Appulse is the least

apparent distance Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle between the two Line of sight, sightlines, or between two point objects as viewed from an observer. Angular distance appears in math ...

between one

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

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, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

s together in the sky, or of the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

to a

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

or planet while the Moon orbits

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, 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 Conjunction may refer to: * Conjunction (grammar), a part of speech * Logical conjunction, a mathematical operator ** Conjunction introduction, a rule of inference of propositional logic * Conjunction (astronomy), in which two astronomical bodies ...

, 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 March equinox to the (hour circle of the) point in question above the earth. When paired w ...

ecliptic longitude The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodi ...

. 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 or ...

(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'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...

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 ...

, or

eclipse An eclipse is an astronomical event that 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 ce ...

, 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