What is a blue moon?
A blue moon usually means the second full moon in a single calendar month. It happens about once every two to three years, which is where the phrase once in a blue moon comes from.
A lunar cycle lasts about 29.5 days, a little shorter than most calendar months. Occasionally, a full moon falls at the very start of a month, leaving time for a second one before the month ends. This second full moon is called a monthly blue moon.
An older definition, called a seasonal blue moon, refers to the third full moon in a season that has four instead of the usual three. The monthly meaning became popular after a 1946 article in Sky & Telescope magazine misread the older rule, and the mistake caught on.
Is it actually blue?
A blue moon is almost never blue in color. However, smoke or ash from wildfires and volcanic eruptions can sometimes make the Moon look bluish. After the 1883 eruption of Krakatoa in Indonesia, people around the world reported seeing blue moons for months.
Did You Know?
February can never have a monthly blue moon, because it is shorter than a full lunar cycle. In some years, February has no full moon at all.