1. The problem statement, all variables and given/known data This is just a general question. When using Kepler's second law, which radius am I supposed to use to sub into r? Is it the radius of the object (ex. Earth's is 6.38e6 m) or the radius of orbit (ex. Earth's is 1.49e11 m)? 2. Relevant equations C = (GM)/(4pi^2) = (r^3)/(T^2) 3. The attempt at a solution I don't know the answer. I've seen solutions that involved using the radius of the object AND the radius of the orbit but I don't know when to use which. I understand that the answer to this may be painstakingly obvious... but I really would appreciate any responses.