Magratheans, anyone?

November 2, 2009 6:06 PM

How do I discover what day/night and seasonal cycles are like on a hypothetical unusual planet?

I would like to be able to find out how, given, say, an earth-sized globe, with, say, two suns, and four moons, and a retrograde orbit.... or, wait, suppose my world is flat, like Terry Pratchett's Discworld; how fast does a miniature sun have to orbit to give the impression of normal daytime? What if the world is a cube; what does sunrise look like? What if the world is like a ball where the top half spins clockwise and the bottom half spins in the opposite direction; how do days and nights appear? What about if our Earth spun around the east-west axis instead of around the North/south one? What about a Dyson sphere, or a ringworld like the one in Halo? Spinning or stable? How long is a day, a year, a season? What's the climate like? Etc.!!

I'm not so much interested in the physics of how such a thing could exist, as how it would appear to people living on it. Obviously a lot of magic or fudging could be involved, but if there's something cool like "night only comes once every hundred years" or "a day is about an hour long, so the sun seems to whip across the sky like a comet" I would like to know it.

Is there a computer simulation program that could maybe help with this, or perhaps a well-written book that could make some of this understandable to the layman? Ideally I would have a pet mad scientist whom I could say, “Design me a planet that's shaped like a donut and tell me how it works!”... any volunteers?

posted by The otter lady to science & nature (5 comments total)

These are not hard questions in general, and you definitely do not need a computer simulation to figure out the basic sunrise/sunset behavior. Some careful thought and some pen and paper will do it. You might want to consult any basic astronomy textbook to get some ideas though; it'll help if you first figure out for yourself how the earth and moon work. A basic astronomy textbook should also help you get straight the effects of a planet's orbit from the effects of its spin.

Climate is a different matter. The atmospheric composition starts to become a critical factor, so figuring out much by hand is pretty rough.

(The planet with northern and southern hemispheres rotating in opposite directions is kind of funny. The number of days each hemisphere saw in a year would differ by one. )
posted by kiltedtaco at 6:29 PM on November 2

A real example of this is Uranus.
posted by rickim at 6:31 PM on November 2

You want to read the essay "Bigger Than Worlds" by Larry Niven. (You can see *most* of it here in Google books preview, and an overview of Megastructures in SF at his site.) You might also want to read his novel, Ringworld, which introduced the concept.

For a ringworld, you need an inner rotating ring of shades to provide night, and when there is not shade overhead, it's "noon" all day long (the sun stays directly overhead).

The "easy" way to make a Discworld is an Alderson Disc -- think a solar-system sized record or CD. The sun bobs up and down through the hole in the center.

A cube world that had oceans like ours would be mostly spherical where the water is concerned, with six seas (one on each face), and eight super-gigantic mountains that poke up out of the atmosphere (the corners).

Climate on most artificial worlds would be whatever the designers want it to be.
posted by fings at 8:53 PM on November 2 [1 favorite]

A world where the north and south [*] halves are rotating in opposite directions would have severe air turbulence near the equator. Night and day would appear mostly the same, except for half the world, the sun rises in the west and sets in the east. If the two halves are rotating at exactly the same speed but in opposite directions, a day measured noon-to-noon would take slightly longer in the half that moving opposite the direction the planet is revolving around the sun. Alternately, if you want the days to be the same length in both halves, one half has to spin a little faster than the other.

If Earth was turned on its side, for an east-west axis, at the equinoxes, things would look largely like they do now. At the solstices, half the world would be in darkness, and half in light, for days on end. That's because the axis remains pointed in the same direction (north star), as the planet revolves around the sun. Think of what things are like now on Earth in the far north (Alaska, Norway, etc), where winter has days of total darkness, and summer has days of total sun, but extend it all the way down to the equator. (Earth at present has an axis tilt of 23.5°, what you are proposing is a 90° tilt.)

Ringworlds have to rotate if you want to simulate gravity. Otherwise, you need to "glass" over the top to hold in the air, and everything floats. (See also, "The Smoke Ring" by Larry Niven.)

Non-rotating Dyson spheres have no gravity, unless you happen to have some form of artificial gravity we haven't invented. Rotating Dyson spheres would have "gravity" (centripetal acceleration) that is strongest at their equator, and feel like they have an uphill slope as you move away from the equator, while the effective gravity would drop off to zero at the poles.

[*] There is no top/bottom for a planet floating in space.
posted by fings at 9:24 PM on November 2

You could be your own mad scientist here, and make yourself a model. A bright incandescent bulb for each sun, and a ball (or cube or disc) as your planet. Mark a dot on the ball to show where your person/sentient being lives, and try it out in a darkened room. I'd start by modeling the earth -- see what happens to the dot as you move the ball around the bulb in a darkened room, spinning it about a tilted axis that doesn't change orientation, and use this to account for what you observe everyday -- shortened days during the winter, differences between hemispheres, path of the sun through the sky etc. Then you could add a moon, and check to see that you can account for moon phases. Then you can add an extra sun, 5 extra moons, change the shape of the planet, introduce a wobble to the disc, whatever.
This won't help with the physics of accounting for whatever motion you introduce, nor will it help with the climate issues.
A planet with counter-rotating hemispheres would have some great advantages in terms of travel! Hop sides (not trivial), take a nap, and hop back again when you're at the longitude you want.
posted by Killick at 6:50 AM on November 3