Occulations

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. 

Occultations are regular, predictable events because planets, dwarf planets, moons, and asteroids orbit the Sun and pass in front of stars in the background.

The term occultation is most frequently used to describe lunar occultations. These are not uncommon occasions when the Moon passes in front of a star, planet, or other heavenly body during the course of its orbital motion around the Earth. Occultations of bright stars and planets typically occur a few times per year, often clustering with several occultations of the same object in successive months, since the Moon traces roughly the same path across the sky each month.

But occultations also occur when a moon passes another moon, or a planet passes another planet. Mutual occultations or transits of planets are extremely rare. The most recent event occurred on 3 January 1818, and the next will occur on 22 November 2065. Both involve the same two planets: Venus and Jupiter.

Saturn occultation CC BY-SA 2.0 at, Link

A grazing occultation of Rhea (a large moon of Saturn)
by another of its moons, Dione. Image: assini–Huygens