Conjunction: what a funny-sounding word it is, yet in the field of astronomy this phenomenon can give us some wondrous night-sky sights, ranging from naked-eye views through to binoculars and even telescopic viewing.
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There are usually a few beautiful conjunctions to see in the night sky - or early morning sky for that matter.
For more conjunctions and stargazing advice, listen to our Star Diary podcast, which reveals what to look out for in the evening and morning skies over the coming weeks.
A conjunction of the moon with Venus and Jupiter, Thailand, 28 November 2019. Credit: Chakarin Wattanamongkol / Getty Images
What is a conjunction in astronomy?
Generally speaking a ‘conjunction’ is the name given to two or more celestial objects close together in the night sky.
The most commonly observed conjunctions involve the Moon, often as a crescent in the evening or morning sky, along with any of the bright planets – Venus, Mercury, Mars, Jupiter or Saturn.
You can also see conjunctions between the Moon and bright stars or even between the planets themselves, so there is quite a range of possible combinations.
Some involve more than two objects, such as when two planets are in conjunction and are joined by the Moon.
There are also times when incredibly close conjunctions set two objects in the same telescope field of view, or in really special cases, show Venus or Mercury transit across the face of the Sun.
You may already have come across the term in astronomy guides, yet if we went by its strictest definition then some events called conjunctions would probably not qualify.
A conjunction of the Moon, Venus, Mars and Spica, Azul, Argentina. Credit: Stocktrek Images/Luis Argerich/Getty
To be precise: a conjunction is a line-up of at least two celestial objects in the sky relatively close together that share the same Right Ascension (RA) or ecliptic longitude in the sky.
RA is the equivalent of longitude on Earth but projected onto the celestial sphere
The ecliptic is the plane of Earth’s orbit and appears to us as the apparent path of the Sun across the sky. Ecliptic longitude is measured along the ecliptic eastwards from the spring equinox
Even within astronomy there are different meanings of the word conjunction.
When a planet, either outer or inner, lies on the other side of the Sun to Earth it is said to be at superior conjunction
When an inner planet lies between Earth and the Sun it is at inferior conjunction
Tony Titchener captured this beautiful image of a crescent Moon and Venus on 27 February 2020. Tony captured it on 27 February 2020 from Seaford, Sussex, UK using a handheld Nikon Coolpix 520 bridge camera. Credit: Tony Titchener
When do conjunctions occur?
Often a conjunction will occur during daytime or when the objects are below the horizon, and this is where the definition becomes more relaxed.
If the objects are very bright, such as a crescent Moon and Venus, then daylight viewing can be possible, but if the objects have set below the horizon they won’t be visible.
So conjunction can be applied in quite a loose context to refer to objects that are viewable above the horizon in twilight or at night, even if they are not, at that point, at the exact moment of conjunction.
If the objects are at their closest, then this is known as an appulse: the minimum separation between two bodies that occurs just before or after true conjunction.
Conjunctions really capture our attention, which makes them ideal targets for public stargazing events, or for inspiring young astronomers and newcomers to look up at the night sky.
They are also easy to capture with a smartphone camera, giving more people the chance to preserve the moment and share with friends or on social media.
Mars Jupiter Conjunction by James Robertson, Lake District, UK. Equipment: Zwo asi120mc-s, Canon zoom lens at 18mm
Moon, Venus, Jupiter and Mars Conjunction 8 Oct 2015 by Peter Louer, Tenerife. Equipment: Canon 700D 18-55mm Lens
Moon, Venus, Jupiter & Mars Conjunction 10 Oct 2015 by Peter Louer, Tenerife. Equipment: Canon 700D, Samyang 10mm Lens
Moon & Co by Jonathan, London. Equipment: Sony
Planetary Trio Conjunction Venus Jupiter Mars by Martin Marthadinata, Surabaya, E. Java, Indonesia. Equipment: Nikon D5000, 50mm lens
Venus Mars and Jupiter by John Brady, W. Lancashire, UK. Equipment: Canon Powershot SX60 IS
Jupiter, Venus, Mercury by Colin Brumfitt, Moreton Beach, Wirral, Merseyside, UK. Equipment: Sony a100, tripod.
