The full Moon has a case of the blues tonight — it’s known as a Blue Moon.

There’s a passel of blue moon definitions. The phrase “once in a blue moon” is applied to something that happens rarely. In the calendar, a Blue Moon can be the 13th full Moon in a year, or the third of four full Moons in a calendar season. Today, Blue Moon usually refers to a second full Moon in a calendar month. And that’s what we get tonight.

There’s a slight difference between the Moon’s cycle of phases and the length of a month. A lunar cycle averages 29 and a half days. So in a month that has 30 days, a full Moon on the first of the month can be followed by another on the 30th. Months with 31 days can have full Moons on the 1st or 2nd, and again on the 30th or 31st.

The rarest of all blue moons are those that actually look blue. They happen when a big volcanic eruption or a major wildfire spews tiny bits of ash high into the sky. The ash scatters red light and allows bluer wavelengths to shine through — giving us a truly blue Moon.

And if you wonder where the modern definition of Blue Moon came from, we’re partially to blame — or credit. Writer Deborah Byrd discovered the definition in a 1946 magazine article, and used it in a Star Date episode in January 1980. The article was mistaken, but Byrd found no evidence of that. The definition later was used in the board game Trivial Pursuit — ingraining the Blue Moon in popular culture.
 

Script by Damond Benningfield

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