Stellar occultation

An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy (see below). It can also refer to any situation wherein 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 computer-generated imagery) wherein foreground objects obscure distant objects dynamically, as the scene changes over time.

The term occultation is most frequently used to describe 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 µrad/s, has a very thin atmosphere and stars have an angular diameter of at most 0.057 arcseconds or 0.28 µrad, 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 these occultations to more easily be observed and timed.

The Moon's orbit is inclined to the ecliptic (see orbit of the Moon), and any stars with an ecliptic latitude of less than about 6.5 degrees may be occulted by it. 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. Occultations of Aldebaran are presently only possible by the Moon, because the planets pass Aldebaran to the north. Neither planetary nor lunar occultations of Pollux are currently possible. However, in the far future, occultations of Pollux will be possible, as they were in the far past. Some deep-sky objects, such as the Pleiades, can also be occulted by the Moon.

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