Occultations occur when one celestial object passes in front of another celestial object. For example, when the Moon or a planet or asteroid passes in front of a background star, light from the background star is prevented from reaching the Earth. A shadow of the Moon is cast by the star onto the Earth, and this shadow sweeps across the Earth at roughly the same speed as the Moon is moving. A special case of an occultation is a Total Solar Eclipse, in which the Moon passes in front of the Sun, obscuring it from view.

Planetary Occultations occur when planets or minor planets (asteroids) pass in front of background stars. Typically, these events are much shorter than Lunar Occultations because the apparent size of a planet or asteroid is much small than the apparent size of the Moon in our sky. However, some of the most important discoveries in planetary astronomy have been made via planetary occultations – e.g. the rings of Uranus, and the atmosphere of Pluto.