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.
1-3 July: Venus and M44
In the early evening twilight have a go at a quite tricky encounter as Venus lies close to star cluster M44 and passes it on 2 July and 3 July.
It is highly likely that the cluster won’t be visible in the bright twilight but it’s worth looking for it! Mars lies the other side of the cluster to Venus and a bit further away, so if you don’t spot the cluster then see if you can spot Mars.
6 July: Moon and Pleiades
In the early onset of morning twilight look over towards the eater-northeast for the crescent Moon lying below the Pleiades star cluster, M45. If you look around 3am onwards you will see them rise and not long afterwards be joined by the Hyades star cluster and Aldebaran, the eye of the Bull.
8 July: Mercury and the Moon
Mercury moves into the morning sky but is very low in the morning twilight. On 8 July look towards the northeast morning twilight horizon for the crescent Moon with Mercury, roughly 45 minutes before sunrise. Mercury is always brightest at the end of its morning apparition, so it improves before it drops back into the solar glare later in the month.
8-20 July: Outer planets
With Venus and Mars in the evening twilight and Mercury in the morning twilight we find the giant outer planets Saturn, Jupiter, Neptune and Uranus spread out through the night, giving us a chance to view all in one night!
11-12 July: Venus, Mars and the Moon
The slim crescent Moon lies to the right of Venus with Mars to the left of Venus. Then Venus and Mars meet on 12 July for a conjunction where Venus will guide you to the fainter Red Planet for a great visual and photo opportunity. For added interest the slim crescent Moon lies to their upper left.
11-18 July: Neptune
Requiring at least 7×50 binoculars or larger or a telescope, from 11-18 July Neptune lies close to the star HIP 116402 ,which is mag. +7.2, slightly brighter than mag. +7.8 Neptune.
HIP 116402 is the left hand star of a diamond of stars that Neptune passes through over the next few months and this diamond of stars lies below the Circlet asterism of Pisces.
17 July: Pluto
One for a large telescope now, as Pluto reaches opposition and so is viewable all night. It is magnitude 14.3 so needs a good chart and large telescope indeed! It lies in Sagittarius in a quite bland area of sky to the west (right) of M75.
For more info, read our guide on how to observe Pluto.
21 July: Mars, Venus and Regulus
See how long you can spot Mars as it drops deeper into the evening twilight, as Venus moves away to the left from it. On 21 July Venus lies above right of the star Regulus. This star is normally an easy naked eye bright star but deep in the twilight is a different story, so the bright planet may be your only chance to spot Regulus before it follows Mars and is lost to view.
24 July: Moon, Jupiter and Saturn
The Moon is full on the morning of 24 July but later that evening it still looks ‘full’ to the naked eye and now lies below left of Saturn as they rise. Look about 11pm towards the southeast and Jupiter will be above the horizon too. Next evening the Moon lies to the lower right of the giant planet.
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 July 2021
- 1 July Last Quarter Moon forms shallow triangle with Iota Ceti and Neptune
- 4 July Moon lies to right of Uranus (morning)
- 16 July Moon forms triangle with Porrima and Spica (evening)
- 18 July Moon lies above Alpha Librae (evening)
- 20 July Moon lies to upper left of Antares (evening)
- 28 July Moon lies below and a little left of Neptune (morning)
- 31 July Last Quarter Moon lies close to Xi Ceti (morning)