(NEXSTAR) – If you’ve been watching the sky on these dark December nights you may have noticed two of the brightest objects creeping nearer to each other. Jupiter and Saturn are about to appear closer in the sky than they have in 400 years.

The two planets will be so close that they will appear to be touching, separated by one-fifth the diameter of a full moon.

“They’re not close in space – they’re still hundreds of millions of kilometers apart from each other,” said NASA Astronomer Henry Throop. “But they appear as two points very close in the sky in fact they’re so close that if you extend your pinky at arms length you’ll be able to cover both planets with just your pinky finger.”

When celestial bodies align, astronomers call it a conjunction, but since this one involves our solar system’s two biggest gas giants, it’s technically a “great conjunction.” Because the event is landing on a holiday week, many have begun calling the formation the “Christmas Star.”

Online stories about the event have gone viral, and NASA responded by creating a video primer for the event, which peaks on Dec. 21. According to Throop, this event should be visible to almost anyone in the U.S. with a clear view of the horizon.

“In order to find the planets in the sky, it’s really easy if you just look up. If you can see the sunset – that means you are looking to the west, the southwest if you are in the northern hemisphere – then you’ll be able to see Saturn and Jupiter. You want to go outside maybe 45 minutes after sunset. Let the sky darken a little bit. You don’t have to wait until night time. And then look toward the sunset,” said Throop, an astronomer at NASA headquarters. “You’ll have about an hour of time where you can observe Saturn and Jupiter.”

Throop says Jupiter will look like the brightest star in the sky, It’s not a star, of course, it’s simply reflecting the sun. Saturn will be slightly fainter and found above and to the left of Jupiter.

Throop says the planets are so bright that even people in light-filled cities should be able to see them and potentially even take high-quality photographs. If you can’t make it out Monday night he says the solar system’s two largest planets will appear nearly as close on the 20th and 22nd.

“Anybody can go out and see these planets,” said Throop.

If you are planning to snap pictures, NASA has created a guide to photographing the great conjunction.

The 21st will be the closest Jupiter and Saturn have appeared to most since March 4, 1226. There was a closer conjunction of the planets some 400 years ago, but that event was not visible to many of us here on Earth, according to Space.com.

Great conjunctions happen on average every 19.6 years. NASA says after this month’s event, you’ll have to hang in there until 2080 to catch the next great conjunction of similar proximity.