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.
The last time people would have noticed this conjunction this close would have been in 1226, during the Middle Ages, almost 800 years ago.
It’s also happening on the December solstice, adding even more to its rarity, she said.
The two planets have been approaching one another in the Earth’s sky for a while now, Oliver said. For over a week they will be separated by less than the diameter of a full moon, with Dec. 21 being the closest approach.
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.”
Jupiter and Saturn will be at their closest alignment about 1:20 p.m. eastern 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 together when it starts to get dark, or shortly after sunset, for Boston viewers, before it pops below the horizon, Oliver said.
The next time to get a chance to see Jupiter and Saturn this close together, but higher in the sky, would be March 15, 2080, Oliver said.
In other words, don’t miss out.
“2020 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 than what we’re giving them credit for.”