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OCCULT - v.t. Conceal, cut off from view by passing in front, (usu.
Astron., of concealing body much greater in apparent size than concealed body).

OCCULTATION - n. [f. L occultare frequent. of occulere; cf. celare hide]

Occultations occur when one celestial object passes in front of another celestial object. For example, when the Moon 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.

Timing the instant at which an occultation occurs (i.e. the shadow of the occulting body just sweeps across an observer) is one of the most important and accurate measurements amateur astronomers can make. Many important discoveries have been made in this way.

Total Lunar Occultations occur when the Moon passes in front of background stars. They occur every night of the year, and an observer with a small telescope can typically expect to see a hundred or more annually. The exact number will depend on the observerís geographical location, telescope size, and factors such as the brightness of the star and the phase of the Moon.

A Grazing Lunar Occultation occurs when the northern or southern edge of the Moon passes very close to a background star. To us, the star looks like it is just grazing along the edge of the Moon (just like in the graphic at the top of this page). On such occasions the star is alternately hidden behind the mountains on the edge of the Moon, and then reappears in lunar valleys. Someone on the Earth watching the star will see it disappear and reappear as the mountains and valleys of the Moon move past the star.

Planetary Occultations occur when planets or minor planets (asteroids) pass in front of background stars. Such events occur less often than lunar occultations, mainly 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 atmosophere of Pluto.

Although not strictly occultations, occultation observers are also encouraged to time eclipses of Jupiter's four major satellites. In a Jovian Satellite Eclipse one of the satellites of Jupiter will pass into the shadow that Jupiter casts in space. Timing when the last speck of the satellite's light disappears from view provides information about the position of the satellite relative to Jupiter. Observations of these events have materially contributed to the planning of NASA's spacecraft missions to explore the Jovian system.

This page was last updated on 11 March 1999.
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