For the first time in nearly 800 years, a rare Christmas star will light up the sky at the start of this month’s winter solstice.

It’s a planetary alignment that last happened in the year 1226, according to Rice University astronomer Patrick Hartigan.

As they make their orbit around the sun, Jupiter and Saturn will slowly grow closer together over the next two weeks until they are almost completely aligned, according to NASA.

“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 Hartigan. “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.”

When they align this closely, the two planets will appear to form a single bright star, also know as the “Christmas Star.”

If Western Washington’s weather cooperates and the sky clears, you can look to the west about 45 minutes after sunset and see the two planets drawing closer together from now until the solstice, Dec. 21, when they will be at their nearest point, and then watch them begin to pull apart.

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Planetary alignments aren’t all that uncommon, said Jason Matter, program director with the South Florida Amateur Astronomers Association. Jupiter and Saturn align usually about every twenty years or so, but this upcoming occurrence is particularly striking because of how close to each other the two planets will appear to the naked eye.

By Dec. 21, Jupiter and Saturn’s orbits will be so closely aligned — less than a 10th of a degree as seen from Earth — that they will appear as one. The planets will be actually hundreds of millions of miles apart. From here, the distance between them will be about the thickness of a dime held about a foot away from your eyes.

These conjunctions happen periodically due to the way Earth, Jupiter and Saturn orbit around the Sun, according to Matter. As all three planets orbit at different speeds, at random occasions during their orbits around the sun, the planets will appear to be close to each other.

The best time to witness this year’s Christmas Star will be to look southwest as the sun sets. For the next two weeks, the planets will be closer and closer each day until Dec. 21, then will appear to swap positions and move apart. A telescope with a wide enough field of view — about 2 degrees — should be able to see both planets with some of the planets moons. Through binoculars, the event will look like a double star, Matter said.

NASA predicts the next “great conjunction” of this magnitude won’t happen for another 60 years or so, then again about 320 years after that.