Appulse is the least apparent distance between one

celestial object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...

and another, as seen from a third body during a given period. Appulse is seen in the apparent motion typical of two

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

s together in the sky, or of the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

to a

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

or planet while the Moon orbits

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, as seen from Earth. An appulse is an apparent phenomenon caused by perspective only; the two objects involved are not near in physical space. An appulse is related to a conjunction, but the definitions differ in detail. While an appulse occurs when the apparent separation between two bodies is at its minimum, a conjunction occurs at the moment when the two bodies have the same

right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...

or ecliptic longitude. In general, the precise time of an appulse will be different from that of a conjunction. Objects which exhibit

retrograde motion Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession ...

(such as planets) occasionally display an appulse event without an associated conjunction event. In these cases, the two objects appear to approach each other, but turn away before reaching a momentary coincidence of right ascension. When the celestial bodies appear so close together that one actually passes in front of the other, the event is classified as a

transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1980 film), a 1980 Israeli film * ''Transit'' (1986 film), a Canadian short film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countrie ...

occultation 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 f ...

, or

eclipse An eclipse is an astronomical event which occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ...

, and not an appulse. Appulses are naked-eye events for general observers when involving bright planets and the Moon. They facilitate finding faint objects when such objects are involved. Very close appulse events provide an opportunity to witness two objects together in the same telescopic field of view, which is an appealing curiosity in amateur telescopy, e.g. Jupiter-Saturn appulse of December 13, 2020.

References

{{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Astrometry Astronomical events of the Solar System

See also

References