A busy year for skywatchers – including supermoons, meteor showers, and cometary flybys – is about to conclude with a rare sight. On Monday, December 21, Jupiter and Saturn will get close enough together in the night sky that they’ll appear to form a single planet. It’s known as a “great conjunction” by astronomers and is a unique celestial event that occurs once every 20 years or so. This particular conjunction will be even more special, and not just because it will happen during the winter solstice. According to astronomers, Jupiter and Saturn will appear to be less than a 10th of a degree apart, the closest they’ve been in almost 400 years. (The last time it occurred was in 1623 during the age of Galileo.)