Venus, Jupiter, Moon & Mercury by Richard Sass, Cloudcroft, New Mexico USA. Equipment: Nikon D-40, 18mm lens, Tripod
Jupiter & Moon by Brian.M.Johnson, Hove, UK. Equipment: Canon 50D.
Crescent Moon & Jupiter Close Conjunction by Anthony Burley, Redditch, Worcs, UK. Equipment: Skywatcher 150P, Nikon 5100
Crescent Moon & Jupiter Close Conjunction by Anthony Burley, Redditch, Worcs, UK. Equipment: Skywatcher 150P, Nikon 5100
The Moon and Jupiter Meet by Steve Jarvis, Churwell, Leeds, UK. Equipment: Olympus E410, Celestron 80mm Travelscope.
Christmas Conjunction: Jupiter & Moon by André Gonçalves, Vieira do Minho, Braga, Portugal. Equipment: SkyWatcher 80ED, Canon 1000D
Moon and Jupiter by Humberto Cecim, Brazil. Equipment: 114/1000 Reflector Greika, Fujifilm Finepix S2800 HD
Occultation of the Moon and Jupiter by Alastair Willis, Shire of Augusta, Margaret River, Western Australia. Equipment: Nexstar 8se, Olympus FE-100.
46 Hour Old Moon and Jupiter by Brian R Bugler, Worth Matravers, Dorset, UK. Equipment: Canon 5D mk II, 70-200 f2.8L zoom lens.
Moon with Jupiter by Philip Pugh, Chippenham, UK. Equipment: Nikon D3200, tripod
Moon-Jupiter Conjunction by John Bell, Haversham, Milton Keynes, UK. Equipment: Canon 5D mk2, 200mm f2.8 lens.
Conjunction of Waning Moon and Jupiter by Sarah & Simon Fisher, Bromsgrove, Worcestershire, UK. Equipment: Canon 600D, 300mm
Conjunction of Jupiter and Waning Moon by Sarah & Simon Fisher, Bromsgrove, Worcestershire, UK. Equipment: Canon 600D, 300mm
Moon, Jupiter and Galilean Moons by Steve Brown, Stokesley, N. Yorkshire, UK. Equipment: Canon 600D, 250mm lens, tripod.
Waxing Moon & Jupiter by Sarah & Simon Fisher, Bromsgrove, Worcestershire, UK. Equipment: Canon 600D, 300mm
Jupiter and the Moon in Conjuction by John Foster, Plymouth, UK. Equipment: Nikon D3200, Evostar 102/1000 frac, EQ3 mount.
Planets, Moon and More by Alfredo Balreira, Rio Tinto, Portugal. Equipment: Canon Eos 1000D, Tripod.
Conjunction over Brisbane by Teale Britstra, Brisbane, Queensland, Australia. Equipment: Canon 600D, 18-55mm lens
Venus, Jupiter and the Moon by Graham Green, Brighstone, Isle of Wight. Equipment: Canon EOS 600D, Canon EF 14mm lens, Astronomik CLS CCD filter, AstroTrac.
Sunset, Jupiter, Venus and Crescent Moon by Jenny Budden, Wimborne, UK. Equipment: Nikon D200, 18-200 lens.
The Moon Jupiter & Venus over Tenerife by Peter Louer, Tenerife. Equipment: Canon 700d, 55mm Lens
Moon, Venus and Jupiter Rising by Steve Brown, Stokesley, N. Yorkshire, UK. Equipment: Canon 600D, 250mm lens, tripod.
Jupiter - Venus conjunction and Cookham Dean Church by Roger Palframan, Cookham Dean, Berkshire, UK. Equipment: Nikon D300s, 16-85mm (~40mm).
Jupiter and Venus March 2012 by George Zealey, Effingham, Surrey, UK. Equipment: Sony Alpha A390, Sony 18-75mm lens.
Conjunction by Mohammad Reza Ghorbanzade, Babol, Iran. Equipment: Fuji Film Finepix S4000.
Dawn conjunction of Venus, Jupiter and the Pleiades by Jonathan Green, North Shore, Auckland, New Zealand. Equipment: Canon 60Da, 28-80mm Canon lens.