December 21st is going to be a very special day for all astronomy enthusiasts. For the first time since the Middle Ages, the Solar System’s two largest planets, Jupiter and Saturn, will appear so close to each other they’ll look like one super-bright point of light.

The truth is, they won’t be close at. Jupiter and Saturn are separated by vast distances as they orbit the Sun, but now they will overlap and look like a double planet.

The science behind it

You may be wondering, how exactly this works. Basically, Jupiter orbits the Sun faster than Saturn, therefore they are drawing closer and closer to each other each day. On December 21st they will be separated by only a fifth of the diameter of the full moon, appearing to look like a double planet from the Earth’s point of view. The event happens every 19.85 years and is known as the great conjunction.

“Alignments between these two planets are rather rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear to be to one another,” said Patrick Hartigan, an astronomer at Rice University. “You’d have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky.”

The rare event will be the first Jupiter-Saturn conjunction since the year 2000, and the closest Jupiter-Saturn conjunction since 1623.

How to observe the great conjunction

The big event will be observable from anywhere on Earth where the sky is clear on December 21st, about 45 minutes after sunset. Meanwhile, you can start watching the planets now as they will be drawing closer to each other in the coming weeks. Don’t miss this rare event as the two planets won’t be this close to each other again until 2080!