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Questions about inhabited moons (or at least one)

  1. Dec 4, 2013 #1
    Hi All,

    I've got an idea brewing in my head that involves a inhabited moon. My original idea was a place that had day, night, and Really Dark Night(tm). It's a fantasy setting, so there won't be people tottering around in space, but I'd really like it all to make at least some modicum of sense. If the moon needs to be made out of handwavium or something, I'm fine with that.

    Among my many flaws, this project has reinforced that I am just not smart enough to figure out much on this topic myself, which means I was super glad to stumble across this site. Without further ado, here's a giant pile of questions. I'll really appreciate any insight you can offer.

    1. Is it possible to have a moon with an Earth-like climate?
    2. If so, what would be required in terms of size, atmosphere, stuff I can't think of, etc.?
    3. What would gravity be like on this moon? Is it feasible to have Earth-like gravity?
    4. If the night side of the moon faces the day side of the planet, would it receive significant illumination? Does this depend on the size of the planet?
    5. At night, what would the planet look like in the sky? Could it fill the sky, or does that imply things about distance/size that are infeasible?
    6. What sort of orbital periods should I be thinking about for the planet and the moon?
    7. If the planet the moon orbits eclipses the sun, would that have significant climatological effects?
    8. Are there other factors that might affect weather or seasons beyond those on Earth?
    9. What effects should I be aware of from the gravity of the sun and planet both acting on the moon?
    10. Are there any other things I should know about moons here?

    Thanks for any help you can offer. I'll be happy to clarify anything I need to, and I'm sure I'll be back with more questions later!
    Last edited by a moderator: Dec 5, 2013
  2. jcsd
  3. Dec 4, 2013 #2
    Sorry, I totally put this in the wrong forum. :-(
  4. Dec 5, 2013 #3
    don't worry it seems to have gotten moved.

    1. Yes it is Physically possible provided either handwavium mass, or the Moon in question is the size of earth. Either way my research on the topic says the the moon would be tidally locked, since it would likely have to orbit a gas giant. Other wise the gravity of the Moon would cause a Binary planetary system should it be orbiting some thing Earth-sized.

    2,3, 5. Pick some thing, Either Determine the Parent or Moon, and then work back from that the Models to work from are available online.

    4. Depends on what the Parent plant is made of, and the distance to which it orbits. Likly yes it would provide at least some.

    6. this is more or less a factor of the masses of the bodies involved, (orbital radius is involved as well) in our own solar system there is the Earth's Moon that Circles every 28ish days, and then there is moons like titan that circle every 16ish days.

    7. Depends on many factors, the biggest one is how long. the Longer the more significant the changes, otherwise expect some major tidal effects and some nasty storms caused by a cooling atmosphere.

    8. No idea my self.

    9. as I mentioned there would be major tides, probably measured in Meters (sea life would be adapted to this) even tens of meters, Winds cased by the massive high pressure zone on the Parent-Planet side, and a smaller one cased by the Sun.

    10. Depends on how "Hard", but probably.
  5. Dec 19, 2013 #4
    1) Yes, sure. Basically in habitable zone, and big enough.
    2) What is the range of "Earth-like enough" for you?
    In terms of size, Mars has traces of flowing liquid, having been around for a long time. Mars almost held atmosphere and water. My inference is that a planet or satellite needs to be only slightly bigger than Mars to have Earth-like climate - a planet closer to Mars than to Earth in size still can have Earthlike climate.
    3) Yes. It simply needs to be about Earth size.
    4) Yes. It depends on the size and distance of the planet. And its climate/albedo.
    A simple thing is - new Moon gets 43 times more light from full Earth than Earth gets from full Moon. The illumination from a planet goes mostly up from there (though there are options depending on the albedo and phase of the planet).
    5) The maximum size of the planet is constrained by the Roche limit of the satellite... you can press it if the satellite has very high density, and planet low density
    6) Two limits. Roche and Hill ones.
    If a satellite is too near planet, it is torn apart by tides into a ring like Saturn rings. This can actually be stated in terms of orbital period. A ring outer edge orbits at 14 hour period.
    Sure, a dense planet can orbit even closer. But earthlike climate and atmosphere is another matter. Air from the top of atmosphere would escape into rings well before stones from ground start falling off to sky.
    If the satellite is too far out, it is perturbed by the star. But Moon, with 27 day orbit, is on a stable orbit. It is strongly perturbed by Sun, its apsides and nodes are rapidly changing, but it is a stable orbit with constant period, eccentricity and inclination.
    My rules of thumb: if a satellite has orbital period under 14 hours, compute atmospheric escape into rings. If the orbital period is over a month, compute perturbation by star. If anywhere in between, don´t worry.
    7) Eclipse is rather shorter than night.
    Just think of Earth geosynchronous orbit. A night is 12 hours. Geosynchronous orbit is 260 000 km long, and Earth shadow about 13 000 km wide. So you are speaking of absence of warmth of Sun for 12 hours at night and 1,2 hours during eclipse.
    Nasty storms? Don´t think so. 12 hour nights do not cause very nasty storms - they cause modest day and night breezes. Eclipses will cause breezes that are milder and shorter than day and night ones.
    9) Tides.
    The strength of tides is actually fairly arbitrary. The uncertainty here is the eccentricity of satellite orbit. The satellite might be on an eccentric orbit causing high tides like Moon or Io. Or it might be on a very exactly circular one, in which case there would be mainly the solar tides.
  6. Dec 19, 2013 #5
    Thank you, everyone, this was immensely helpful!

    Now, for some questions about illuminations. Here's a picture of what I think will happen in this highly simplistic arrangement:


    So, if the moon is between the sun and the planet, then it's pretty obvious. The sun side is bright, the planet side is darker.

    Now, if the moon is off to the side (I'm sure there's a word for this), then am I right that, roughly, one quadrant is in darkness, one quadrant has only sunlight, and the other half are illuminated by the sun and the planet? Would that be noticeably brighter than just having sunlight?

    Finally, when the planet eclipses the sun, are all parts of the moon going to be equally dark, or will planet/space side be more illuminated? Would the answer change if the planet were a gas giant?
  7. Dec 20, 2013 #6
    You are forgetting the fourth quarter, both in text and on your picture.
    At most, it is a few % of sunlight. It does not create a very strong and clear difference at the edge where sun is high. And there will not be sharp shadows because the planet is big. If you look at the shadow of a sharp edged obstacle, it will have only a slight contrast between sunlit shadow and fully lit part, and the edge broadens and smears out very rapidly.
    But the matter is very different near terminator. The illumination on horizontal surfaces by Sun on horizon, plus twilit sky, is estimated as 0,6% of full sunlight - twilit sky alone, with Sun under horizon, is 0,3 %. So there the half phase planet high in the sky does outshine the sunlit twilit sky.
    It depends, basically, on twilight into umbra. Which is very hard for me to derive from first principles.
  8. Dec 20, 2013 #7
    8a) I'm not sure whether it counts as "climate" - because of tidal lock on one hemisphere one would see the planet all the time and the surface would be lit all the time either by the star or its reflected light. On the other hemisphere one would never see the planet and would have to got used to darker nights.

    8b) On Earth seasons are caused by axial tilt, however for practical purposes Earth orbit is circular enough not to affect it significantly. You can play with ex. negligible axial tilt and higher eccentricity of orbit.

    8c) There should be some additional moons visible with naked eye which would presumably affect local mythology/calendar. The main big ones should be presumably synchronized with this "main" one.

    It seems that I already once answered a similar question:
    Last edited: Dec 20, 2013
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