Effect of axial tilt on a planet

[woodelf]

the big question here is how are the landmasses of this world arranged? if the landmasses were more polar, or all in a single hemisphere, then the planet would be very temperate - warm and comfortable, though a fair ammount of hurricanes would spawn.

if, however, more than 70% ish (forgive the lack of a presciese number here, it's been a long time since the geology class i learned about this stuff in) of the equator's area were covered in landmass, the land would reflect the light back into space, along with all of the heat that comes with it. the oceans would get colder (signifigantly - 10 degrees or more), and the whole thing would trigger a Day After Tomorow-like ice-age - except that the ice would cover even more of the planet's surface, reflecting even more sun back into space... etc etc.

and yes, this did happen on earth, a few billion years ago. the only way the earth got out of it was the fact that the ice age killed off most of the alge in the water, and over a few million years, volcanism put out enough CO2 to start a good greenhouse effect going on.

It did? Point me to some sources--i was under the impression that we didn't know what happened more than ~1bya (approx. formation of Rodinia). [btw, that's info-seeking, not authority-challenging]

Anyway, how does what you're saying here interact with the idea that more polar land mass promotes the formation of larger polar icecaps, increasing albedo and causing cooling, and the idea that part of why the Cretaceous/Jurassic/Triassic periods were so warm was the high concentration of land in the temperate-to-tropical region, and almost nothing polar?

Or does that fall into the "all in a single hemisphere" exception, above? [I originally read that to mean N or S, given the context of latitude discussions surrounding it, but maybe you meant E or W, too?]

[And, yes, i'm gonna give that paper a quick read. But an executive summary would be nice. ]

 

[woodelf]

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?

Actually, the weather wouldn't so much be absent, as more localized, and driven by geographic features.

The primary drivers of Earth's weather patterns are thought to be, in no particular order:

• climate-zone temperature differences (hot air in the tropics rises and is displaced by cool air from the sub-tropical/temperate zone)
• geography (the Tibetan Plateau, the Rockies, the Mongolian highlands, and the distribution of land and water across the climactic zones may be why we have the specific weather/climate patterns we do, and not something else)
• ocean currents--which are driven by the prior two
• varying daily sunlight intensity and duration (i.e., seasons), which acts primarily by altering the first point above seasonally. The direction of the Earth's rotation comes in here, too, and is why the jetstream blows in the direction it does.

It may be that the ultimate driver of weather patterns for the globe is

• 1. Water cools in the N Atlantic, becoming denser and sinking.
• 2. This draws warmer waters north from the tropics, specifically the Gulf Stream (due perhaps just to accidents of geograph)
• 3. the interesting part, and the part i don't understand, is that, for whatever reason (probably partly coriolis effects), this doesn't simply form a neat closed loop, but instead drives a series of massive ocean currents that travel across the S Atlantic, around the tip of Africa, and do a massive loop around the Indian and Pacific oceans. Now, which particular part is the chicken in this chicken-and-egg scenario is hard to know--and probably breaking any one of the links would break the whole system.

Another interesting data point: it may be that part of why the Earth was much warmer during the dinosaur period was because of no polar landmasses, and therefore much less polar ice, meaning both more water in circulation (i.e., higher humidity and therefore more greenhouse effect), and less albedo (ice is very reflective, so the more of the Earth is covered in ice, the more sun energy that gets reflected back into space rather than absorbed).

 

[woodelf]

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)

[Note that these are a layman's approximations; real climatologists use different break points.]

You get more extreme weather up to the point where those bands start crossing, then things get confusing. 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.

Actually, wouldn't the tropics and arctic (as climactic zones) shrink, and the sub-arctic and sub-tropical regions grow? Right now, everything between the Tropics of Cancer and Capricorn get direct sunlight at least one day of the year. But that's also the reason where the lowest angle of incidence the sun has during the year is 47deg. Increase the tilt to 40deg, and the solar incidence at the Tropic of Cancer would now vary from 0deg to 80deg. IOW, i'd expect greater seasonal variation at that latitude than we generaly characterize as "tropical" climate. Sure, it'd be plenty warm in the summer--but so are inland temperate areas in our own world.

 

[woodelf]

Would rings have the same stabilizing affect?

In theory, yes. In practice, though, the mass just wouldn't be there. Think of it like this: the entire asteroid belt, spanning well over a hundred million linear miles, and with a width in the hundreds of thousands or even millions of miles, has a total mass thought to be less than that of pluto (i.e., less than the Moon). And, as an interesting data point, the single largest asteroid (Ceres) accounts for roughly 1/3rd of that mass. Rings are very similar to the asteroid belt, but on a smaller scale.

