On December 21 — coincidentally the winter solstice — the two largest planets in our solar system will appear to almost merge in Earth’s night sky.

During the event, Jupiter and Saturn will sit just 0.1 degrees apart, or a mere one-fifth the width of the Moon. The sight will likely leave many casual observers wondering “What are those large, bright objects so close together in the sky?”

In fact, Jupiter and Saturn will be so close that you will be able to fit them both in the same telescopic field of view. That’s an incredibly rare occurrence. The last time Jupiter and Saturn were this close together was in 1226 A.D., at a time when Genghis Khan was conquering large swaths of Asia, and Europe was still generations away from the Renaissance.

Conjunctions happen when two celestial objects appear to pass close to one another as seen from Earth. Of course, those objects aren’t necessarily physically close, they simply look that way because of their orbital alignment.

Conjunctions are extremely common, though. Throughout the year, the Moon regularly appears alongside a number of planets and bright stars. And two or more planets come together relatively frequently. However, having two bright planets like Jupiter and Saturn pair up is extremely rare.

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