On December 21st, a ‘Christmas Star’ will shine its brightest in 800 years. 

It will, in fact, not be a star but a rare phenomenon which occurs when two planets – Jupiter and Saturn – get very close to one another and appear to nearly collide.

It’s called the “Great Conjunction,” or the moment when the two planets appear just a tenth of a degree apart. It will be the first time such a close conjunction has been observable since 1226, according to the EarthSky website.

But while they may appear very close to us, according to NASA, “while the two gas giants may appear close, in reality they are hundreds of millions of miles apart.”

The event has been dubbed the “Christmas star”.

“This will still be quite a striking sight, but you will need to look fast as both planets will set shortly after sunset,” NASA’s website says.  Those looking to see the star will want to look above the southwestern or western horizon after sunset, experts say.

These conjunctions happen every 20 years or so, but NASA officials said the 2020 event is “the greatest great conjunction between Jupiter and Saturn” since the 17th century — and won’t happen again until the year 2080.

WATCH a livestream of the Great Conjunction below, courtesy of Lowell Observatory. Beginning at 7 P.M. EST on December 21, Lowell astronomers and educators will share spectacular live views of Jupiter and Saturn through observatory telescopes while discussing the nature of conjunctions