Ooo! Here's another fun analogy. Try spinning around with a big bucket of water on a string, and then with a hulahoop around your waist. Which one imparts more centrifugal force to stabilize your spin?

 

[woodelf]

Boring.

Minimal weather patterns, pretty much the whole planet would have an even day-night cycle. Take out the moon and you don't even have to worry about tides and volcanos. So, this is the option you'd go with if you want an excuse not to worry about any of those things.

Um, you can have volcanos without tilt or moon--tectonics causes most of them on our planet, frex. Also, weather patterns are largely driven by the difference in day-night temperature, the difference in solar energy at different latitudes, and geography. Eliminating tilt and eccentricity would pretty much change seasonal variation in weather, but it wouldn't eliminate weather.

 

[woodelf]

Eh, for this setting, I'm thinking more of a Mars-like cold desert setting. Very little surface water at all.

More radical weather, in all seasons. Large masses of water have much greater thermal mass than large masses of land, and thus cut down on rapid/large weather changes. It's why Boston and Seattle have much less seasonal variation than Milwaukee & Minneapolis (respectively), despite being at similar latitudes. Take away the oceans, and both climate and weather change more rapidly and have greater extremes. Also, the day-night temperature variation would probably be greater, just like in Earth deserts--less humidity to mollify the change.

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.

The variance on the distance from the sun to the ground due to tilt is essentially zero. The important parts are the change in angle of incidence (same amount of energy spread over more area), and amount of atmosphere traveled through.

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.

Strictly speaking, yes. But in the real world, it gets more complicated: Earth's axial tilt produces "differences" that are orders of magnitude greater than those produced by its minimal eccentricity. It's probably most accurate to say that both axial tilt and orbital eccentricity contribute to the formation of seasons, and the magnitudes vary. But it's gonna take a lot of eccentricity to equal a fairly moderate amount of tilt. Earth's tilt means that the angle of incidence of sunlight in the temperate zones shifts from, say, 20deg to 65deg from summer to winter. That's gonna cut the energy content by, what, 60% [ballpark calculation]. To duplicate that level of change with just eccentricity, you'd need to have the planet be ~1.6x as far away in winter as summer, or an eccentricity of 0.27. Pluto's eccentricity is 0.24, and the Earth's is 0.0167.

 

[Dannyalcatraz]

Me If you compare the seasons between the Northern and Southern hemispheres, you'll find that their summers and winters aren't as harsh South of the equator, and that's a function of eccentricity of orbit

Woodelf jumped on me for this, but- someone else already corrected me on this earlier in the thread- (I had it bass-ackwards):

Warehouse23 (a planetary geologist):
* 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.

Did I mention that my 1 semester of Astronomy was 17 years ago?

 

[Nyeshet]

Probably not; the mass of rings is minimal compared to a good moon -- our moon is roughly 25% of the mass of the Earth. Rings can vary from the ephemeral structures around Jupiter to something more like the magnificent structures of Saturn, but in any case, they'd likely be a fraction of 1% of the mass of the host planet.

Rings are cool, though. Imagine the mythology of a world that has a ever-present, unmoving series of big silver bands that go across the entire sky, visible day or night.

Very nice, until you realize that meteors are falling with alarming regularity - perhaps every other day around the equator.

The image of the ring as seen from the ground would not look like a gentle ring or even a mass of multitudes of mini-'moons'. Well, yes, it might seem like that latter at its most outter visible edge. But its inner edge would seem to burn. Rarely would an hour pass without a flash of light as a house size chunk of rock - or larger - falls from the sky, burning up as it does so. I imagine the entire inner edge of the ring would seem to shimmer faintly from the near continuous burn up of rock ranging from the size of your finger to the size of your body.

The land at the equator would resemble a moon - especially if the world is rather dry. Pock marked from thousands of asteroid and meteor strikes, the heat from the impacts would more than make up for the shade offered by the continuous cloud of meteors surrounding the equator - at least at a local level. I imagine the lack - or at least extreme irregularity - of heat(ing) at the equator might partially throw off the whole seasonal shift in winds, etc.

Oh, and I seem to recall the moon being something like 1/6 the size of the earth but only 1/81 the mass. Perhaps I mis-remember? That's about 1.23% - a major difference from your stated 25%. Of course, even that is massive compared to the ring.

