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.
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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.
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.
28 May: Venus, Mercury
Venus and Mercury are in conjunction on 28 May. Use Venus as a guide, as Mercury will now be much fainter in the bright evening twilight.
31 May: Moon, Saturn and Jupiter
The Moon lies below Saturn on 31 May but next morning on 1 June it lies below Jupiter.
1 June: Moon, Jupiter and Saturn
Starting with the early morning twilight sky around 3am roughly south, we find the gibbous Moon below Jupiter, it having been below Saturn the previous night.
When conjunctions like this occur at the start of the month, they repeat at the end of the month too, due to the roughly 29.5 days it takes for the phases of the Moon cycle to complete. So, the the Moon lies below Saturn on 27 June, below and between Saturn and Jupiter on 28 June then to the lower left of Jupiter on 29 June.
9 – 12 June: asteroid Vesta
Vesta is faint at mag. +7.3 but gives us a challenge as it passes below the Leo triplet of galaxies. It lies in the bright evening twilight over in the west around 11:30pm to give a little darker sky background.
We like challenges, and this is a double challenge with the faint Vesta near to Messier 65 and Messier 66 which are ~ mag. +9 smudges with the harder to spot (and probably too difficult) NGC 3628.
For the story of Vesta’s discovery, read our history of the so-called Celestial Police.
10 June: partial solar eclipse
Don’t forget about the 10 June solar eclipse. You could say this is a special form of conjunction between the Moon and the Sun! In arctic regions this will be an annular eclipse but in the UK and other regions, viewers will be treated instead to a partial solar eclipse. It’s also quite convenient as the eclipse begins around 10am and lasts until last contact at 12:24 BST.
Remember: don’t look at the Sun without specialist solar glasses or approved filters. Safety is paramount at all times. Projecting an image of the Sun onto a card is quite safe using a small telescope too, meaning you can watch as the Moon’s disc creeps across the Sun. Find out how in our guide to making a solar projection screen.
11, 12 June: Moon and Venus
A very thin crescent Moon will be seen near to Venus. On 11 June the Moon is only 1% illuminated so quite difficult to see in a bright evening twilight sky, about 30 minutes after sunset. As a helpful guide, Venus will lie to the Moon’s upper left. On 12 June the crescent Moon lies to the upper left of Venus, and will be a little easier to spot.
13 June: Moon and Mars
A slightly thicker crescent moon lies close to and above right of Mars for an encore.
22 June: Mars and the Beehive
From 22 June Mars lies on the edge of M44 the Beehive cluster. On 23 June it’s right in the cluster, and then on 24 June it’s on the other edge of M44. This is a really challenging event as the star cluster will be almost lost in the bright twilight. Look for Mars in the west-northwest, low down around 11pm BST and see if you can spot both the cluster and planet.
Mars is now losing the race against the approaching sun and during June drops lower in the evening sky into the ever-brightening twilight. As it does so the distance between it and Venus is shrinking as they head towards a conjunction in July 2021.
The last week of May sees the start of Noctilucent Cloud season, so it’s well worth keeping an eye out towards the north for these ‘Night Shining’ ethereal clouds. For more info, read our guide to Noctilucent Clouds.
More conjunctions in June 2021
- 3 June: Moon lies below Neptune (morning twilight)
- 15 June: Moon is close to Eta Leonis and above right of Regulus (evening)
- 17 June: Moon lies close to Nu Virginis and also forms triangle with Denebola and Vesta (evening)
- 18 June: First Quarter Moon lies close to Porrima (evening)
- 19 June: Moon lies above Spica (evening)
- 21 June: Summer Solstice / Moon lies to the left of Alpha Librae (evening)
- 22 June: Moon lies left of Graffias and Omega Scorpii with Antares to their lower left (evening)
- 24 June: Full Moon lies close to Kaus Borealis in Sagittarius (evening)
- 30 June: Moon lies to lower right of Neptune and close to Psi Aquarii (morning)