An “occultation” is defined as the complete or partial obscuration of an astronomical object by another. A solar eclipse is, strictly speaking, an occultation.

Lunar occultations

When the Moon passes in front of a star (or occasionally a planet or asteroid), a total occultation occurs. On average, every 44 days a star brighter than 1st magnitude is occulted, whilst every two hours a star brighter than 7th magnitude is occulted. Stars always disappear at the eastern edge of the Moon (away from Mare Crisium) and reappear up to more than an hour later at the western edge. Stars disappear almost instantly; planets fade gradually over a few seconds. Double stars can disappear stepwise: first dimming as one component is covered, then vanishing completely a fraction of a second later (several double stars have been discovered in this way). The Sky Guide lists predictions for lunar occultations of stars, planets and asteroids, for the year.

When a star moves tangentially to the limb of the Moon, and is occulted for a very short period only – a few minutes, or even seconds – a grazing occultation is said to occur. Because the limb of the Moon, as seen from the Earth, is in fact the outline of numerous mountains and valleys, there may be several disappearances and reappearances, which are not only fascinating to observe, but which also may be accurately timed to yield valuable data on the relative positions of star and Moon, as well as on the shape of the Moon. Some of this data cannot readily be obtained in any other way. Details of grazes are available from the Director of the Shallow-Sky Section.

Minor planet occultations

Occasionally, an asteroid may move in front of a star as seen from Earth. Such an event is of great interest since the size and shape of the asteroid can then be measured. The duration of these occultations rarely exceeds 30 seconds. If the minor planet has a moon, it may be seen (or even discovered) during such an occultation. Predictions for occultations of stars are given in the Sky Guide.

Planetary occultations

As seen from Earth, the occultation of one planet by another happens, on average, once every 33 years. The previous mutual occultation was in 1818 (Venus occulting Jupiter) and the next event will be in 2065 (also Venus and Jupiter).