The orbit of the Moon is, while stable, highly complex, and as such still studied by lunar theory. Most models describe the Moon's orbit geocentrically, but while the Moon is mainly bound to Earth, it orbits with Earth, as the Earth-Moon system around their shared barycenter. From a heliocentric view its geocentric orbit is the result of Earth perturbating the Moon's orbit around the Sun. It orbits Earth in the prograde direction and completes one revolution relative to the Vernal Equinox and the fixed stars in about 27.3 days, and one revolution relative to the Sun in about 29.5 days.