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It’s coming up in December and it’s a once-in-a-lifetime view.

Jupiter and Saturn will appear to look like a double planet just after sunset on Dec. 21, as the two celestial bodies will be in conjunction, in an event sometimes called the “great conjunction,” said Amy Oliver, a spokeswoman for the Harvard-Smithsonian Center for Astrophysics.

“Most adult people have never seen a conjunction like this, and they won’t have an opportunity to see this again,” Oliver said in a telephone interview Wednesday.

Although this conjunction happens once every 20 years, the one on Dec. 21 is rare: The two planets will appear to be on top of one another because they will be only .1 degrees — approximately one-fifth of the moon’s diameter — away from one another, Oliver said.

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The last time the two planets appeared this close together was in 1623 — 14 years after Galileo created his first telescope, Oliver said. However, people weren’t able to view because it was close to the sun.

“Only astronomers would have really known, but they didn’t have social media to tell people, ‘Hey! Look at this!’ ” she said.

The last time people would have noticed a conjunction this close would have been in 1226, during the Middle Ages.

This year’s occurrence on the December solstice adds to its rarity, Oliver said.

A zoomed in simulated photo of Jupiter, left, and Saturn, right, at 5:20 p.m. Dec. 21.
A zoomed in simulated photo of Jupiter, left, and Saturn, right, at 5:20 p.m. Dec. 21.Stellarium

The two planets have been approaching one another in the Earth’s sky for a while now, Oliver said. For more than a week they will be separated by less than the diameter of a full moon, with Dec. 21 being the closest they come together.

If you look up now, you can already see it in action.

“It’s like teenagers at a high school dance: They’re getting closer and closer together,” Oliver said. “It’s been a year of watching this, of them getting closer, and now they’re going to have a close slow dance.”

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Jupiter and Saturn will be at their nearest alignment about 1:20 p.m. EST Dec. 21 and it will be visible in the daytime sky with a “really good pair of binoculars or a backyard telescope,” weather permitting, Oliver said.

The two planets will still be visibly close when it starts to get dark, or shortly after sunset, for Boston viewers, before it drops below the western horizon, Oliver said.

The next time to get a chance to see Jupiter and Saturn this close, but higher in the sky, would be March 15, 2080, Oliver said.

So, don’t miss the chance this month.

“Two thousand twenty has been a great year for astronomy and lots of really wonderful things have happened in the night — and daytime — sky,” Oliver said. “In part, we’re so very focused on everything that has not been so great about 2020 that we’re forgetting to take in these moments that are a lot bigger.”


Breanne Kovatch can be reached at breanne.kovatch@globe.com. Follow her on Twitter at @breannekovatch.