The Great Conjunction of Jupiter and Saturn
It’s been nearly 400 years since the two largest planets in the Solar System have appeared this close together.
If the skies are clear, you should look up at the night sky in the early evening on 21 December 2020, as Jupiter and Saturn will appear as one bright double planet, a sight that has not been seen for nearly 400 years.
The planets are not actually any closer to each other than normal, but will appear 0.06° apart. They will still be over 400 million miles apart, which is nearly four and a half times the distance between the Earth and the Sun, but they will appear close in the sky due to the orbits of the two planets lining up.
What we see in this “great conjunction” is the faster orbiting Jupiter overtaking the slower orbiting Saturn from our viewpoint on Earth.
Due to the differing speeds of Jupiter and Saturn, a conjunction between the two planets happens every 19.85 years, however, they are not often as close to each other as they will be on this occasion.
Previous close conjunctions of this nature have been obscured due to close proximity to the Sun, such as the conjunction on 16 July 1623, which occurred just 13 years after Galileo used an early telescope to observe Jupiter’s first four moons.
The last time a great conjunction was this close and easy to see we have to go back to 04 March 1226, when England was ruled by the Plantagenet King Henry III.
Luckily, we won’t have to wait quite as long for the next one, and the next time the planets will once again be just 0.06° apart in our night sky is on 15 March 2080.
So, for some of our younger readers it might be a twice in a lifetime event.
How To Watch
The planets will move to within 0.06° at 13:30 GMT, during daylight hours. However, the great conjunction will only be visible after sunset.
To view from the UK, head outside on 21 December around 40 minutes after sunset (15:53 GMT). This will allow time for the skies to be dark. Darker locations are best for viewing, so try to stay away from city lights.
Face South West (many mobile phones will have a compass on them if you need to check for the correct direction).
You should be able to see the rare ‘double planet’ shining about 10° above the horizon.
You’ll have roughly a 90-minute window to see this conjunction.
If you go outside over the nights before 21 December you will gradually see Jupiter and Saturn move closer and closer together in the night sky.
What Else Can You See?
Whilst you don’t need any equipment to see the great conjunction, this is also the perfect time for astronomy. Check our What’s in the Night Sky video for December on our YouTube channel.
Through a good set of binoculars, or even a low-power telescope, Jupiter and Saturn should be in the same field of view but clearly separate from each other. It may also be possible to spot Saturn’s rings and even the largest moons of both planets.
If you have a telescope, 21 December is a perfect evening to look at the First Quarter Moon. You should be able to see fine detail of the craters at the point where the Moon fades from light to dark, known as the terminator.
Also on the evening of 21 December the Ursids Meteor Shower will peak. The meteors will radiate from the constellation of Ursa Minor high in the sky, but can appear anywhere so keep your eyes peeled.