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 in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
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 ...
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 ...
to a star
A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...
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 surf ...
, 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
Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle between the two sightlines, or between two point objects as viewed from an observer.
Angular distance appears in mathematics (in par ...
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 pai ...
or 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 b ...
. 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'' (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
See also
*Opposition (astronomy)
In positional astronomy, two astronomical objects are said to be in opposition when they are on opposite sides of the celestial sphere, as observed from a given body (usually Earth).
A planet (or asteroid or comet) is said to be "in oppositio ...
* Syzygy (astronomy)
In astronomy, a syzygy ( ; ) is a roughly straight-line configuration of three or more celestial bodies in a gravitational system.
Overview
The word is often used in reference to the Sun, Earth, and either the Moon or a planet, where the lat ...
References
{{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Astrometry Astronomical events of the Solar System