Jupiter and Saturn came together in a ‘once in a lifetime’ show on the 21st of December.

The two planets will appear together above the western horizon around an hour after sunset on December 21.

This event, known as a great conjunction, happens about once every 20 years. But the closeness of the two planets makes this a very rare conjunction.

With only about a 10th of a degree separating the two planets – that’s a fifth of the with of the moon – this is the closest they will have appeared in the sky to each other in nearly 400 years. The last time it occurred was in 1623. 

They will be so close together that you will be able to see them in one eyepiece of a telescope, says Andrew Jacob, curator at Sydney Observatory. ” “You would be lucky to see this once in a lifetime,” he says. The two planets won’t appear this close to each other again until 2080. While the planets look close to each other from earth, in reality they are separated by vast distances as they orbit the sun.

From earth, we see the planets move from east to west across the sky along the same narrow band as the Moon and the Sun.

While it takes Saturn nearly 30 years to complete one orbit, Jupiter takes nearly 12 years, so we see Jupiter lap Saturn every 20 years. Just how far apart they appear each time depends on how the tilt of the two planet’s orbits line up.

By December 21st, Jupiter and Saturn will appear in the evening twilight about half an hour after sunset, but they will only be visible for an hour before disappearing below the western horizon. “The further north you are, the longer they are in the sky,” Dr Musgrave says.

While the planets will look very close wherever you are on December 21, the actual conjunction occurs at precisely 13.33 UTC, by which time it has set over most of Australia and just sinking out of view from Perth.

With your naked eye look for two dots – the smaller Saturn appearing to chase the larger, brighter Jupiter – towards the western horizon. On December 21 the two planets come so close you may not be able to see the gap between them, Dr. Musgrave says.

“But the fact that you can look at a dot and say ‘that’s two separate planets fairly close to each other’ is going to be amazing” he says.

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