how long does an orbit around the moon take
It's the amount of time between perigees. Edit: more explanation on that, since I'm not sure what frame of reference your physics question relates to. If you ignore the earth's orbit around the sun, the moon takes 27.32 days to go 360 degrees around us. In other words, the time it takes to cover 360 degrees of the celestial sphere. But since we orbit the sun, the moon has to travel farther than 360 degrees to reach the same point, with respect to the earth and sun.