Conjunction

Definition

Two bodies are in conjunction when they have the same ecliptic longitude, or in some almanacs the same right ascension, as seen from the Earth. They then appear close together in the sky, although they may be at very different distances. Meeus uses longitude for conjunctions with the Sun and right ascension for conjunctions between planets; the instants differ by up to a few hours.

Conjunction does not mean that the bodies touch or line up exactly: at conjunction, one usually passes north or south of the other, by an amount given by their difference in latitude or declination. The smallest angular distance, which can occur a little before or after the conjunction itself, is called an appulse. If one body actually passes in front of the other, it is an occultation or, for a planet crossing the Sun, a transit.

Conjunctions with the Sun

A planet in conjunction with the Sun is lost in its glare, often for several weeks. Inner and outer planets behave differently:

• Inferior conjunction: Mercury or Venus passes between the Earth and the Sun. The planet is at its closest, shows its dark side and changes from evening to morning sky.
• Superior conjunction: Mercury or Venus is behind the Sun, at its farthest, fully lit but invisible.
• Outer planets (Mars to Neptune) only have conjunctions behind the Sun, opposite to their oppositions.

Conjunctions with the Sun matter for spaceflight too: when Mars is behind the Sun, about every 26 months, radio contact with Mars probes is degraded and mission teams pause commanding for about two weeks.

Conjunctions between planets

Planets all move close to the ecliptic, so they regularly pass each other in the sky. Fast inner planets, Venus especially, have several conjunctions a year with the others. Bright pairings low in the twilight, sometimes with the crescent Moon, are among the most photographed sky events.

The rarest are the great conjunctions of Jupiter and Saturn, which recur every 19.86 years, the synodic period of the pair. On 21 December 2020 the two giants came within 6.1′ of each other, less than a fifth of the Moon's diameter: the closest since 1623, and the closest observable since 1226. The next great conjunction, on 31 October 2040, will be wider, a little over 1°.

How Meeus computes conjunctions

For a conjunction between two planets, Astronomical Algorithms tabulates the geocentric right ascensions of both bodies over five days and interpolates the instant when the difference is zero, together with the difference in declination at that moment.

For conjunctions with the Sun, oppositions and greatest elongations, Meeus gives a faster method: a mean instant from the planet's synodic period, then periodic corrections:

JDE0 = A + B · k        (k integer, B = synodic period in days)JDE = JDE0 + periodic terms in M and T

The tables give A and B for each planet and event, so that inferior and superior conjunctions of Mercury and Venus or conjunctions and oppositions of the outer planets can be dated within a fraction of a day over several thousand years, without computing any positions.

Conjunctions on AstroAlgos

In the all-sky disk and the night-sky view, set the date of a conjunction from the table above and the two bodies appear side by side. The heliocentric view shows why: at inferior conjunction, Venus and the Earth are on the same side of the Sun; at superior conjunction they are on opposite sides.

Explore it on AstroAlgos

• All-sky disk — see planets pairing up in the sky
• Heliocentric view — the geometry of a conjunction
• Opposition — the opposite configuration
• Elongation — angular distance from the Sun