Do you remember what you were doing 20 years ago?

That’s how long it’s been since Jupiter and Saturn were as close together in our night sky as they will appear this month.

This planetary ‘kiss’, which will occur on 21st December 2020, will be their closest pairing since 1623 – nearly 400 years! To the unaided eye, the two giant planets will appear to almost touch, forming a ‘double planet’. At their closest, they will be only 0.1 degrees apart – just 1/5 of the diameter of the Moon. This extra-close conjunction won’t be matched again until March 2080.

Of course, the two worlds never really touch, not even close. Jupiter orbits the Sun at a distance about five times greater than Earth, and Saturn about ten times the Earth-Sun distance. Nonetheless, the view from here on Earth looks like spectacular near miss of the two largest planets in our solar system!

What are Great Conjunctions and why do they occur?

Astronomers use the word ‘conjunction’ to describe the apparent meetings of celestial objects on our sky’s dome. But because they occur relatively rarely, conjunctions of Jupiter and Saturn are referred to as ‘great conjunctions’.

Conjunctions are all about perspective and line of sight. If we imagine the 8 planets to be marbles rolling around a central point on a tabletop, with the outer ones moving more slowly than the inner ones, it’s easy to visualise the occasional lining up of 3 or more marbles.

Great conjunctions occur just once every 19.86 years on average. Conjunctions involving the Moon and the inner planets are much more frequent because these bodies move more quickly along their orbital paths. The Moon takes less than a month to circle Earth whereas Jupiter takes 11.86 years to orbit the Sun, and Saturn 29.46 years. Notwithstanding retrograde motion, this orbital dance results in Jupiter catching up with Saturn, ‘overtaking’ it, and leaving it behind about once every 20 years.

Caption: This image, looking down on the Solar System from above the north pole, shows the alignment of the planets on 21st December 2020. The distances are to scale; the sizes are not. 

How will I identify Jupiter and Saturn in December 2020?

You don’t have to wait until December 21st to view these planets in the night sky! They are easily naked eye visible and can be seen tonight, and every night until the end of the year (weather permitting), so start watching from your back yard! Careful observers will perceive the reducing separation night by night, especially as they get closer together. Not long after sunset Jupiter, the brighter of the two, will become visible part way up the western sky. Jupiter appears brighter than any star and cannot be missed. Saturn, pale yellow and much fainter, though still brighter than any background stars close-by, is soon seen above and to the right of Jupiter. A thin sliver of Moon will sit above the pair on the 17th making a beautiful sight.


Caption: Moon, Jupiter, Saturn: 10:00pm 17 December 2020

As December progresses the pair will gradually sink towards the west-southwest horizon. They’ll be lost to Sun’s glare in early January and will re-appear on the opposite side of the sky, before sunrise, later in February. 

What else can I see with the naked eye on these warmer nights?

The Andromeda Galaxy (M31) is the most distant object visible to the unaided human eye. Visible only to those in northern New Zealand (the further north the better), early December is your last opportunity to catch a fleeting glimpse of the galaxy some 2.5 million light years from Earth. Looking quite close to the horizon, just east of north just after it gets properly dark with no Moon and no town lights, you might be rewarded.

The Andromeda galaxy will return to our pre-dawn skies in June, east of northeast – about the same time as Matariki.

Caption: Andromeda Galaxy (brightness enhanced): 10:00pm 10 December 2020

Speaking of Matariki, summertime is the most comfortable time to spot this beautiful glittering star cluster (also known as the Pleiades). In June we must brave the cold and rise early to spot the cluster low in the northeast. But, over the summer months it is easily seen after dark, partway up the sky. In December it can be found in the northeast. The key to finding it is Orion’s belt, sometimes referred to as the base of the pot, and known also as Tautoro. These three stars are easily identified and lie practically parallel to the horizon in December and January. An imaginary line drawn through these stars to the right takes us to brilliant Sirius and one of equal length to the left leads us to Matariki.

Caption: Matariki (Pleiades), Orion, Sirius: 10:00pm 17 December 2020

For those keen on early mornings there is something for you too. Early to mid-December is the season for the stunning Geminids meteor showers. You can find out more and see a time lapse video of these beautiful lights in the sky in one of our previous blogs. Happy summer stargazing, keep looking up and we’ll see you again in 2021.

Caption: The Geminids shower as seen from Perth, Australia. Credit: Jacqui Hunt Unique Photography