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I'm building a campaign scenario for a tabletop rpg. Well that's not really important, the thing is that as the title says, the word where the characters are is actually a moon/satellite of a gas giant.

I don't know how would this affect life on said planet.

I'm guessing solar eclipses due to being on the shadow of the planet would be a regular issue. Would they be able to see the sun through the gas giant? (it's not solid so... Maybe??)

How would the gas giant affect the sea tides?

Also, this one goes a little more into physics, what kind of orbit should this moon have? I mean, should it orbit on the same plane as the planet? Or would this be bad due to solar eclipses? Should it have a more angled orbit then??

If you happen to think of something else that would be relevant please do tell.

Edit: I don't think that my question is a copy of the one suggested by Serban Tanasa. That one goes into great detail (and lots of physics) in how could a system like that be possible, how could the planet be captured, how stable would it's orbit be, etc... And what I ask is, well, this kind of binary system is already there, what does people on the ground see and experience? I hope is different enough for this question to be worth it.

Edit 2: I guess no, the planet is not tidally locked to the gas giant. If it where only half the planet would be able to see the gas giant, so... I've decided it's not tidally locked. (Haven't thought about that one previously)


marked as duplicate by Serban Tanasa, Monty Wild, Gilles, Nick2253, bowlturner Mar 17 '15 at 23:44

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  • $\begingroup$ A lot of your questions depend on if your moon is tidally locked so the same side always faces toward the gas giant - can you add if that's the case or not? en.wikipedia.org/wiki/Tidal_locking $\endgroup$ – Dan Smolinske Mar 17 '15 at 22:01
  • $\begingroup$ Added the info you requested Dan ^^ $\endgroup$ – Helwar Mar 17 '15 at 22:16

Eclipses: Since you're not tidally locked, your moon is going to be a fair distance from the gas giant. This means that while it will still be very large and a dominant presence in the sky, it probably won't be too insanely large. Because it's a fair ways away, you don't need to worry much about the orbit - even a relatively small inclination will prevent most eclipses (the gas giant will pass under or above the sun instead of through it). You could design the orbit so you get regular eclipses, but that's unlikely to occur naturally. When eclipses do happen, they will likely be longer than on Earth (I suspect the gas giant will be larger in the sky than Earth's Moon), and you will be able to see the sun through portions of the gas giant's atmosphere, but the majority of it will fully block light.

Tides: The gas giant will be the dominant presence on the tides, but your sun will still have some impact. Tides will probably be similar to that of Earth, but with larger differences between low and high - having more of the tides from one source should increase the amplitude of tidal frequencies.

Geologic activity: You will probably get more volcano and earthquake activity than on Earth. It won't be by an incredible amount, like if you were tidally locked, but the presence of a close gravitationally dominant body is going to stir things up more than what happens on Earth. This will mean some geologic features, like mountain peaks, might be more exaggerated compared to what we see on Earth. There will still be a lot of variation, though - an older mountain range will still be worn down, it's just that the newer peaks might be higher.

Weather: I am not qualified to answer this definitively, but I suspect that you will get some weather generated by the gas giant's tidal forces. So you might get more extreme storms. This will interact with other climate forces though, so I have no idea how it will turn out - it might be more hurricanes and stronger storms. Or it's even possible that the gas giant could be at the right frequency to act as a moderating influence, where it actually reduces the power and frequency of storms.

Navigation: Your people will be able to use the gas giant as a navigation aid. Importantly, they'll be able to do this for portions of the day, as well as at night, and the gas giant is hugely visible compared to stars. The combination of the gas giant + either stars or the sun will make navigation easier, although it won't be trivial - ships will still need things like charts and time keeping, and overland navigation will still require some level of expertise.

Natural Life Cycles: on Earth lots of natural cycles tie roughly to lunar orbits. I suspect on the moon of a gas giant, any native life might have cycles that tie to the orbital period of the moon. Depending on the distance, this could be a much greater or smaller period (although not too much smaller since you're not tidally locked).


Considering Jupiter, you could see THIS LINK which states how long it takes many of the moons to orbit Jupiter, as well as their distance from Jupiter.

For orbit direction, it states:

The outer 33 moons all orbit Jupiter in a direction opposite (except for J/2003 J20) to that which Jupiter spins, which leads scientists to believe they are captured asteroids.

They should experience regular eclipses,According to Dan in comments, slight variations cause eclipses to be rare, but how often depends on their distance from the planet and the direction of their orbit around the gas planet compared to the orbit of the gas planet around the star, as well as how long it takes them to orbit. I would think it unlikely that they orbit in a way which makes them never have solar eclipses, but it is possible - you could really set it up however you want for a rpg.

How that affects life is dependent on how often you'd like there to be eclipses. Consider the following scenarios:

The world has an extremely long orbit, and is eclipsed for half. This means there are long periods where the world will be mostly dark and cold, these times are greatly feared and must be prepared for.

The world rotates at a pace which causes half the world to be considered "evil" by the other half.. the light half vs. the dark half

The world rotates and orbits at a pace which means half the world always has Jupiter in sight while the other half has never seen Jupiter - causing wild religious beliefs, etc.

In general, think of the themes you'd like to use for your game and fit your world to that, rather than building your world and then creating a story for it, because you don't know which world decisions will benefit your story most.

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    $\begingroup$ There's actually very few orbits that get regular eclipses. In general the distance of the moon, combined with the inclination ("tilt") of the orbit means that it's pretty rare for everything to line up like that. $\endgroup$ – Dan Smolinske Mar 17 '15 at 22:04
  • $\begingroup$ I thought that if all the elements (sun, gas giant, planet) where in the same plane, and the planet rotates around the gas giant, it would get "behind" it rather regularly. So, that's not the case? My physics are a little out of date then... $\endgroup$ – Helwar Mar 17 '15 at 22:12
  • $\begingroup$ The thing is that "same plane" in planetary terms is a pretty loose concept, due to the distances involved. A few degrees changes things significantly. The Earth, Moon and Sun are all in the same plane, but there's enough slight variation that things only line up rarely. $\endgroup$ – Dan Smolinske Mar 17 '15 at 22:14
  • $\begingroup$ Thank you too, DoubleDouble, you gave me some pretty good ideas to play with :) $\endgroup$ – Helwar Mar 17 '15 at 22:32
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    $\begingroup$ The thing is, Jupiter's closest moon is farther away from Jupiter than our Moon is from Earth. The closest non-tidally locked moon is 30 times further from Jupiter than Earth's Moon (this is actually much further away than I thought). Now, Jupiter is nearly 40 times larger than the Moon in radius (it's much, much more massive), but you can see how the combination of factors means it won't be as large in the sky as you'd think. $\endgroup$ – Dan Smolinske Mar 18 '15 at 18:06

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