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 some amazing conjunctions coming up in the latter half of 2020, so read on to find out how to view them.
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.
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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
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.
Below we’ll look at some fascinating conjunctions coming up over the next few months between planets, the Moon and stars.
In September 2020 there are several interesting conjunctions, beginning with a close encounter between the Moon and Mars. Because Mars is approaching opposition, it currently appears bright in the sky.
At 04:00 BST (03:00 UT) on 6 September, mag. –1.9 Mars appears due south, and 1.3˚ from an 86%-lit waxing gibbous Moon. As the morning progresses, the apparent distance between both worlds reduces so that by 06:00 BST (05:00 UT) they appear 0.6˚ apart.
If the weather is clear, try to stay with them into the day. At 07:00 BST (06:00 UT) with the Sun up, binoculars or a telescope should still be able to show Mars just 11 arcminutes north of the Moon’s northern limb.
On 14 September, the apparent movement of the Moon will have it located further east and close to the brilliant planet Venus. Shining at mag. –4.0, Venus will be hard to miss in the early hours.
At 05:00 BST (04:00 UT) the 14%-lit waning crescent Moon will sit 3.8˚ north of Venus. As an added bonus, just before the morning dawn, look out for the stars of the Beehive Cluster 1.6˚ south of the Moon (see opposite).
More on this conjunction below.
On 18 September, there’s a tricky meeting between a 2%-lit waxing crescent Moon and mag. 0.0 Mercury. You’ll need a flat western horizon for this pair though as they will be extremely low.
On 24 and 25 September it’s the turn of the gas giants Jupiter and Saturn to get a visit from the Moon. Both planets themselves are coming together for what’s known as a ‘Great Conjunction’, which is set to occur on 21 December (more on this below).
Currently they appear slightly under 8˚ apart. On the evening of 24 September, both planets form a bent line with a 57%-lit waxing gibbous Moon, located 6.5˚ west of mag. –2.3 Jupiter at 21:00 BST (20:00 UT).
The next evening, the now 68%-lit Moon will appear further to the east, forming an equilateral triangle with Saturn and Jupiter.
The Moon, M44 and Venus, 14 September
See it with: Naked eye and binoculars
How to see it: Look eastward in the very early morning, around 4am
Normally the most observed conjunctions are seen in the evening sky, but you’ll need to set your alarm clock for this one as it’s best seen around 4am towards the eastern horizon. The slim crescent Moon and dazzling Venus lie either side of the wonderful star cluster M44, the Beehive Cluster.
The Moon and Venus will be viewable with the naked eye and you may be able to glimpse the cluster, but the view through binoculars will be special indeed, enhancing the appearance of the Beehive. This is one of the occasional conjunctions that involve a deep-sky object, so it’s definitely worth making the effort provided you have clear skies.
Mars and the Moon, 2, 3 & 29 October
See it with: Naked eye and binoculars
How to see it: Look eastward around 8pm
The Red Planet is just 11 days away from opposition and at its best for Northern Hemisphere viewers for this conjunction. On 2 October the Moon – just one day past being full – lies to Mars’s lower right. Look towards the eastern horizon from about 8pm onwards as both rise higher and the Moon creeps closer to Mars.
The pair will be closest just as they set the next morning. You will find that 7×50 and 10×50 binoculars give good views throughout the night, and for a bonus look out for the Moon and Mars lying close together yet again on the 29th in the early southeastern evening sky.
Jupiter and Saturn, 21 December
See it with: Naked eye, binoculars and telescope
How to see it: Early evening bright twilight towards the southwest horizon
Christmas is an interesting time of year for a conjunction of bright planets to occur in the evening twilight, often giving rise to ‘Christmas Star’ reports. These are usually prevalent when Venus is visible at Christmas, but Jupiter is the next brightest planet and it’s the one that will be catching our attention.
On 21 December, after months slowly approaching each other, Jupiter and Saturn meet up for a spectacular ‘great conjunction’. They will appear so close in the sky that for a naked-eye view they may look like a single, bright object. They will be low in the evening twilight and will set quickly, so a good uncluttered southwestern horizon is essential in order to view this conjunction.
Binoculars will separate them into two objects with Saturn, the fainter of the two, lying above the mighty Jupiter. However, if you can use a telescope then aim it at them before they get too low. You will not only see them as discs, but may even see Saturn’s rings and Jupiter’s belts in the same view, along with the four Galilean moons of Jupiter and Saturn’s largest moon, Titan. This will be a brilliant conjunction as the year draws to a close.
Paul Money is BBC Sky at Night Magazine’s Reviews Editor. Pete Lawrence is a co-host of The Sky at Night. This article originally appeared in the May and September 2020 issues of BBC Sky at Night Magazine.