The orbit of the Moon is, while stable and known, highly complex, and as such still studied by lunar theory. Most models describe the Moon's orbit geocentrically since the Moon is mainly bound to Earth, but it also orbits together with Earth, as the Earth–Moon system, around their shared barycenter. Furthermore from a heliocentric view its geocentric orbit is the result of Earth perturbing 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.