Jupiter and Saturn, two huge gas giants, will appear closer to each other on Monday than they have in nearly 400 years.
In what Canadian astronaut Chris Hadfield has described as a “planetary kiss”, the event will be visible from Britain on 21 December and coincides with the Winter Solstice.
Once every 20 years, the two planets – the largest in our solar system – appear to meet in the skies above Earth. These events are called the “great conjunction”.
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This year, the conjunction will be far closer than normal, with the two gas giants only 0.1 degrees apart (a fifth of the diameter of the Moon).
It’s the closest the two planets have come to each other in the sky since the great conjunction of 1623.
During typical great conjunctions, Jupiter and Saturn come within about one degree (the width of two Moons) of each other.
Patricia Skelton, astronomy education officer at Royal Observatories Greenwich, writes: "In the early evening Jupiter and Saturn will be so close together on the sky, only around 0.1 degree apart, that instead of seeing two separate points of light we may only be able to make out one very bright point.
"When this happens between any other two astronomical objects it is called a conjunction, but because of the historical importance of Jupiter and Saturn, when they appear close together in the night sky it is called a great conjunction."
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On 21 December, Jupiter and Saturn will look like two bright stars that are almost touching, as seen by the human eye.
In binoculars or a wide-field telescope, they will still appear quite close to each other.
A telescope under high magnification will reveal the cloud belts of Jupiter and the rings of Saturn, as well as several of their moons, all at the same time.
Lowell Observatory in Flagstaff, Arizona, will be covering this great conjunction with a free interactive livestream program on YouTube.
To see it from Britain, you might have to wait until the Sun goes down, warn experts from Sky at Night.
The magazine’s Jamie Carter writes: “The main event will have Jupiter and Saturn separated by a mere 0.06′ – 6 seconds of arc – at 13:30 Universal Time on Monday, 21 December, 2020. That’s 13:30 GMT, so during daylight hours in the UK.
“Sunset will take place at 15:53 in London, 16:06 in Cardiff, 15:59 in Belfast and 15:39 in Edinburgh. About 45 minutes after sunset observers should look 10 degrees above the south-southwest horizon to see Jupiter and Saturn shining almost as one.”
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