On Tuesday (Aug. 20), a rare blue moon will appear in the night sky. Contrary to popular belief, the moon doesn't actually appear blue. But what is a blue moon exactly?

There's been some confusion over exactly what the definition of a blue moon is. The common and most accepted definition is that it's the second full moon in a given month.

However, according to Sky & Telescope the term blue moon was originally used in the Maine Farmers' almanac to describe an extra moon in a given season:

At last we have the "Maine rule" for Blue Moons: Seasonal Moon names are assigned near the spring equinox in accordance with the ecclesiastical rules for determining the dates of Easter and Lent. The beginnings of summer, fall, and winter are determined by the dynamical mean Sun. When a season contains four full Moons, the third is called a Blue Moon.

The Weather Channel will be showing off the moon's unique appearance tonight. If you're unable to step outside and enjoy it firsthand, you can check it out here.