1. The problem statement, all variables and given/known data A planet orbits around the Sun with an orbital period of 14.5 years. It is observed it has a moon. At opposition, the moon has an almost circular orbit, and a radius of 7 arcminutes as seen from the telescope. Its orbital period around the planet is 3 days. What is the mass of the planet? 2. Relevant equations Newton's ver Kepler's 3rd law: p^2 = [4pi^2/(G*M1+M2)]*a^3 G=6.67x10^-11 m^3/kgs^2 3. The attempt at a solution I used the small angle formula to get "a"; at oppositon, a=(7")(1deg/60")(1.5x10^8km)(2pi/360deg)= 3.05x10^5km p=72hrs*(3600sec/hr) (Mplanet+Mmoon)= [4pi^2*a^3]/[G*p^2] Am I doing this correctly? How do I find the mass of the moon since it doesn't state the relative mass is small enough to be negligible. Where does the orbital period of the planet come into play?