Appulse is an astronomical term that refers to the closest approach of one celestial object to another, as seen from a third body.  Usually it refers to the close approach of two planets together in the sky, or of the Moon to a star or planet as the Moon follows its monthly orbit around Earth, as seen by an observer located on Earth.
An appulse is related to a conjunction but the definitions differ in detail. Whereas an appulse occurs when the separation between two bodies is at its minimum, a conjunction occurs at the instant when the two bodies have the same right ascension or the same ecliptic longitude. In general, the precise time of an appulse will be different to that of a conjunction.  It is possible in some particular cases for an appulse to occur but no conjunction.
Where the celestial bodies come so close together that one actually passes over the other, the event is known as an occultation.
An appulse is an apparent phenomenon caused by perspective only: there is no close physical approach in space between the two objects involved and scientists insist that appulses have no direct effect on the Earth. They can be interesting naked-eye events for general observers when they involve bright planets and the Moon.
- Her Majesty's Nautical Almanac Office and United States Naval Observatory (2012). "Appulse". Glossary, The Astronomical Almanac Online. Retrieved 2012-07-08.
- Jean Meeus (1991). "Astronomical Algorthms". Astronomical Algorthms (Willman-Bell Inc., Richmond, Virginia).
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