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An occultation occurs when the Moon covers and uncovers a more distant object such as a planet, star or an asteroid. See the FAQ below for more information about occultations.
Within any one night most objects that can be seen in the night sky do not change their relative position to each other. The pattern they form will remain steady throughout the night. The Moon is relatively close to us - only 400,000 kilometers (240,000 miles) from Earth and it revolves around the Earth about once a month. Although the Moon also rises and set almost the same as the stars, it does change its position relative to the stars and the planets.
As the Moon rotates around the Earth, moving across the background of the stars and planets, it will occasionally cover up a planet and then uncover the same planet. This "immersion" and "remersion" of the planet (or a star) is an occultation.
In order to see an occultation, but the planet or star, the Moon's edge and you must be in a straight line. Generally there is a path on the Earth from which a particular occultation can be seen. Software is available to do these calculations. For an occultation to be determined within a few seconds, it is necessary to know your location to a high degree of accuracy. SkyWatch provides warning of occultations of Planets to its subscribers. Once the time of an occultation is known, it's easy enough to see it because it will take place at the Moon's edge.
Any object that lies in the Moon's path and beyond the Moon may be occulted. Fortunately, most of the interesting objects within the Solar System are along the ecliptic - a line indicating the path of the Sun, Moon and the planets. The Moon will not cover up and cause an occultation of a planet or star ever time it revolves around the Earth, because sometimes the Moon will the object and be above or below the object's path.
The timing of occultation provides important information about the structure of the Solar System and the location of stars. Amateur astronomers have provided a valuable service to the professionals by making accurate timing of these events.
Others just like to see a star disappear or reappear on schedule. If the Moon is not full, the star will simply disappear at the correct time. At worse occultation can be used to impress your friends. At best it will increase your understanding on how the universe actually works.