Perturbations from Sun and Moon

The Sun and Moon also pull on satellites. The Earth's equator and hence also the orbits of geostationary satellites are inclined by 23 deg in respect to the orbital plane of the Earth. Therefore the gravity of the Sun changes the orbital plane of geostationary satellites.

Together with the attraction of the Moon this causes the inclination to oscillate between 0 and 15 deg once every 54 years. During this time the inclination vector, which is perpendicular on the orbit plane and shown in green here, draws a complete circle.