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.
2 April: Moon and Antares
In the morning sky on 2 April, the waning gibbous Moon lies above left of red star Antares in Scorpius. Because this occurs at the start of the month, about 27 days later (i.e. the orbital period of the Moon) the Moon will lie to the upper right of Antares on 29 April. On that date it will lie between Antares and the next brightest star in Scorpius, Graffias or Beta Scorpii.
6, 7 April: Moon, Saturn, Jupiter
Spot the waning crescent Moon below Saturn on 6 April, then Jupiter on 7 April in the early morning twilight, 1 hour to 40 mins before sunrise over in the southeast.
22 April: Lyrid Meteor Shower
The Lyrid meteor shower peaks in daylight, so with a Moon present in the sky it is best to view on the evenings of 21 and 22 April.
25 April: Venus and Mercury
From 25 April, Venus and Mercury make an appearance low in the northwest to west-northwest about 40 minutes after sunset. Try to catch both inner planets together in the evening sky.
In the early hours of 30 April the Moon occults mag 3.2 Theta Ophiuchi. If you enjoy getting up early (or staying out late!) then use binoculars or a small telescope to watch for the disappearance on the bright limb, then a little later on the reappearance on the dark limb.
Don’t be confused with a fainter star appearing first on the dark limb, as this will be a mag. +6 star! For more info, read our guide to occultations.
Other conjunctions in April 2021:
4 April Last Quarter moon lies close to Nunki, Sigma Sagittarii (morning)
13 April Crescent moon lies to left of Uranus (evening twilight)
18 April Moon lies to upper left of Mebsuta, Epsilon Geminorum (evening)
19 April Moon lies to the left of Pollux and close to Kappa Geminorum (evening)
20 April First Quarter moon lies in Cancer above Beehive cluster (evening)
22 April Moon lies to right of Sickle asterism and Regulus (evening)
25 April Moon lies to right of Porrima in Virgo (morning) / Moon lies above Spica (evening)