Watch for the 'Christmas Star' as Jupiter and Saturn come closer than they have in centuries

 The two largest planets in our solar system are coming closer together than they have been since the Middle Ages, and it's happening just in time for Christmas -- hence the nickname of the "Christmas Star."

While it's not an actual star, the two planets will certainly make a bright splash in the night sky.

On the night of December 21, the winter solstice, Jupiter and Saturn will appear so closely aligned in our sky that they will look like a double planet. This close approach is called a conjunction. The fact that this event is happening during the winter solstice is pure coincidence, 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 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."

If you're a stargazer, you've likely noticed Jupiter and Saturn have been getting closer together since the summer. And they're currently visible in our night sky, inching ever closer to one another.

Between December 16 and 25, they will become even cozier. Look for the Jupiter-Saturn conjunction low in the western sky for about an hour after sunset each evening during this time.

"You can imagine the solar system to be a racetrack, with each of the planets as a runner in their own lane and the Earth toward the center of the stadium," said Henry Throop, astronomer in the Planetary Science Division at NASA Headquarters in Washington.

"From our vantage point, we'll be able to be to see Jupiter on the inside lane, approaching Saturn all month and finally overtaking it on December 21."

How to watch

"On the evening of closest approach on Dec(ember) 21 they will look like a double planet, separated by only 1/5th the diameter of the full Moon," Hartigan said. "For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening."

While these two planets may appear close, they are still hundreds of millions of miles apart, according to NASA.

Hope for clear skies because the conjunction will be visible around the world, with the best perspective for those near the equator.

"The further north a viewer is, the less time they'll have to catch a glimpse of the conjunction before the planets sink below the horizon," Hartigan said.