Conjunction (astronomy)

In astronomy, a conjunction occurs when two astronomical objects (such as asteroids, moons, planets, or stars) appear close together in the sky, as seen from a particular viewpoint, usually Earth. It is a line-of-sight effect; the objects are not necessarily physically close to each other in space.

Conjunctions can involve any two celestial objects, but are most commonly used to describe apparent close approaches between planets, or between a planet and another celestial body such as a star or the Moon.

There are two main types of conjunction:

• Inferior Conjunction: This can only occur with inferior planets (Mercury and Venus), when they pass between the Earth and the Sun. An inferior conjunction can be further divided into superior inferior conjunction (when the planet is on the far side of the Sun) and inferior inferior conjunction (when the planet is between the Sun and Earth). During inferior inferior conjunction, the planet may be seen as a transit across the Sun's disk.

• Superior Conjunction: This occurs when a planet passes behind the Sun as seen from Earth. Any planet can experience a superior conjunction.

The term "opposition" is used when two celestial objects appear opposite each other in the sky. This is the situation when an outer planet (Mars, Jupiter, Saturn, Uranus, or Neptune) is closest to Earth and appears brightest, as it is fully illuminated by the Sun.

The term "great conjunction" is used for conjunctions of the planets Jupiter and Saturn. Because of their slow orbital periods, these are rarer, occurring approximately every 20 years. The last great conjunction occurred in December 2020.

The closeness of a conjunction is measured in angular separation, often in degrees or arcminutes. A very close conjunction is often referred to as an "appulse". Conjunctions are predictable events, based on the known orbital parameters of the celestial objects involved. They are often listed in astronomical almanacs and software.