(NEXSTAR) — The world will soon see a planetary phenomenon that hasn’t occurred in 800 years.

On Dec. 21, the date of the December solstice, Jupiter, the largest planet in our solar system, and ringed Saturn, the second-largest, will line up perfectly to look like a double planet in the night sky.

The two planets will be so close that they will appear to be touching, separated by one-fifth the diameter of a full moon. It will be the closest they’ve appeared since March 4, 1226.

“The two planets have been brilliant highlights of the night sky for much of this year, and are now getting closer together in advance of their super close pairing in mid-December,” NASA said. “Be sure to watch as they draw a little nearer to each other each week.”

When celestial bodies align, it’s called a conjunction, but since this one involves our solar system’s two biggest gas giants, it’s known as the “great conjunction.”

Though the two planets will look close together, they actually will be more than four times the distance between Earth and the sun.

Great conjunctions happen on average every 19.6 years.

You can catch next month’s rare sight — look for a single point of bright light — in the western sky just after sunset. This super-close Jupiter-Saturn alignment isn’t expected again until March 15, 2080.