Jupiter, meet Saturn

The two largest planets in our solar system are coming closer together than they have been since the Middle Ages, and it happened just in time for Christmas -- hence the nickname of the "Christmas Star."

While it's not an actual star, the two planets will certainly make a bright splash in the night sky.

On the night of December 21, the winter solstice, Jupiter and Saturn will appear so closely aligned in our sky that they will look like a double planet. This close approach is called a conjunction. The fact that this event is happening during the winter solstice is pure coincidence, according to NASA.