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Occultation

In Islam the occulation is the name given to the disappearance of the Twelfth Imam. See also occult (disambiguation).

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An occultation is an astronomical event observed when a non-luminous celestial 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. An occultation of the Sun by the Moon is commonly called a solar eclipse, though that use of the term eclipse can be confusing. For instance, in the system of Jovian satellites, a satellite may be eclipsed (much dimmer because Jupiter's shadow prevents any sunlight falling on it) or occulted (hidden from us because Jupiter lies on our line of sight) or both.

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 Antares. The first magnitude star Aldebaran is somewhat further from the ecliptic and can only be occulted by planets in a few thousand years.

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.

It is also possible for a planet to occult another planet. This last occurred in 1818 and will occur next in 2065.

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.