Later this month a rare event will occur in the heavens, as Jupiter and Saturn appear closer together in the skies than they have for nearly 400 years.
Although the planets will appear to come close together, from our perspective, they are still over 650 million km apart in space, so it’s a conjunction, not a collision.
The coming together of the two planets is formally known as a Great Conjunction and happens just under every 20 years, but normally, the gap between them is still fairly wide.
Between 16th to 25th December 2020, the distance between the two planets will be less than the width of the full moon, and they will keep getting closer until the day of the Winter Solstice, 21st December when the gap will be around a fifth of the width of the full moon.
The last time the two planets were that close in the night sky was in July 1623, when James I was on the throne of England and Scotland. It wasn’t actually visible from England then, and the last time a conjunction this close was visible from England was all the way back in 1216, when Henry III was on the throne.
On the date of the closest approach, the two planets will appear low in the western sky as a very bright star for about an hour after sunset each evening, although as the planets will be very close to the horizon, and sinking down behind it, the chance of seeing them will be best about 45 minutes after sunset – so around 4:40pm.
At the London latitude, the planets will be about 5 degrees above the horizon, which is about the width of three fingers – so the best way to see them is to find somewhere high, and above the street lights, face south-west, hold out your hand horizontally and that’ll give you a rough idea of how much above the horizon you want be looking.
You’re looking for two close fairly bright steady stars. A pair of binoculars will help if you have them, or even a telescope if you own one – and you can pick up a relatively decent beginner telescope with smartphone camera adaptor for £75.
As the planets come closer over a number of days, if the sky is clear in the days leading up to the 21st Dec, go out anyway, just in case the 21st is cloudy. But if the 21st is clear, then obviously, that’s the magic date to go out. It will be much harder to spot after that anyway as by then they will be too close to the horizon.
If you miss it entirely, the next chance to see them together will be in 2040, and they won’t be this close together until 2080.