertaining (and extremely well written) fictional examination of the effects on an Earth-like planet with a very eccentric orbit, see the Helliconia trilogy by my old acquaintence Brian Aldiss. {The poster formerly known as 87.81.230.195} 2.217.209.178 (talk) 13:38, 25 November 2019 (UTC)[reply]

So, "seasons" are a higher-order climate phenomenon and it's really hard to extrapolate from our Earthly experience... they obviously involve the planet's orbit and its axial tilt, but they also involve its atmospheric dynamics and its average temperature and so on.

The first-order item to consider, if we're speaking of a generic planet in a generic orbit - is the insolation - incoming solar radiation - and the equation of time that describes how insolation changes over a long period of time - let's call that a "year" cycle or a single orbit. If the planet is earth-like, its day-night cycle occurs hundreds of times per orbital-revolution; its axial tilt precesses very slowly (thousands ir millions of years go by with no meaningful precession); the orbit is nearly perfectly circular; and so on; the dominant "seasonal" change is caused by axial-tilt only.

If the planet is Venus-like - meaning that its day-night cycle is more like its orbital period - everything "seasonal" that we infer about its axial tilt gets dwarfed by other factors. If the planet is Jupiter-like, its nine-hour atmospheric rotation is so hard to describe that the variability in gas circulation probably affects local long-term temperatures and weather more than the orbit. Uranus, as we now know it, rotates prograde and nearly perpendicular to the rest of our solar system; and its temperature profile against latitude indicates that solar radiation does not directly dominate the seasons: the hottest parts of the planet get the least sunlight. Gas convection is really complicated when the gas blob is the size of an entire world and self-gravitates! And if the planet is as eccentric as Pluto, or Halley's Comet, the variable insolation due to orbital radius change - direct distance from the Sun - has a bigger impact than axial tilt. On certain moons of the big gas-giants, like the moon Io, the most severe long-term climate cycles are caused by tidal heating, and not by the variable Solar radiation itself.

The summary, then, is that every world is very unique; it happens that on Earth our dominant seasonal variation is caused by our orbit's weirdest peculiarity; but other worlds have their own weird peculiarities. The task of the planetary scientist is to rigorously and open-mindedly study which parameters, out of all the zillions of parameters into all the equations of physics, predominantly define the nature of any particular celestial body.

Nimur (talk) 15:52, 25 November 2019 (UTC)[reply]

The other thing with the potential on the seasons for the outer planets is that the seasons are caused by the variance in solar radiation due to axial tilt, and that variance is a function of the amount of solar radiation to begin with. The less overall solar radiation a planet receives, the less axial tilt "matters" to variances in solar radiation. If the earth were located out by Jupiter's orbit, with the same axial tilt, it would get less variation in its seasons. --Jayron 32 19:22, 25 November 2019 (UTC)[reply]

I once read somewhere or other that on a planet with tilt more than 54° the poles get more total insolation than the equator. I don't quite know how to confirm that! If that's the case, we can imagine a planet at exactly 54° where the average temperature is the same at every latitude, but the severity of seasons increases with latitude. —Tamfang (talk) 02:08, 1 December 2019 (UTC)[reply]