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.
There are usually a few beautiful conjunctions to see in the night sky – or early morning sky for that matter – each month, and below we’ll look at some of the best coming up over the next few weeks.
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.
Read more observing guides:
- Deep-sky astronomy: a beginner’s guide
- How to observe and record meteor showers
- How can I stop planets drifting out of my field of view?
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.
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
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.
For more on this, read our guide on how to photograph a conjunction.
Upcoming conjunctions to see in the night sky
Below we’ll look at some fascinating conjunctions coming up over the next few weeks between the Moon, planets and stars.
10 January 2021: Jupiter, Saturn, Mercury
Fancy a real challenge? Look for a trio of planets in the bright evening twilight on 10 January shortly after sunset towards the south-west horizon. Here we find Jupiter, Saturn and Mercury, but will the fainter two be visible? Use naked eye and binoculars to see this conjunction, but to be truly safe make sure the Sun has set first.
11 January 2021: Venus, crescent Moon
An early morning challenge now. Catch a slim crescent moon to the right of brilliant Venus low down in the southeastern sky at around 7:30am UTC just before sunrise.
14 January 2021: Crescent Moon, Jupiter, Saturn, Mercury
Four days later the slim Crescent Moon joins Jupiter and Saturn in the evening sky whilst Mercury lies to the upper left of Jupiter. Saturn lies deeper in twilight and may well be lost to view.
21 January 2021: Mars, Uranus
Look in the evening sky towards the south, high up around 7pm UTC for the Moon to the left of Mars and Uranus. Mars and Uranus are in conjunction and so it’s a good time to spot the more distant and fainter Uranus lying below the orange/red Mars. This conjunction can be seen in binoculars. The Moon will be included in the view of 7×50 binoculars, but a pair of 10×50 binos will show Mars and Uranus better.
23 January 2021: The Pleiades and the Hyades
Two evenings later look for the Moon lying between the star clusters M45, the Seven Sisters and the Hyades which have the bright orange star Aldebaran which technically is not a part of the Hyades!
Other conjunctions in January 2021:
3 January 2021 Moon lies to upper right of Denebola and Vesta (late evening)
6 January 2021 Last Quarter moon lies close to Porrima in Virgo (morning)
7 January 2021 Moon lies to left of Spica (morning)
8 January 2021 Thick crescent moon lies above Alpha Librae (morning)
10 January 2021 Crescent moon lies to the left of Antares (morning twilight)
17 January 2021 Crescent moon forms triangle with Neptune and Psi Aquarii (evening)
27 January 2021 Moon forms a line with Castor and Pollux (evening)
28 January 2021 Full Moon lies in Cancer near to Beehive cluster (evening)