Occultation

An occultation is an astronomical event observed when a non-luminous body passes between the observer and a more distant body which is either luminous or shining by reflected light. If the more distant body is partially or intermittently concealed, then the event is termed a grazing occultation.

The term is most frequently employed to describe those, relatively frequent, occasions on which the Moon passes in front of a star. To a pedant, a total solar eclipse is an occultation of the Sun. The distinction is more important in contexts such as the system of Jovian satellites. In this case a satellite may be eclipsed (invisible because Jupiter's shadow prevents any sunlight falling on it) or occulted (hidden from us because Jupiter lies on our line of sight).

There are three first magnitude stars that are sufficiently close to the ecliptic that they may be occulted by the Moon and by planets: Regulus, Spica and Aldebaran. The first magnitude star Antares is somewhat further from the ecliptic and according to some sources can undergo occultation on very rare occasions.

Stars may be occulted by planets as well as by the Moon. The rings around the planet Uranus were first discovered when it occulted a star in 1977.

Early radio astronomers found occultations of radio sources by the Moon valuable for determining their exact positions, because the long wavelength of radio waves limited the resolution available through direct observation.

In Islam the term is also applied to the disappearance of the Twelfth Imam.