Effect of axial tilt on a planet

[Warehouse23]

As far as producing a Mars-like world goes, here's some things to think about:

* Mars and Earth have very similar axial tilts (at present) (25.2* versus 23.5*).

* The technical term for axial tilt is "obliquity."

* Mars, unlike the Earth, has experienced rapid changes in obliquity over the past millions to billions of years. The moon keeps Earth's obliquity to within +/- about 5* of its current state. Lacking a large moon, Mars' obliquity has varied between 0-90* obliquity during its existance, and likely, as much as 0-60* within the past several million years (all these calculations are statistical, as the fine dynamics of planets is still something which is rather difficult to compute).

* There is a lot of discussion in the Mars community over what the planet was like during past periods of high obliquity. When the poles face the sun for large portions of the year, volatiles, such as water and C02 seem to migrate towards the equator, which is largley in shadow at high obliquity. Exactly how quickly the polar ice deposits collapse and migrate is still not well constrained. This makes for a nice backdrop to gaming, though, if you want your world to be suffering from sudden droughts and floods in unpredictable ways.

* Just a reminder: seasons are a function of a planet's obliquity. At present, the northern hemisphere of Earth is actually closer to the sun during winter than during summer. It's just angled at such a way that the sun is lower in the sky, producing less intense radiation on the surface (in Watts/square meter) which produces the cold season. At 0 obliquity, "seasons" would be driven by eccentricity (the degree of circularity of the planet's orbit). Although there are eccentricity effects on climate (on Earth and Mars), seasonal variation is dominated by obliquity.

 

[cmanos]

More extreme temperatures, longer days during summer months with a lower latitude for 'midnight sun'. Depending on where you are and how steep a tilt you may have weeks or months of 'night' or 'day'.

 

[radferth]

Wouldn't it be easier/more fun to decide what weather you want your world to have, and then decide on why later?

 

[Umbran]

It also matters what plane the tilt is in - Earth's is essentially in line with its orbit, and it'd be very different if it was essentially in line with its orbital radius.

Not quite.

The Earth's axis points at Polaris (with some small precession with a 10,000 year period). In other words, the direction the axis points is (on human timescales) pretty much fixed relative to the distant stars. That means the axis is not fixed with respect to the orbit. Sometimes the tilt is along the orbit, sometimes it isn't. This is normal, a consequence of conservation of angular momentum.

Assuming, for a moment, that the planet's year is much longer than it's day...

As you increase axial tilt, the arctic (and antarctic) circle moves toward the equator. The tropics move toward the poles. Eventually, when you reach a tilt of 90 degrees, the tropics are at the poles, and the arctic circle is at the equator. You still have equinoxes. But at the solstices, you get an entire hemisphere that gets and entire planetary rotation of daylight, the other an entire rotation of night.

Water or not, you'll still develop storms. As you move into summer, you're getting more and mor of the planet in constant daylight, and more in constant darkness. The temperature differences will be huge. For gases, temperature differences mean pressure differences - and that means wind. Rain is a side effect for places that have water, but you don't need the water to have a windstorm.

If the day is not much longer than the year, things get a bit more complicated.

 

[BiggusGeekus]

Wouldn't it be easier/more fun to decide what weather you want your world to have, and then decide on why later?

Not for some.

Some of people (e.g. me) want places that "could have been". Worlds that could potentially exist in the infininte universe. The why of it kind of has to come first. If you want a world with radical weather and make a post to that effect you'll get a variety of answers, many of them not plausible because they rely on magic to work. That's fine, but some people (e.g. me again) have an easier time suspending disbelief if everything works according to natural law.

You also get some cool ideas this way. For example, if you know how the plate tectonics of your world works you can be inspried to create a strip of mountians that are highly volcanic which would make for a pretty neat adventure area.

Obviously, story is the most important thing. Nobody's suggesting that you get embroiled into the geo-physics of a world to the point where you forget story. Mordor has a square mountain range, but nobody ever gave Tolkien a hard time about it.

 

[Janx]

Yep. More extreme seasons - up to a point. Too much and you get no real seasons at all.

Axial tilt also determines climate bands -
Tropics = Equator -> Equator + tilt (23.5 S -> 23.5 N on Earth)
Subtropics = Tropics -> Tropics + 1/2 tilt (35.25 S > 23.5 S, 23.5 N > 35.25 N)
Temperate = Subtropics -> Subarctic (54.75 S > 35.25S, 35.25N > 54.75 N)
Subartic = Arctic - 1/2 tilt -> Arctic ( 66.5 S > 54.75, 54.75 > 66.5)
Arctic = Pole - tilt -> Pole (90 S > 66.5 S, 66.5 N > 90 N)

The first post I've seen that gets it. The tilt affects climate effects in REGIONS. Take earth. The tilt of the earth has little effect at the equator. The sun remains the same distance pretty much no matter what. But farther north, the tilt has a greater impact, longer days, more atsmosphere/angle to block UV & light, greater variance on distance from sun when tilt is sunward or not affects climate and season.

 

[Dannyalcatraz]

According to my old college astronomy book:

Length of day is determined primarily by planetary rotation. Venus' day is actually longer than its year.

Seasons are caused by the eccentricity of the planet's orbit. The more eccentric (elliptical or otherwise disturbed) the orbit, the more pronounced the differences in the seasons. Earth's orbit is mildly elliptical, while Mercury has the most eccentric orbit of any planet (though still less than objects like comets).

Axial tilt affects the intensity of the seasons and produces differences in the seasons between northern and southern hemispheres.

 

[Miln]

So let me see if I've got some of this straight.
If you have a planet with no tilt (the poles are perfectly perpendicular to the plane of orbit), you would have the following:

1. No seasons to speak of
2. Little to no weather, except what you have from the motion of the planet around the sun.
3. Different regions of the globe would have different climates, but little seasonal variance in each area.

Would the poles be ice covered?

Am I even close?

 

[GuardianLurker]

@Miln:

1) Check
2) Sort of. You'll still get weather, but it will be much more predictable, and more strongly influenced by local environments. When weather does vary, it will tend to be more localized.
3) Check.

4) The poles may or may not be ice covered, it depends on the global average temperature. Note that geologically, our current temperatures are much *colder* than typical. Note that without the ice caps, there would be much more water in the weather cycle, and (since we're talking a warmer world) the storms would be much more energetic.

 

[Numion]

Seasons are caused by the eccentricity of the planet's orbit. The more eccentric (elliptical or otherwise disturbed) the orbit, the more pronounced the differences in the seasons. Earth's orbit is mildly elliptical, while Mercury has the most eccentric orbit of any planet (though still less than objects like comets).

Are you sure? It sounds strange .. I thought that the tilt is the main reason for seasons. Wikipedia agrees.