In a rare celestial ‘Great Conjunction’, ttwo largest planets in our solar system are coming closer together than they have been since the Middle Ages. This close approach is called a conjunction, and it’s happening just in time for Christmas — hence the nickname of the “Christmas Star.”
Between December 16 and 25, look for the Jupiter-Saturn conjunction low in the western sky for about an hour after sunset each evening during this time.
On the night of December 21, the winter solstice (the shortest day of the year), the two planets appeared so closely aligned in our sky that they looked like a double planet, even though they were, in fact, more than 730 million kilometers (400 million miles) apart. Jupiter overtook Saturn on December 21 and they swapped places in our sky.
That this event is happening during the winter solstice is a pure coincidence, according to NASA.
Jupiter, which is the larger planet, takes 12 years to revolve around the sun, while Saturn takes 29 years. The next Great Conjunction between the two planets will not occur again until 2080
“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 one another,” said astronomer Patrick Hartigan, a professor of physics and astronomy at Rice University in Houston, in a statement.
“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.”
Between 0 and 3000 CE, or Common Era, only seven conjunctions were or will be closer than this one — and two of those were too close to the sun to be seen without a telescope, according to Hartigan. So, yes, this is an incredibly rare event.
If you missed this conjunction and want to see the planets with the same proximity, just higher in the sky, wait until March 15, 2080 — it won’t happen again until after 2400.
The original article appeared in CNN.