Starship Asterisk*

I know that Mars had four seasons just as Earth does, but when are they? So it is late spring now in Earth's northern hemisphere, and autumn i its southern, but do these n anyway correspond to the Martian seasons or are they on an unrelated, independent cycle? What season is it in Mars' northern and southern hemisphere now?

I am not the right person to answer that question. However, like on the Earth, one hemisphere on Mars must have summer while the other one has winter. That is because the tilt of the Martian axis is almost exactly the same as the tilt of the Earth's axis, about 24-25 degrees. So when one hemisphere is "leaning in" towards the Sun and has summer, the the other hemisphere is "leaning away" from the Sun and has winter.

However, Mars has a more elliptical orbit than the Earth. So if one hemisphere leans away from the Sun while Mars is at its furthest point from the Sun, then that hemisphere is going to have a pretty severe winter. Of course winters on Mars are always severe!

But I seem to remember that the ice cap at the south pole of Mars is bigger than the ice cap at the North pole. This suggests that, like on the Earth, Mars is at its maximum distance from the Sun at the same time as its southern hemisphere is leaning away from the Sun.

But if you ask me which hemisphere has summer or winter right now, I have no idea. Bear in mind that Mars could possibly be at the stage of equinox right now, so that neither hemisphere has either summer or winter.

Ann

A year on Mars (the time it takes to orbit the Sun) is about 687 Earth days. But a Solar day (sol) is about 40 min longer than an Earth day, so a year on Mars is about 668.6 sols. Mars has four seasons, just like Earth, but they obviously do not coincide with Earth's. The Northern hemisphere of Mars is just entering Spring and Opportunity is again on the move after the Martian winter.

Wikipedia: Timekeeping on Mars

http://asterisk.apod.com/viewtopic.php? ... 51#p175251

I thank you both. So, bystander, spring is just beginning on Mars' northern hemisphere, while here spring is quite advanced. All right, and if I understand your information correctly that is the merest coincidence. So if a Martian year is 687 Earth days long, are the seasons about twice as long as well? I suppose Next May 28 (2013) it may be a different season on Mars, not so nearly aligned with Earth's?? I'll try to work it out for myself, but it looks complicated. So complicated that I almost hesitate to ask about seasons on other planets. I understood what Ann said about Mars' similar tilt and how that causes seasons, is it impossible then for untilted planets to even have seasons? I don't know that any planet has no tilt at all. In any case, what else is known about seasons on the other planets of the Solar System?

Epictetus wrote:
I understood what Ann said about Mars' similar tilt and how that causes seasons, is it impossible then for untilted planets to even have seasons? I don't know that any planet has no tilt at all. In any case, what else is known about seasons on the other planets of the Solar System?

Again, I'm not the expert here. As far as I can understand, however, if a planet's axis of rotation is at right angles to its orbital plane, the planet should not experience seasons.

As for the other planets and their seasons, I'm sorry to say that I'd advise you to google. However, I can tell you that one planet you ought to check out is Uranus.

http://en.wikipedia.org/wiki/Uranus wrote:
The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. Its north and south poles therefore lie where most other planets have their equators.

So Uranus is orbiting the Sun pole first, at least some of the time!

I can't help thinking that the man who seems ready to slide downhill on his back, feet first, is doing a fair imitation of Uranus!

By the way, Uranus does have seasons:

http://en.wikipedia.org/wiki/Uranus wrote:
Terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox.

Ann

Ann wrote:Again, I'm not the expert here. As far as I can understand, however, if a planet's axis of rotation is at right angles to its orbital plane, the planet should not experience seasons.

It would not experience seasons caused by a change in the position of the Sun in the sky over its year. But if it were in an orbit with any significant eccentricity, it would experience seasons because of its changing distance to the Sun. Even the Earth, with a nearly circular orbit, sees its seasons affected slightly by this effect. Mars, with a much greater orbital eccentricity, has its axial-tilt seasons significantly modulated by its large difference in aphelion and perihelion distances.

Okay Chris, so at what point in Earth's year (what month?) is it at aphelion? Does the month shift over the years? And how much does it actually affect the weather/climate?
Thanks, Ann, I will try to learn more about Uranus. I had thought of its rolling ball orbit, but I was hoping one of you would extrapolate on that topic.

Well, Epictetus, it's a pity that I'm the one trying to answer your question, since I am the great galaxy/blue stars nerd on these boards. I'm far less of a solar system nerd, but I'll do my best.

I am almost certain that the Earth reaches perihelion in January and aphelion in July. That explains why the South Pole is colder than the North Pole here on the Earth. The seasonal effect caused by the Earth's tilt counteracts and alleviates the effect of aphelion and perihelion in the Northern Hemisphere, where most people live. In the Southern Hemisphere these two effects work in tandem, making winters colder and summers hotter.

As for Mars, it seems obvious to me that seasons have to be almost twice as long as the seasons on the Earth. That's because the Martian year is almost twice as long as the Earth's year, and there is no way that, say, the northern hemisphere of Mars could have summer twice during one Martian year. But as Chris points out, the orbit of Mars is considerably more elliptical than the orbit of the Earth. For a planet with a very elliptical orbit, the effects of being at perihelion or aphelion will have a much greater effect on the planet's temperature than the tilt of its axis, unless that tilt is extreme. For Mars, I believe that the tilt of the Martian axis has a greater effect than the ellipticity of its orbit, but the elongated shape of the orbit of Mars will affect the Martian seasons more than the much less pronounced ellipticity of the orbit of the Earth affects the seasons here.

As for minor planet Pluto, the operative word is C-O-L-D. Even so, Pluto must experience considerable seasons, due to the strong ellipticity of its orbit. Some years ago Pluto was at perihelion, and then it was closer to us than Neptune. What happened when Pluto was at perihelion was that its atmosphere became more substantial, due to the fact that certain volatiles that are usually frozen on Pluto sublimated and turned into an atmosphere.

A strange world of the solar system is Venus. The Venusian orbit is extremely circular, and the Venusian day is almost as long as its year. I don't know anything about its axial tilt (but you can google it), but I believe that the very thick atmosphere of Venus probably spreads the killer heat of our sister planet pretty evenly all over its surface. I doubt that Venus experiences any noticeable seasons at all.

Ann

Thanks again, Ann. I have looked up Venus' axial tilt, and here's what I got:
http://www.universetoday.com/36123/axis-of-venus/
Apparently Venus rotates "backwards" because it is upside down. And, you're right, it experiences no season but a constant temperature.

Another interesting planet is Jupiter, whose axial tilt is almost zero! Well, according to http://en.wikipedia.org/wiki/Jupiter, its axial inclination is 1.305° to Ecliptic, which is its orbital planet (or so I think). So while the other planets are "leaning", Jupiter is "standing upright".

But Jupiter's orbit is more elliptical than the Earth's orbit, so it might experience seasons. On the other hand, Jupiter's extremely thick atmosphere probably makes sure that the inner parts of Jupiter don't feel any seasonal changes. Perhaps the moons of Jupiter experience seasons, however!

Ann