Jupiter and Saturn will appear to look like a double planet just after sunset on Dec. 21, as the two planets will be in conjunction, also known as 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, this one is even more rare: the two planets will appear to be on top of one another because they will be .1 degrees — approximately one-fifth of the moon’s diameter — away from one another, Oliver said.

The last time the two planets appeared to be this close was in 1623, or 14 years after Galileo created his first telescope, Oliver said. However, people back then weren’t able to see it due to how close it was to the sun, blocking the view.

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

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