[Cranstonvm]

For a game idea I have, I need to find what height of orbit a moon needs to be to circle an earth like planet every 3 days. I have not been able to find this out and would appreciate the help.

[Palladion]

Wow, I actually remember how to do this... sort of...

Given an equivalent speed to our current moon of 1,020 m/s...

Speed x Time = 2 π R (where R is the radius from the center of the planet)
R = Speed x Time / 2 π
R = 1,020 m/s x 259,200 s / 2 π
R = 42,078,020 m = 42,078 km

Earth's radius is approximately 6,371 km (from center to surface)...

Hope that means something to someone... heh.

[FastJack]

Orbital speed has many variables, not just the "height" from the planet. But maybe Wikipedia can get the ball rolling.

[The Wyrm Ouroboros]

... yeah.  Wow.

Depends on the planet's mass; depends on the mass of the satellite/moon.  You'd get a hellish tidal effect with a 3-day period, though, one which would probably tear your planet - or moon - apart Very Quickly.  If you want realism for a Terra-weight planet and a Luna-weight satellite that close, give it instead a) rings and b) steady meteor showers.

Otherwise, make it up.

[Cranstonvm]

The moon is about the size of  Phobos. 22.2 km diameter 1.08x10e16 kg

[Palladion]

The moon is about the size of  Phobos. 22.2 km diameter 1.08x10e16 kg

How fast do you want it to travel?  3 days is 259,200 seconds and our moon travels approximately 1,020 m/s relative to the Earth.  If it travels faster, the orbit is higher than I indicated, if it is slower, then it is closer to your planet than indicated.  The math is correct, you just need to alter the constants (in this case, the speed).

Whether it is physically possible?  No idea, my Bachelors of Mathematics avoided Physics problems.

(Check the math about moon speed: http://answers.yahoo.com/question/index?qid=20100311154046AA7oHtN )