The main problem with a ring for stability of obliquity is that the mass is not centralized - it is spread out over a massive volume of space. As such, it offers little if any stability, especially since it is continuously shrinking as its pieces fall to the world below. Given a few score million years any ring formed around a world like the earth would mostly vanish - at least to a low enough level not to be visible from the surface of the planet.

[Edit]Oh, I see a few issues (moon mass, ring inadequecy, etc) have already been dealt with. Still, it is fun to think about the realistic consequences of having a ring around a world. Eberron should be a lot more interesting - at least around the equator. ^^[/Edit]

 

[Nyeshet]

Here are some links that might be of interest to you:

Stars and Habitable Planets
http://www.solstation.com/habitable.htm

Twin Earths: An Inquiry into the Posibility of Binary Habitable Planets*
http://www.jabpage.org/posts/twnearth.html
* You can guess how complex / 'interesting' my homebrew world is, ne? ^^

Planet Designer **
http://www.planetdesigner.org.uk/
** Remember to set the moon(s) distances (relative to their planet) in AUs!! It used to take km, until he modified the form to deal with binary planets a couple years ago. I still have mixed feelings about the change. Its lack of recognition for more than a few points after the decimal also increases the difficulty of this unless you use scientific notation.

http://www.bumply.com/astro.html
This link is useful to collaborate with the above link. Some differences - sometimes notable: such as habitable zones for stars; one uses mass, one uses apparent luminosity for the planetary distance, and the two sites agree only if you manage to compute the apparent luminosity very very finely before you input it. That the Planet Designer seems to have a low threshold for places after the decimal place means I often use this link instead for things like Habitable Zones.

Star Gen (builds solar systems, but entirely random)
http://fast-times.eldacur.com/StarGen/RunStarGen.html

World Builders (good for the basics, an actual college couse in LA)
http://curriculum.calstatela.edu/courses/builders/index.html

- - -
These might also be of interest to you:

Medieval Demographics Made Easy (based on historical tax records, mostly from france during the late middle ages, IIRC)
http://www.io.com/~sjohn/demog.htm

Population Center Demographic Calculator (another version of the prior, with D&D racial breakdown)
http://www.lucidphoenix.com/dnd/demo/business0.asp

Kingdom Populator (uses the above link to produce stats for an entire kingdom! )
http://www.lucidphoenix.com/dnd/demo/kingdom.asp

- - -

Oh, if anyone is interested, the world I'm working on is larger than earth (r=11262 km), but has about the same land mass. Its a binary planet - each with a moon - circling an F3 type star (~1.29 solar masses) which has a very small, very distant companion red dwarf (~180 to 200 AUs away, IIRC). The binary planet itself is around 1.68 AUs, but since the star has a habitable zone of 1.47 to 2.15 AU that's not really a problem. The larger world is a bit over three earth masses, while the smaller world (about half the radius of the larger) is about three-quarters the earth's mass.

Nearly all of the land is in the northern hemisphere, focused into three main continents that will likely become a pangaea mass continent sometime in the next 50-100 million years. The other planet is not yet developed as a concept (beyond the fact it is habitable). The moons are both smaller and much less massive than the earth's moon - more like exceedingly large captured asteriods, really, just large enough from the ground not to be confused with stars. They're the only unrealistic point, so far. I expect the only reason they're still in orbit (not tossed out of it) is due to divine intervention. Perhaps another divine intervention necessity: I'm still not sure if the main world (nearly eightfold earth's surface area and only just over threefold its mass) is feasible. I don't want a hollow world, but perhaps a world with less iron / heavy metals at its core would be a reasonable explanation, perhaps.

No posted maps yet. I'm still determining ocean current patterns, basic wind patterns, and plate techtonics (so I can be sure I have the mountains right). I have ideas for the cultures that will later form and the histories of their peoples, but for now I'm still working on its geologic past and its realism. I'm thinking that perhaps elves and humans developed on different continents. Wild magic exists, so perhaps a small tribe of human precursors was somehow shunted to the other continent in ages past and developed along different lines. Once in a million years doesn't seem to unlikely for so massive a surge, I think . . . . I'm still creating the magic system, however, so I'm not yet sure how realistic this idea is yet.

 

[Umbran]

Um, you can have volcanos without tilt or moon--tectonics causes most of them on our planet, frex.

Careful. Tectonics is not the root cause. Tectonics are a whole bunch of things we glom together that are all caused by our having a thin crust over a hot, molten, fluid mantle.

So, the root cause is the hot rock not that far under your feet. What keeps the rock hot? Mostly radioactive decay and tidal friction from the Moon.