We are the moon.

[ecliptic]

Lunar videogames took place on a moon of a planet.

 

[Lonely Tylenol]

George Lucas certainly likes them.
It would be pretty difficult physically, but under the right circumstances largish moons like Ganymede and Titan could have decent atmospheres, at least fora while. The biggest problem with smaller worlds is keeping those light water molecules from wandering off into space. Earth has a little extra edge in that the structure of the atmosphere tends to trap water more efficently than just the gravity would do alone, and if you had a really good "cold trap," maybe a Ganymede-to-Marsish-sized body could hold some decent amount of non-ice water. The lower you keep the temp, the easier it is to do, like maybe a body with very frozen poles and a temperate equator. A really dense body would help as well, some late-generation star system, heavy-metal star, that sort of thing.
Or you can wave it off as magic.

Atmosphere is a tough thing to accomplish. Apparently, science tells us that the Earth had no atmosphere for quite a while because the solar wind stripped it clean until the magnetic field around the earth stabilized enough to deflect the high-energy charged particles that were sandblasting the planet, so to speak. After that, we had a mostly reducing atmosphere, which suddenly changed to an oxidizing atmosphere shortly after the appearance of photosynthetic organisms.

So a moon is going to need a magnetic field, at least. That is, of course, if you're using real-world physics to handle cosmology.

 

[Gez]

My campaign world, Erdelane, was sundered long ago into two equal halves. Edhel and Rhane. These twin planets are each the moon of the other. Edhel is the world for mortals, and Rhane the world for deities. Or so it seems.

Of course, I didn't bother much about physics. Neither of the twin planets are tide-locked to the other, the cycle of days and night is normal, etc.

 

[J_D]

Tides would depend on if your orbit is circular or elliptical. A circular orbit would give very minimal tides, likely tied to the sunlight cycles (as one side of the planet is exposed to light, it turns away from the planet, and vice versa). An elliptical orbit would give far slower tides, possibly closer to seasonal, but they'd be fairly dramatic as the moon pulls closer to the planet and then farther away.

Actually, the ellipticity of the orbit in and of itself has nothing to do with tides. It's all about gravity and rotation. Remember that the strength of the gravitational force is inversely proportional to the square of the distance. The closer you are, the stronger the pull, and the farther away the less strong the pull is. Now, remember that planets and moons are not point objects - they take up space. Tides come from the fact that the gravitational pull a body exerts on the nearest side of another body at a distance is a bit stronger than the pull that same first body exerts on the farthest side of the other body. That difference in gravitational pull is what a tide is.

On a solid body, this tidal force acts over time to slow down the rotation of the distant body. Angular momentum is a quantity that is conserved, though, so the lost angular momentum must go somewhere and it does - into the orbit between the two bodies. As the rotation of the bodies slow, the size of their orbit increases. Eventually both bodies will either be tidelocked to each other (i.e. show each other the same face constantly), or the orbiting body will leave the orbit and go off on its own. Without the moon, the Earth would spin a lot faster and our day would be a lot shorter. Over time, our day is slowing and this is driving the moon away from us. The moon is tidelocked to the Earth already because the Earth's gravity is much stronger, but eventually the Earth will be tidelocked to the moon as well. That, or the moon will get far enough from the earth to leave orbit. I'm not sure which would happen first.

When a world has a fluid on the surface, though - liquid oceans, for example - the tidal force causes the fluid to bulge both toward and away from the body exerting the tidal force. There will be "bulges" on the near side and far side relative to the more rigid solid surface, and shallow points in the middle. These bulges stay aligned with the tide-generating body, and the planet rotates underneath them. Thus, there are two high tides and two low tides each day.

The only possible effect the moon's eccentricity would be is to affect the severity or degree of the tidal difference over time. When the moon is closer, it'll exert a greater tidal force thus the high tides will be a bit higher and the low tides will be a bit lower. When the moon is farther away, the difference between high and low tide will be lessened. The moon's eccentricity will affect the degree of the tides, but has nothing to do with the cause of the tides. I'm not going to go into the math here, but the way the math works out the degree of the "tidal force" is inversely proportional to the cube of the distance, which is different than just the basic gravitational attraction between two masses. To get a tidal force twice as strong the moon would need to be eight times closer. What I can't say for sure is whether the "tidal force" is directly proportional to the height in feet of the ocean tides.

 

[tarchon]

Apparently, science tells us that the Earth had no atmosphere for quite a while because the solar wind stripped it clean until the magnetic field around the earth stabilized enough to deflect the high-energy charged particles that were sandblasting the planet, so to speak.

The existence of an extremely dense atmosphere on a smaller and almost totally non-magnetic planet, closer to the sun, presents certain problems for the idea of solar-wind erosion as a dominant factor in atmospheric evolution.

 

[Ferret]

Thanks for all the help, so anything smaller then a gas giant shouldn't be able to have a moon earth size. I should be able to have normal climate and seasons, what happens when it touches outside atmosphere? Gets colder?

