NOW LIVE! Today's the day you meet your new best friend. You don’t have to leave Wolfy behind... In 'Pets & Sidekicks' your companions level up with you!

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Message: <blockquote data-quote="Steverooo" data-source="post: 1925273" data-attributes="member: 9410">The day depends upon the length of time it takes the moon to orbit the gas giant (since it will probably be tidally locked).  The year will be that of the gas giant's.  Since it must be within the life zone, use a year approximately equal to ours... a little longer if you want a colder planet, a little shorter for a warmer one.  The "month" is pretty much an arbitrary distinction.  If you have a world with a 400-day year, then you could have 10 40-day months, for instance...Not necessarily...  Occultations of the sun by the gas giant (GG, hereafter) can occur as often as you like.  It depends upon the degree of inclination of the moon's orbit to that of the GG's!  If they have the same orbital inclination, then an occultation would occur once per "day" (the amount of time that it takes the moon to orbit the GG once).  If the moon has a tilted orbital inclination, BUT RETURNS TO THE SAME POSITION IN SOME MULTIPLE OF THE "DAY", then occultations could occur twice a "day", or three times a "year", or whatever else you like - within reasonable limits...  Remember; in order for the moon to rotate very fast, it has to be very close to the planet, and at some point, you reach the "Roche Limit", and the moon breaks apart into asteroids...  Also, the farther out you get, the longer it takes to rotate (the longer the day gets), and at some point, the moon is beyond the GG's penumbra (shadow).Of course, if the moon has a highly eliptical orbit, and doesn't return to a certain location (with respect to the GG) in a fixed period, then you're talking some serious orbital mechanics mathematics (and a high Knowledge (Nature)) to figure it out...  Kinda like what we have here on Earth, eh?  <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=";)" title="Wink    ;)"  data-smilie="2"data-shortname=";)" />So the short answer is: It all depends!  Mass of GG, GG's orbital distance from the sun, Sun's type and mass, moon's orbital inclination, speed, mass, density... blah, blah, blah!  Best to just pick what you want, and describe it to your players.  Nearly anything is possible.Myself, I'd put the GG fortuitously right smack-dab in the center of the Life Zone, make the GG large enough that it is a "Brown Dwarf" (or "almost sun"), which sheds light even when the sun is blocked, and put the moon far enough out that there is only one occultation in - say - a thrity-six-hour day.  Does that fit what you want?What about it?  Do you mean "Is it dark?"  Like with Earth, there'd be stars, at night, but how dark it got would depend upon what "phase" the GG was showing...  When "full", it would fill most of the sky (glowing dully, if you use the Brown Dwarf idea).  In other phases, it would shed less or more.But yes, there would be some light at night, especially on the GG side, if it's a Brown Dwarf (BD, from now on).  On the far side, tidally locked AWAY from the GG, there would be starlight when in the GG's shadow, or daylight when not.  At points in between, there would be more/less of one or the other.  At the center of the side tidally locked towards the GG, it would be directly "overhead" ALL THE TIME!  Quite a sight, no doubt!If by that you mean "Can the moon be the size of the Earth?", then yes.What would you like them to be like?  If you want stronger tides, make the GG bigger and closer.  If you want lesser tides, make the GG smaller and/or farther.  If you want more irregular tides, make the orbit more elliptical (it does make a difference, as inverse square gravity IS affected by the elipsicity of the orbit)!  But again, you can have what you want, within reason.About the size of the Earth, I assume (if you want a breathable atmosphere).  You could go for a heavy metal planet (smaller and denser), and/or go smaller and with a bit less gravity...There's math to help you out, if you want it...  No calculus needed, just High School stuffs...  You can handle it!If you want to borrow/lift/steal a setting, the <a href="http://home.earthlink.net/~ad2300/products.htm#AS" target="_blank">Aurore Sourcebook</a> for the 2300AD Sci-Fi RPG had a world called Tithonus, which was a BD, orbited by Aurore, a tidally locked world with about 0.75 Gs of gravity.  You won't be able to use the setting, but the planetary info and descriptions might help you out...</blockquote>

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