A blue moon is the second full moon in any calendar month, occurring approximately every 2 1/2 years. It is rare and the moon’s color does not change. In earlier centuries, it referred to the third full moon in a season with four full moons.
The phrase “once in a blue moon” is often used to refer to an event that occurs very rarely, but what exactly is a blue moon? Simply put, a blue moon is the second full moon in any calendar month. Note that despite the name, the color of the moon does not change during a blue moon.
The average lunar cycle lasts approximately 29.5 days and of course most months have 30 or 31 days. It is therefore clear that blue moons are relatively rare. For a blue moon to occur, a first full moon must fall at the beginning of a month so that the next full moon (29.5 days later) can rise during the same month.
On average, there are about 2 1/2 years between blue moons. Obviously months that have 31 days are much more likely to have blue moons than those that have 30 days. February, the only month with fewer than 30 days, never has a blue moon.
About four times a century, a calendar year has two blue moons. 1999 was the most recent year with two blue moons. The 21st century will have the following double blue moon years: 2018, 2037, 2067 and 2094.
There is little controversy as to what a blue moon actually is. We described the modern version, but in earlier centuries the term blue moon referred to the third full moon in a season that has four full moons.
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