What Makes Tonight’s Blue Moon Blue?

Tonight is a “Blue Moon”, but unlike the picture above it won’t actually be blue. So where does the name from? Let us explain.

There are actually two definitions for a Blue Moon, and neither of them has anything to do with its color. The first defines a Blue Moon as the third full moon in a season with four full moons. If there’s four full moons in one summer, the third is a blue moon.

The other definition says that a Blue Moon is defined as the second full moon in one month. That definition came about accidentally when an article appeared in Sky & Telescope magazine in 1943. This has become the common definition, even though it was a mistake. The first definition predates it and came in 1937 in the Maine Farmers’ Almanac.

Tonight’s Blue Moon is “Blue” because of the 1937 definition, and it will be the last Blue Moon until the year 2015, so get out there and enjoy it.

(via Space.com, image via FrozenInLight)

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