The Moon in conjunction with Venus and Jupiter, with the Very Large Telescope in the foreground. Image © Y. Beletsky, ESO, 2009.
Close approaches of the two planets are called great conjunctions because they are the rarest of all conjunctions between the planets that are visible to the naked eye.
The rarity of great conjunctions is due to the slow motion of Jupiter and Saturn across the sky. Among the planets that are visible to the naked eye, they are the two most distant from the Sun, taking 11.86 years and 29.5 years respectively to orbit it. As the two planets gradually move through the constellations at different speeds, they follow almost the same path across the sky, called the ecliptic. Periodically, Jupiter catches up with Saturn and overtakes it, resulting in a great conjunction, on average once every 19.6 years.
Not all of these great conjunctions are equally dramatic. Sometimes they happen when the planets are too close to the Sun to be observable, as happened in 2000. And at other times they may not pass any closer than five degrees (the width of ten full moons) apart. The great conjunction of December 2020 will be exceptionally favourable, when the two planets will pass within a mere six arcminutes (a fifth of the diameter of a full moon) of each other. This will be the closest approach of the two planets since 1623, and they will not come so close again until their 2080 great conjunction.
Conjunctions vs Appulses
Technically, the word conjunction refers to the moment when the two planets share the same right ascension – or sometimes, ecliptic latitude – though the moment more likely to be of interest to astronomers is when the two planets appear closest in the sky – technically, an appulse.
The table below list the moments of close approach for all great conjunctions between 1950 and 2299. The times may differ slightly from those shown on other websites, which instead catalogue the moment of shared right ascension.
List of great conjunctions
|18 Feb 1961 23:40 UTC||0°13.8'||34.9°||20°56'S||More information »|
|31 Dec 1980 15:27 UTC||1°02.8'||90.6°||1°42'S||More information »|
|21 Dec 2020 18:20 UTC||0°06.1'||30.1°||20°28'S||More information »|
|15 Jun 2041 23:44 UTC||4°19.5'||112.5°||4°19'S||More information »|
|07 Apr 2060 23:32 UTC||1°07.4'||41.9°||18°25'N||More information »|
|15 Mar 2080 01:39 UTC||0°06.0'||43.5°||18°03'S||More information »|
|18 Sep 2100 22:37 UTC||1°12.9'||29.5°||7°14'S||More information »|
|15 Jul 2119 21:04 UTC||0°57.0'||38.1°||20°47'N||More information »|
|14 Jan 2140 16:03 UTC||0°14.5'||23.3°||17°11'S||More information »|
|20 Dec 2159 23:19 UTC||1°11.8'||50.9°||11°14'S||More information »|
|04 Oct 2178 15:23 UTC||6°11.1'||109.6°||21°22'N||More information »|
|07 Apr 2199 22:59 UTC||0°25.0'||49.9°||14°02'S||More information »|
|07 Sep 2238 13:26 UTC||0°39.4'||68.1°||22°21'N||More information »|
|23 Mar 2239 04:38 UTC||0°45.3'||89.7°||22°38'N||More information »|
|06 Feb 2279 09:14 UTC||1°09.6'||81.2°||16°05'S||More information »|
|22 Mar 2299 18:52 UTC||7°01.6'||105.4°||22°37'N||More information »|