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Concept
Occultation
Summary
An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks from view (occults) an object in the background. In this general sense, occultation applies to the visual scene observed from low-flying aircraft (or ) when foreground objects obscure distant objects dynamically, as the scene changes over time.
If the closer body does not entirely conceal the farther one, the event is called a transit. Both transit and occultation may be referred to generally as occlusion; and if a shadow is cast onto the observer, it is called an eclipse.
The symbol for an occultation, and especially a solar eclipse, is (U+1F775 đźťµ).
The term occultation is most frequently used to describe lunar occultations, those relatively frequent occasions when the Moon passes in front of a star during the course of its orbital motion around the Earth. Since the Moon, with an angular speed with respect to the stars of 0.55 arcsec/s or 2.7 ÎĽrad/s, has a very thin atmosphere and stars have an angular diameter of at most 0.057 arcseconds or 0.28 ÎĽrad, a star that is occulted by the Moon will disappear or reappear in 0.1 seconds or less on the Moon's edge, or limb. Events that take place on the Moon's dark limb are of particular interest to observers, because the lack of glare allows easier observation and timing.
The Moon's orbit is inclined slightly with respect to the ecliptic (see orbit of the Moon) meaning any star with an ecliptic latitude between –6.6 and +6.6 degrees may be occulted by it. Three first magnitude stars appear well within that band – Regulus, Spica and Antares – meaning they may be occulted by the Moon or by planets. Occultations of Aldebaran are in this epoch only possible by the Moon, because the planets pass Aldebaran to the north. Neither planetary nor lunar occultations of Pollux are currently possible, however several thousand years ago lunar occultations were possible.
Official source
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