Jupiter and Saturn are about to appear closer in the sky than they have for 800 years, lining up as a ‘double planet’

• Jupiter and Saturn will line up in the night sky on December 21st. It will be the closest period they have appeared in the last 800 years.

• The two planets move in alignment, or conjunction, every 20 years. But this year they will be so close that they will seem like a “double planet”.

• The last time Saturn and Jupiter were both so close and visible was in 1226.

• Here are the best ways to see this conjunction.

• Visit the Business Insider home page for more stories.

Jupiter and Saturn are about to line up perfectly in the night sky: the two planets will get so close on December 21st that they will seem to touch.

The last time they looked so closely from Earth’s perspective was nearly 800 years ago, on March 4, 1226.

An astronomical event in which celestial bodies align is called a conjunction. As this conjunction involves the two largest gas giants in our solar system, it is known as the “great conjunction”. It happens to some extent once every two decades.

“But it is fair to say that this conjunction is truly exceptional in that the planets come very close to each other,” Patrick Hartigan, professor of physics and astronomy at Rice University, explained on his website.

“In fact, they will be so close that it might be a challenge to separate them with the naked eye for many people,” he added.

Here’s how to see this rare conjunction.

How to visualize a “double planet”

On the night of the winter solstice, Jupiter and Saturn will be separated by a distance in the sky about one-fifth the diameter of a full moon, according to Hartigan. They will appear to form a “double planet”.

Of course, the two planets are much further apart than that – more than four times the distance between the Earth and the sun. But to our naked eyes, they will look like a single bright spot of light.

If you peek through a small telescope, Jupiter and Saturn will appear in the same field of view, along with some of their moons.

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However, it will be difficult to see this conjunction in the United States, Canada and Europe, Hartigan said, due to how low it will be on the horizon in the Northern Hemisphere.

“Viewing conditions are best near the equator, although no matter where you are, there is perhaps an hour or so to observe this conjunction before the planets sink into the haze,” he added.

If you go out at dusk, the time after sunset, and point the telescope at the southwestern sky, you can spot the event, Hartigan said. (Websites such as Stellarium can help you orient your telescope towards the meeting point of the planets.)

“You’ll need a clear southwestern horizon and no low clouds in the distance,” Hartigan said.

He recommends setting up the telescope before it gets completely dark and bringing binoculars.

If the solstice night ends up being cloudy where you are, Hartigan said, don’t worry: the conjunction is an ongoing event from December 17-25. The 21st is just when the two planets will be closest in the sky.

Another conjunction like this will come in 2080

In the last 2,000 years, there have only been twice that Jupiter and Saturn have come closer in the sky than they have this year.

One of these was from 1623, but the glare of the sun made it impossible to see. However, if you miss this rare upcoming conjunction, another one will arrive in 60 years.

On March 15, 2080, Jupiter and Saturn will seem just as close as this year. That event will be much easier to see, according to Hartigan, because it will take place higher above the horizon.

“The main challenge is that you will have to survive another 60 years to see it!” He said.

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