Astronomy conjunctions: upcoming events and how to see them

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.

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.

1-2 May: Jupiter & Venus, Mars & Saturn

On 1 Jupiter and Venus are in conjunction and just 22 arc minutes apart but from 2 May they quickly part company. Including Mars and further away Saturn, there are still four planets in the morning twilight to look out for.

Venus is dropping back towards the Sun, but Jupiter continues to improve as it climbs a little higher each passing morning.

Look for them near to the east with Mars in the east-southeast and Saturn roughly southeast at around 4:40am

2 May: Moon, Mercury, Pleiades

The slim crescent Moon lies next to Mercury, with the Pleiades, M45, to Mercury’s right in evening twilight. Look west-northwest around 9:30pm BST.

Over the week Mercury moves away from the Pleiades but both are dropping lower into the ever-brighter evening twilight.

3 March: Mare Crisium

OK so this one isn't a conjunction as such, but if you happen to have your telescope out, there's a great opportunity to see Mare Crisium, the Sea of Crises, around 3 and 4 March, a day after new Moon.

On the 3 March the terminator bisects the sea and then next evening it is fully illuminated.

The sea has a dark floor with several craters that catch the eye. One of these could be of interest to fans of Star Trek as there is a crater called Picard…no, not Jean-Luc but interestingly it is named after Jean Picard, 17^th century French Astronomer, a member of the Academy of Sciences in 1666.

6 May: Eta Aquarids

On the morning of 6 May the Eta Aquariid shower peaks. The Moon is in the evening sky so won’t interfere, but although this shower is better seen from the southern hemisphere it is worth looking out for any meteors before dawn.

9 May: Moon, Regulus, Eta Leonis 

The moon is at first quarter this evening and lies above Regulus and close to Eta Leonis. The stars of Leo in this part of the sky look like a backwards question mark and are also collectively known as the Sickle asterism. Check out Gamma Leonis, Algieba as it is a nice golden yellow pair of stars just under 5 arc sec apart.

11 May: Vesta, Saturn, Delta Capricorni, Neptune

Fancy a challenge? Check out the spread of bright planets from Venus to Jupiter to Mars and Saturn in the morning twilight, around 4:11am.

They are not the challenge however: Vesta lies below left of Saturn forming a triangle with Delta Capricorni. For a greater challenge, look for Neptune between Jupiter and Mars.

Both will require binoculars and maybe even a telescope to pick out in the twilight.

13 May: Lunar occultation of Porrima

In the early hours look for the Moon gradually creeping closer to Porrima, Gamma Virginis. By 1:49am it will occult the star for just under an hour with reappearance occurring around 2:42am.

Aways look at least 15 mins before and after the stated times, as times do vary across the UK.

Binoculars or better yet a telescope is best for this, the latter useful as Porrima is a close double star. See if you can spot each component being occulted.

14 May: Lunar occultation of Lambda Virginis

If you like watching occultations then on the evening of 14 May around 10:30pm onwards, watch as Lambda Virginis is occulted. Reappearance occurs around 11:33pm but again, check 15 mins before and after to make sure you don’t miss the start and end.

16 May: Total lunar eclipse

We kick off the week with a lunar eclipse. The Moon is in the morning sky and just reaches total lunar eclipse as it is setting so you will need a clear, uncluttered southwest horizon.

The Moon enters the penumbra at 02:32am, enters the darker umbra part of Earth's shadow at 03:27am and is total at 05:11am. Moonset varies across the UK so some places will see the moon set before it reaches totality.

For more info, read our guide to the 16 May total lunar eclipse.

18 May: Mars, Neptune, Jupiter, Saturn, Venus

Time for another attempt at Neptune in bright morning twilight. Mars and Neptune are in conjunction with Jupiter to their left. Use binoculars to see if you can spot Neptune to the upper left of Mars around 4am over in the east. Saturn is to their right whilst Venus is now getting quite low and lies to the left of Jupiter.

22 May: Moon, Saturn

The last quarter moon lies below Saturn on 22 May to end the week. Look around 4am for them along with Mars, Jupiter and Venus before the Sun rises.

25/26 May: Moon, Mars, Jupiter, Venus

We begin the week once again in the morning sky as this is where the main bright planets currently reside. Look around 4am to catch the Moon on 25 and 26 May, as firstly it lies below Mars and Jupiter to form a nice triangle on 25 May, then it forms a shallow triangle with Jupiter and Venus on 26 May.

27 May: Moon, Venus

This morning the very slim crescent moon lies just below Venus, very low above the horizon around 4am. Look out for earthshine too: this is when you can see the night side of the moon as well as the bright daylit crescent side.

29: May: Mars, Jupiter

Mars and Jupiter will be just over 0.5° apart in the morning sky: effectively you could just about fit the Moon between them. Look towards the east to east-souteast sky around 4am for this gorgeous view. Binoculars will be good as well as a telescope, but you can also view this with the naked eye.

End of May: NLCs

The end of May marks the start of noctilucent clouds season so keep an eye on the northeast to northwest horizon for these night shining clouds. They appear silvery blue and appear roughly an hour or so after sunset towards the northwest horizon.

Images of astronomical conjunctions

Below is a selection of images of conjunctions captured by BBC Sky at Night Magazine readers and astrophotographers.

For advice on imaging the night sky, read our guide to astrophotography, and don't forget to send us your images or share them with us via Facebook, Twitter and Instagram.