What is a blue moon?

A blue moon is a rare event that occurs when there are two full moons within the same calendar month. The phrase 'blue moon' has nothing to do with the actual color of the moon. It is believed to have originated from an old English phrase 'belewe moon', which means 'betrayer moon'. The term was first used in 1946 in an article published in 'Sky & Telescope' magazine. In general, a full moon occurs approximately every 29.5 days, which means that there are typically 12 full moons in a year. However, because the lunar cycle is approximately 29.5 days long and most months have 30 or 31 days, it is possible to have two full moons in a single calendar month.

Blue moons are relatively rare, occurring about once every 2.7 years on average. However, they are not actually blue in color. In rare circumstances, the moon can take on a bluish tint, but this is typically due to atmospheric conditions and not related to the term 'blue moon'.