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gas giant day/night cycle


M_thraun

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Hi all, been brainstorming a setting on the moon of a gas giant and was trying to figure out the day night cycle. I came up with four "days" so to speak.

True day: when the moon is between the giant and the sun, facing the sun, pretty much like what we got here.

Shadow day: When the moon is facing the sun but blocked by the giant. I imagine a deep purplish glow from the giant, not unlike dusk.

Shadow night: When the moon is between the giant and sun but facing the giant. Brighter than shadow day because it's getting the reflected light.

True night: Standard earth night, looking away from sun and giant.

What I'm not sure of is how long each would last since the moon not only rotating but also orbiting the giant. Would there be days of only twilight and darkness? If so I have to figure out how the plants survive.

My total astronomy experience comes from a course i dropped after first class (me no good with number), so any thoughts would help. Thanks!

 

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Not an astronomer, but decent at googling. Check this out. Many moons are tidally locked with their planets, it looks like, but I don't know if that's guaranteed or not. If that's true, you're only getting true/true or shadow/shadow, no combination of both. So you're probably not terribly interested in that case.

The gas giant will cause a total eclipse when the moon moves behind it; it's the coldest time on the moon, your True Night and Shadow Day. I don't think you'll be able to differentiate them, unless your moon is really far away or something like that.

Some of Jupiter's largest moons orbit the planet every ~40 hours, ~80 hours, and ~160 hours, respectively. So, it all depends on how you want to (or can) match up your orbital periods against a 24-hour day. It's probably not unreasonable to say that they could have a normal earth day, but different parts of the planet have different kinds of days.

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Any significantly sized moon in close proximity to a planet is going to be tidally locked, unless some pretty extraordinary things are taking place. Personally, I'd put the moon in an orbit with a period equal to the length of your preferred day, and let it's notation around the planet determine the day-nigth cycle. This does mean, as pagerunner pointed out, that different parts of the moon will have different day-night cycles.

If you need some accurate numbers, shoot me a PM in which you specify more exactly what you want (for example, a 24-hour cycle around a saturn-analogue planet), and I can run some calculations for you*.

*I've got a Bachelor of science in Aerospace engineering, and am currently doing my masters with a focus on spaceflight. I wouldn't consider myself an expert on moons, but I've got a working knowledge of orbital mechanics.

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  • 4 weeks later...

So what i'm getting is that if the moon is tidally locked, then one side is going to have normal day/night, the opposite will have daylight reflected from the giant (and be colder?), and places between the two might have something in between.

That opens up some pretty cool possibilities with flora/fauna, and cultures. Thanks!

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