Jupiter and Saturn are getting closer together as seen in the sky – and on Monday 21 December they’ll be closer than at any time in nearly 400 years.
Jupiter is in line with Saturn every 20 years – an event known as a conjunction – but not every conjunction is easily visible. The last time it happened was in 2000, but on that occasion the planets were on the far side of the Sun and were unobservable. In 1980 they could be seen, but you’d have had to get up in the early morning to see them, and Jupiter passed some distance from Saturn, so the event was of minor interest.
But on 21 December 2020, Jupiter and Saturn will be only 6 minutes of arc apart – a tenth of a degree. For astronomers this is an amazing sight, as the two will be visible in the same field of view of a telescope at the same time. In fact, the last time they were closer was in 1623. Although Galileo and a small number of other scientists then had telescopes, they would have had to view the event in bright twilight very close to the horizon, and there are no records of anyone doing this.
This year, although the planets appear close in line of sight, they are actually a vast distance apart. Jupiter is 886 million km away, and Saturn is 1620 million km away from Earth at the time.
What to look for
The two planets are low down in the south-west, and are only visible for a short while following sunset. Jupiter becomes visible about 4 pm and Saturn (to its upper left before the event) a short while afterwards, although you need to know just where they are in order to spot them. By 4:30 pm they are easily visible, as long as you have a low enough horizon in that direction. But they are getting lower all the time, and by 6 pm they will be too low for most people to see. In the north of the UK will have already set. So if you want to see the spectacle, check where the pair are in the sky in advance of the 21st, and find your best local spot to observe them
Jupiter is considerably brighter than Saturn, so with the naked eye its brilliance will dominate the view, and Saturn will appear just as a fainter speck alongside it. Through a telescope they are visible using a low magnification from mid December onwards, but because of the low altitude in the sky both planets are likely to be affected by turbulence in our atmosphere and will not appear as clear and sharp as they normally would.
The four bright satellites of Jupiter are easy to see, and Saturn’s brightest moon, Titan, is also visible. Although the closest approach is on the evening of the 21st, the planets will be close for several days before and after, so take whatever opportunities present themselves rather than waiting for the actual night.
If you miss this conjunction, there’s another close one coming along right behind – in 2080. Here is a full list of past and future conjunctions, compiled using Chris Marriott’s SkyMap program (approximate details only – not to be relied upon for observing purposes!).
21 Dec 2020, 6 min 6 sec at 19:30
28 May 2000 – event took place too close to the Sun to be seen from the UK, and the planets were 1 deg 10 min apart.
On 31 December 1980 they were in the early morning sky and were 1 deg 3 min apart.
19 February 1961, 14 arc minutes separation low in the morning sky before sunrise
16 Feb 1941, 1 deg 18 min.
9 Sep 1921, too close to the Sun to be visible, 57 min.
28 Nov 1901, evening sky, 27 min
19 Apr 1881, behind the Sun, 1 deg 13 min
21 Oct 1861, early morning, 48 min
26 Jan 1842, early morning, 32 min
19 June 1821, 1 min 10; also 2 Dec 1821, 1 min 17
17 Jul 1802, 40 min
4 Nov 1782, 45 min
19 Mar 1762, 1 deg 10m
30 Aug 1742, 28 min
4 Jan 1723, 48 min
22 May 1702, 1 deg 3 min
22 Oct 1682, 17 min
17 Oct 1663, 59 min
23 Feb 1643, 1 deg
16 Jul 1623, 5 min 20 sec, evening sky, about a degree above the horizon from Florence at the start of nautical twilight.
19 Dec 1603, 1 deg
In the future
30 Oct 2040, 1 deg 7 min
(Also a close conjunction of all bright planets and Moon on 8 Sep 2040 just after sunset)
6 Apr 2060 1 deg 7 min
15 Mar 2080, 6 min 01 sec
15 Jul 2119, 57 min
14 Jan 2140, 15 min
21 Dec 2159, 1 deg 11 min
26 May 2179, 51 min
7 Apr 2199, 25 min
30 Oct 2219, 1 deg 12 min
21 Mar 2239, 45 min
2 Feb 2259, 34 min
2 Feb 2279, 1 deg 11 min
12 Jul 2298, 28 min
26 April 2318, 43 min
30 Nov 2338, 1 deg 6
22 May 2358, 18 min
17 Feb 2378, 50 min
2 Oct 2398, 1 deg 6 min
24 Aug 2417, 5min 29 sec, early morning