Are you talking about orbits?
Well if you are, the period T = (2*pi*a^(3/2)) / (sqrt(u))
where a is the length of the semi-major axis
and u (which is actually mu) is the gravitational parameter. For the earth, u = 4E5 km^3/sec^2
Maybe this is not even close to what you're talking about. If not, sorry, I had an astrodynamics test today.
Nick