On the angle of orbit, I'm hopping for a world that is mainly like earth but with common eclipses, perhaps once-twice a year? I supose the angle itself doesn't have to be exact. But kind of years are possible? They would just be once around the sun, and a month around the planet? Ok, so that was simple , silly me.

Any other differences? What does it look like when the planet rises?

 

[Imret]

What does it look like when the planet rises?

As much as I'm not here to pimp sci-fi, Pitch Black is set on a moon orbiting a gas giant (IIRC)...there are some fine shots of what a gas giant looks like in the sky of a world. But basically, foremost...it's huge. Consider that with a (relatively) slight increase in mass, Jupiter could collapse into a star. This would be a bad thing for life in the system, obviously, but it's a handy apocalypse metaplot if you ever need one. But, getting back to the question, you can only see about a quarter of the gas giant at any one time in the sky in Pitch Black - it's larger than the moon in an exponential way, so unless the moon is on a far distant orbit from the giant, you'd probably never see all of it in the sky at once.

For the specifics of the gas giant, check out some good NASA photos of our resident gas giants and run from there. It could be any color you want, but the effect is more or less close to a giant marble hanging over your planet. Strips of color, planet-sized storms that look like swirls of color...hell, it's your gas giant. Could look like anything you want.

Of course, if you want some serious complexity, you could have planetoids orbiting your moon as it orbits a gas giant, or simply other moons of the gas giant that are visible every so often from the inhabited moon.

 

[tarchon]

Thanks for all the help, so anything smaller then a gas giant shouldn't be able to have a moon earth size. I should be able to have normal climate and seasons, what happens when it touches outside atmosphere? Gets colder?

On the angle of orbit, I'm hopping for a world that is mainly like earth but with common eclipses, perhaps once-twice a year? I supose the angle itself doesn't have to be exact. But kind of years are possible? They would just be once around the sun, and a month around the planet? Ok, so that was simple , silly me.

Any other differences? What does it look like when the planet rises?

Well, I wouldn't rule out double planet systems. We only have 4 (+1/2) terrestrial planets to look at, so our experience with the varieties of possible orbital configurations is limited. The moon is thought to be the result of a collision with a Mars-sized object, so maybe with some slightly different luck, we would have had two smaller double planets. Pluto and Charon have about a 10:1 mass ratio (or more) too.

Moons in multi-satellite systems around planets can experience frequent eclipses (really occultations) of the star by the other satellites, though it requires fairly in-plane orbits to get it to happen often. Obviously the central planet is going to eclipse the star rather often in most cases as well.

 

[Steverooo]

I'd like to find out what kind of day, year or month would be had from such an arrangement.

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...

I'd of thought as Lunar eclipses are more common (Yes?) that the inhabitants would be in the dark a lot.

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?

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 at night?

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!

is there such a thing as a full planet?

If by that you mean "Can the moon be the size of the Earth?", then yes.

What would tides be like?

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.

What size would the planet have to be?

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...

Anything else I should be aware of.

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 Aurore Sourcebook 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...

 

[Steverooo]

Thanks for all the help, so anything smaller then a gas giant shouldn't be able to have a moon earth size. I should be able to have normal climate and seasons, what happens when it touches outside atmosphere? Gets colder?

What is "it"? Be specific, man! What happens when the moon's atmosphere contacts the GG's? VERY, VERY BAD THINGS! World-killing storms! Friction! The moon falling into your GG! (Not to mention probable poisoning of all life on your moon!)

On the angle of orbit, I'm hopping for a world that is mainly like earth but with common eclipses, perhaps once-twice a year?

Earth-sized is okay. With a tidally locked world, though, the month and day are the same length... once around the GG is both the month and day. The year is once around the sun...

Assuming the GG is about in Earth's orbit (and yes, it is theoretically possible to have a stable gas giant within the life zone - one of the supposed extra-solar planets is supposed to be in just such a position), then the year would be 365.247 or so of OUR days long. The "day" length would depend upon the masses, orbits, etc., of the GG & moon.

If you want only two occultations a year, then you either need to make the moon far enough away to NOT be tidally locked, or give it a greatly inclined orbit and keep the tidal locking.

I supose the angle itself doesn't have to be exact. But kind of years are possible? They would just be once around the sun, and a month around the planet? Ok, so that was simple , silly me.

Using our Earth as your model, the year would be about 365.25 days long, the month about 28 days, and the DAY also about 28 days long... Two weeks of solid daylight (of varying degrees), followed by two weeks of night...

Of course, if the moon's orbit is titled, like Earth's is, then... Like the "Midnight Sun" in Alaska, some locations would see the sun "roll around" the sky, a bit, making the lengths of day and night variable, depending upon your location upon the planet...

Any other differences? What does it look like when the planet rises?

Earth sees a lot more than two eclipses a year (full or patial) SOMEWHERE upon the planet, most years (IIRC), but it varies. There are many years where there are NONE! So, again, it all depends.

Note that, with tidally locked worlds, where the month and day are the same length, that the GG will go through ALL of its phases once per "month-day"! At the "nearside", the GG will always be directly overhead, but parts of it won't always be visible, just like the "dark of the moon" on Earth! (It will still block out the stars, though!)