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 – 31 December: Venus

Venus is still viewable in the evening twilight and initially looked as if it was heading for a close encounter with Saturn but instead it drops back into the brightening evening twilight moving away from the ringed planet. Jupiter and Saturn now lie in the SW evening skies whilst Mercury joins Venus for the last week of the year.

1-3 December: crescent Moon

Morning twilight finds the slim crescent close to Spica on 1 December, then Alpha Librae (Zuben elgenubi) on 2 December, then close to Mars as the red planet emerges into the morning sky on 3 December.

6 December: Comet Leonard & Arcturus

Comet A1 Leonard lies near to Arcturus and is best seen in the morning sky with large binoculars or a telescope and may be mag 9 possibly brightening quickly but estimates do vary.

6 & 7 December: the Moon, Venus and Saturn

The crescent Moon lies either side of Venus in the early evening twilight. On 7 December it also lies below right of Saturn.

8 & 9 December: the Moon, Venus, Jupiter and Saturn

On 8 and 9 December the thick crescent moon lies below and either side of Jupiter. On 8 December it forms a triangle with Jupiter and Saturn. That evening the Moon also lies very close to Zeta Capricorni.

16 December: the Moon & the Pleiades

The evening gibbous moon forms a triangle with M45 and Aldebaren/Hyades but also lies near to Ceres (at mag. +7.5 it might be drowned out).

17 December: the Moon & Tau Tauri

The Moon lies close to Tau Tauri and – from some parts of the UK – occults it. Start viewing from about 9:20pm onwards and note that Tau Tauri has a companion star that is occulted first as a bonus.

18 – 31 December: Mars

In morning twilight Mars is slowly improving visually and lies between Beta and Delta Scorpii.

Next morning it lies very close to Omega 2 sco. By 25 and 26 December, Mars lies due north of Antares. The crescent Moon forms a triangle with Mars and Antares on the last morning of the year for a finale!

25 December: planetary parade

The planets are all on parade for us from Christmas day onwards, as Mercury lies below Venus in the bright evening twilight.

We will be able to see Saturn and Jupiter, then later that evening Neptune, Uranus and finally, in the morning sky, Mars. This is a great chance to view all the planets of the solar system through the course of one night!

December Meteor showers

Traditionally the Geminid meteor shower peaks in the early hours of 14 December, so with the Moon up most of the night it is best to look for Geminids just as the Moon is setting around 3am. This allows a few hours of darkness to catch as many as you can.

In 2021 the Geminid meteor shower will be best viewed in the early hours of 13, 14 and 15 December.

The Ursid meteor shower is circumpolar and peaks on 22 December, but moonlight will wash them out.

A rarely mentioned minor shower is the Sigma Hydrids, which peak on 9 December. It has a low ZHR of just 7, but as the Moon sets early on in the evening, they may be worth looking out for. Just don’t get them confused with any early Geminids!

More conjunctions in December 2021

• 10 December: The Moon forms a line with Psi Aquarii and Neptune (evening)
• 14 December: Moon lies to right of Uranus (evening)
• 18 December: Nearly Full Moon lies between the horns of Taurus (evening)
• 20 December: Moon lies close to Mebsuta, Epsilon Geminorum (morning)
• 20 December: Moon forms triangle with Castor and Pollux (evening)
• 24 December: Moon lies above Regulus and occults Eta Leonis (morning)
• 26 December: Moon lies close to Nu Virginis (morning)
• 27 December: Last Quarter Moon forms triangle with Zaniah and Porrima (morning)
• 28 December: Moon lies above Spica (morning)

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.