A more detailed derivation can be done with general elliptical orbits, instead of circles, as well as orbiting the center of mass, instead of just the large mass. This results in replacing a circular radius, r , with the semi-major axis, a , of the elliptical relative motion of one mass relative to the other, as well as replacing the large mass M with M + m . However, with planet masses being so much smaller than the Sun, this correction is often ignored. The full corresponding formula is: