March 31

moonrise Photo credit: Kirstin Fearnley.

Today in Science

Blue Moon

Today's full moon is a blue moon. Well, that is, depending on whether you go by the old definition or the more recent one.

The current, popular definition of a blue moon (which evolved from a misunderstanding of the original definition and its reliance on the astronomical year versus the calendar year) is the second full moon occurring within the same calendar month. Since there was already a full moon this month on March 1, tonight's is, by this definition, a blue moon. This is the second occurrence of this type this year; Jan. 31 was the year's first blue moon. The next such blue moon will not occur until Halloween 2020.

However, the traditional definition is that a blue moon is the third full moon in a season containing four. (Each of the other three full moons during a season already had their own name. The last full moon before the autumnal equinox, for instance, is known as the corn or fruit moon.) The next blue moon of this sort will occur on May 18, 2019.

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