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.
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.
You can also sign up to the BBC Sky at Night Magazine newsletter for weekly updates on what to see in the night sky.
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.
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.
26, 27 September: Moon, Pleiades and Tau Tauri
On 26 September, look for the Moon in the morning sky from midnight to 4am BST, as it first lies below M45, the Pleiades star cluster, then on 27 September it lies close to Tau Tauri.
3 October: the Moon and Regulus
Look for the crescent Moon above Regulus in the morning sky around 4am onwards as northern parts of England and all of Scotland will see a near miss of Eta but anywhere south of that will see an occultation of the star by the northern part of the Moon. Binoculars or better still, a telescope, will be very useful for this.
8 October: Draconid meteor shower
Tonight the Draconid meteor shower is at its peak and with the Moon a crescent setting early on, as the radiant is circumpolar then meteors may be spotted through the night. Zenithal hourly rate is low at 5 meteors, but outbursts are known to occur for this shower so it’s definitely worth keeping a look out for them.
9, 10 October: Venus and the Moon
Back to the evening twilight and we find Venus is still clinging on as we look towards the south-west about half hour after sunset. The crescent Moon lies above right of Venus on 9 October then it’s off to the left on 10 October. Meanwhile, Antares is faint against the twilight but might just be spotted with care.
10, 11 October: Uranus and Omicron Arietis
Look towards the south-east to south, high up, as Uranus will lie above Omicron Arietis (mag. +5.7) which is a tenth of a magnitude fainter than Uranus. Binoculars and small telescope will help and Uranus may appear slightly greenish in colour.
13 October: the Moon and Saturn
Time for a naked eye view in the early evening as the first quarter Moon lies to the lower right of Saturn, which can be seen on the right-hand side of Capricornus.
The next evening the Moon then forms a triangle with Saturn and Jupiter, which lies above Gamma Capricorni with Delta cap to its left. The next evening, 15 October, the Moon then lies t the lower left of Jupiter with the two stars between them.
15 October: Mercury
Mercury returns to the morning twilight, best morning apparition lasting into November. The innermost planet starts off faint, then gradually brightens reaching greatest elongation west from the Sun on 25 October when it lies 18 degrees from it.
16 October: Comet 67P
Now for a challenge: Comet 67P/ Churyumov-Gerasimenko (made famous by the European Space Agency’s Rosetta mission) is faint at mag. +10 but lies close to M35 in the morning sky, so if you have a telescope then it’s worth either viewing or photographing this conjunction for a great picture of comet and cluster in the sky together.
21 October: the Moon, Uranus and the Pleiades
Look for the Moon a day past full when it lies between Uranus and Mu Ceti in the late evening. Next evening it lies to the lower right of the Pleiades, then forms a triangle with the cluster and Aldebaran on 23 October before passing between the horns of Taurus on 24 October.
22 October: Orionid meteor shower
The Orionid meteor shower peaks this evening, although it will be washed out by light from the Moon.
28 October: the Moon, Castor and Pollux
We always like line-ups in the sky, and on the morning of 28 October the Moon is in a line with Castor and Pollux. Spot it with the naked eye from midnight onwards.
30, 31 October: the Moon, Eta Leonis and Regulus
If an event occurs at the start of the month there is a good chance something similar will happen again at the end of the same month. The Moon lies to the right of Eta Leonis and above right of Regulus on 30 October, then lies to the left of them the next morning. Look from around 2am onwards both mornings with the naked eye.
More conjunctions in October 2021
- 10 October Southern Taurids Meteor shower peak (favourable but low rates)
- 16 October Moon lies close to Tau Aquarii (evening)
- 17 October Moon forms triangle with Neptune and Psi Aquarii (evening)
- 26 October Moon lies to left of Mebsuta, Epsilon Geminorum (late evening)
- 29 October Last Quarter Moon lies near Gamma Cancri and Beehive cluster (morning)
Images of astronomical conjunctions
Below is a selection of images of conjunctions captured by BBC Sky at Night Magazine readers and astrophotographers.