Occultation (redirected from occult)

occultation (ŏk'əltā`shən), in astronomy, eclipse of one celestial body by another, e.g., when the moon lies between a star and the earth. Occultations of stars by the moon are important in astronomy. Since stellar positions are very accurately known, the time and position of an occultation can be used to determine the position of the moon. Alternatively, an observer can determine his or her longitude by comparing the time at which he observes an occultation with a table listing the universal time at which the occultation occurs.

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occultation [‚ä·kəl′tā·shən]

(astronomy)

The disappearance of the light of a celestial body by intervention of another body of larger apparent size; especially, a lunar eclipse of a star or planet.

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Occultation 

in astronomy, a phenomenon in which to an observer on earth one celestial body is hidden by another. Occultations of stars and planets by the moon as it travels around the earth are encountered most frequently. The occultation of the sun by the moon is called a solar eclipse. The term “occultation” is also sometimes used to refer to a transit, in which a heavenly body passes across the larger, and more distant, visible disk of another heavenly body, for example, the transits of inferior planets across the sun’s disk and the transits of planetary satellites across the disks of the planets themselves. With the development of spaceflight and new methods of observation, the concept of occultation has been expanded to include occultations of radio emission sources in space and of celestial bodies by the earth, as observed from space.

In the occultation of stars by the moon—the most frequently observed occultation—the moments at which a star appears and disappears at the lunar limb are detected to within ±0.01 sec by means of photoelectric instruments. Results from many years of observation of the occultation of stars by the moon at various observatories are used to refine the theory of the moon’s rotation about the earth and to study fluctuations in the earth’s rate of rotation about its axis. The latter study is necessary in order to make ephemeris time corrections in studying irregularities of the lunar limb. Observations of the transit of planets across the sun’s disk have made it possible to detect and study the atmospheres of the planets. Radio-astronomical methods of studying occultations of radio emission sources in outer space by bodies of the solar system make possible descriptions of the structure of radio sources.

REFERENCE

Mikhailov, A. A. Teoriia zatmenii, 2nd ed. Moscow, 1954.

V. V. PODOBED