What Is A Blue Moon?

On Friday, Aug. 31, skywatchers will be treated to a Blue Moon. Does this mean the Moon will actually look blue? Unfortunately, no. To understand the celestial event — and the story behind the name — we've prepared a brief Q&A, with a little help from James Applegate, a professor of Astronomy at Columbia University.

What is a Blue Moon?

The term Blue Moon refers to the second full moon in a single calendar month. The first full moon was on Aug. 2. The second full moon will occur on Friday, Aug. 31.

I know the phrase "Once in a Blue Moon" means something very rare. How rare is a Blue Moon?

Blue moons happen every two to three years. Typically, a single month only has one full moon. But sometimes another gets squeezed in because our calendar months and our lunar months aren't an exact match. The time between full moons (the time it takes for the moon to orbit the Earth and cycle through all its phases) is 29.5 days. Our calendar months, on the other hand, usually have 30 or 31 days in them. This means each calendar year has about 11 more days than a lunar year. "The fact that our months are about a day longer than the time between full moons means that if the 1st or 2nd (for a 31-day month) is a full moon, then we get a second full moon (the blue moon) in the same month," says Applegate. This happens roughly every 30 months, or every two to three years.

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