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.

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.

30 May: Mars, Jupiter, Saturn

The major planets lie in the morning sky and although Mars and Jupiter were in conjunction on 29 May, they are still very close together on the morning of 30 May, with Mars to the lower left of Jupiter.

The skies are much lighter now so view from 3am and pick Saturn first, then minor planet Vesta which is viewable with binoculars. Then Neptune, which you must get before the sky gets too light.

Jupiter and Mars are next and as we get to around 3:40 look very low over towards the east-northeast for Venus peeking above the horizon.

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.

2 June: Moon, Castor & Pollux

Look around 22:30 - 23:00 this evening for the crescent Moon forming a right angled triangle with stars Castor and Pollux. Look out for earthshine on the ‘night’ side of the moon as a bonus - it looks quite ethereal.

Castor and Pollux are the Twins of Gemini but there is a second pair of stars to their right: Alpha and Beta Aurigae, better known as Capella and Menkalinan. They are a little more spaced apart.

3 June: Moon, Beehive & Maria

Again at around 22:30 have a look at the crescent Moon, which lies between Pollux and Messier 44, the Beehive cluster. It's a little thicker tonight so you can use binoculars to look out for some of the lunar maria - or lunar seas.

Mare Crisium is on the bright limb. Another darker mare lies below it: Mare Fecunditatis (Sea of Fecundity or Fruitfulness).

Note the crater Messier which has a nice set of rays off to one side. It's worth using a telescope to spot this if you can.

5 June: Moon, Eta Leonis, Regulus

This evening the much thicker crescent moon lies to the lower right of Eta Leonis with Regulus to the lower left of them.

Algieba (Gamma Leonis) lies above Eta, so we have Algieba - a telescopic double star - and Regulus and its companion visible in binoculars as a nice bonus to conclude this week's sights.

6 June: major planets

Saturn, asteroid Vesta, Neptune, Jupiter, Mars and Venus remain in the morning sky with the gap increasing between Jupiter and Mars. So this is a great week to focus on the evening sky and the moon.

7 June: Moon & craters

The Moon is at first quarter and lies in eastern Leo, directly below Denebola (Beta Leonis) this evening. The terminator is near the prominent craters of Tycho and Clavius, both of which will become better to view with a small telescope over the coming days.

8 & 9 June: Moon & Porrima

The Moon lies either side of the close double star Porrima in Virgo.

9 & 10 June: Moon and Spica

The Moon lies either side of Spica (Alpha Virginis).

11 June: Moon & Zubenelgenubi

Look for the Moon very close to Zubenelgenubi (Alpha Librae), the leading star in the constellation Libra. This is another nice double star that can be split using binoculars.

12 June: Moon, Dschubba, Graffias, Antares

This evening the gibbous moon forms right angle triangle with Dschubba and Graffias in Scorpius, with Antares to their lower left.

13 June: Moon & Antares

We start the week with the almost full Moon just to the left of Antares, the heart of Scorpius. Full Moon occurs late next morning with the Moon lying in Ophiuchus, but we see it rise later that evening, by which time the Moon will be in Sagittarius.

14 June: Full Moon

Full moon is poor for crater relief but the best time to view the rays crisscrossing the lunar surface from craters such as Tycho, Copernicus, Kepler and Aristarchus.

It is also known as a Strawberry Full Moon and is a 'supermoon' as the Moon is at perigee on the same day (in fact 11 ½ hrs from Full to perigee).

18 & 19 June: Saturn, Delta & Gamma Capricorni

Saturn is now rising just after midnight and becoming better placed to view. It forms a shallow triangle with Delta and Gamma Capricorni and on the early mornings of 18 & 19 June the waning Moon joins them.

On 18 June the Moon lies just above Zeta Capricorni and to the lower right of Saturn. On the morning of 19 June it lies to the lower left of Saturn.

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.