1. The problem statement, all variables and given/known data There is a planet of mass m_1 orbiting a star of mass m_2. One question is "What is the semimajor axis of the planet's orbit in the coordinate system centered at the star's center", and another is "What is the semimajor axis of the planet's orbit centered at the center of mass of the system. 2. Relevant equations t^2/A^3 = 4pi^2/G(m_1+m_2) ^Kepler's Third Law 3. The attempt at a solution Basically, as far as I can visualize this problem, the semimajor axis does not change when you switch the center of the system. The planet still has to make the same orbit, so the center of said orbit should still be the same. I have an answer, but I can't tell if I'm somehow supposed to modify it for one of these scenarios.