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On Monday night, a truly rare astronomical event will occur: Jupiter and Saturn will align in what NASA's calling the "great conjunction."

Jupiter and Saturn align in the sky once every two decades, according to NASA. Why are they calling this a great conjunction, then? For one, it's been nearly 400 years since the two planets will appear this close to each other in the sky. To us, it'll look like they're a tenth of a degree apart; NASA said that at arm's length, a pinky finger could cover both planets.

A "great conjunction" occurred in July 1623 but it was impossible for humans to see because it was so close to the sun, according to the Associated Press.

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That's not all, though. This conjunction is so special because it's been nearly 800 years since since Jupiter and Saturn's alignment occurred at night — so this time, we can see it happen. "What is most rare is a close conjunction that occurs in our nighttime sky," Vanderbilt University astronomy professor David Weintraub explained to AP.

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The unaided eye will be able to see the planets very close together, even though in actuality they're millions of miles apart in space. NASA broke down how you'll be able to see this once-in-800-years occurrence on Monday, Dec. 21:

• Find an unobstructed view of the sky, such as at a park. Don't worry, city-dwellers — Jupiter and Saturn are so bright, NASA estimates you'll be able to view the conjunction from most cities.

• Look at the sky in the southwest direction an hour after sunset. "Jupiter will look like a bright star and be easily visible," NASA explains. "Saturn will be slightly fainter and will appear slightly above and to the left of Jupiter until Dec. 21, when Jupiter will overtake it and they will reverse positions in the sky."

• If you have binoculars or a telescope, you may be able to see Jupiter's four large moons!

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"I think it's fair to say that such an event typically may occur just once in any one person's lifetime," Weintraub told AP, adding: "And I think 'once in my lifetime' is a pretty good test of whether something merits being labeled as rare or special."

Indeed, a conjunction this close won't occur again until 2080 — so set your timer for an hour after sunset Monday night.

UPDATE: Dec. 19, 2020, 1:18 p.m. EST An earlier version of this post incorrectly referred to the "great conjunction" at several points as the "great conjecture."

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