1. The problem statement, all variables and given/known data From my book-- "In motion of a planet round the sun we have assumed the mass of the sun to be too large in comparison to the mass of the planet. Under such a situation, the sun remains stationary and the planet revolves round the sun. If, however, masses of sun and planet are comparable and motion of sun is also considered, then both of them revolve around their center of mass with same angular velocity but different linear speeds in circles of different radii. The center of mass remains stationary." 2. Relevant equations 3. The attempt at a solution Please explain the bold lines. Why do they revolve with the same angular velocity? Why about the centre of mass? Why does the centre of mass remain stationary? I get that the acceleration of centre of mass should be zero because there is no external force. But why does the COM itself remain stationary?