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Published 18th May 2014 by

A blue moon is a rare event which occurs when there is a second full moon in a month. The phrase has nothing to do with the actual colour of the moon.

A Blue Moon is a rare event, hence the expression "once in a blue moon". The phrase has nothing to do with the actual colour of the moon, although a literal term, meaning the moon appearing with a tinge of blue, may occur in certain atmospheric conditions such as when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere.

A blue moon has traditionally come to mean an additional full moon that appears in a calendar month. This is actually a fallacy originating from the March 1946 issue of Sky & Telescope who misinterpreted the traditional definition.

An average lunar cycle is 29.53 days and there are about 365.24 days in a tropical year. There are about 12.37 lunations (365.24 days divided by 29.53 days) in a tropical year. Normally there is one full moon each month, and each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. These extra days accumulate, so every two or three years, there is an extra full moon. The extra moon falls in one of the four seasons, giving that season four full moons instead of the usual three, and hence a blue moon.

As you can see from the table below, the event is quite rare! An average year contains 12 full moons, where these years feature 13 full moons.

Blue Moon Calendar

You can use this table to see when the next blue moon occurs.

YearFirst Full Moon of monthSecond Full Moon of month
2012August 2, 2012August 31, 2012
2015July 2, 2015July 31, 2015
2018January 2, 2018January 31, 2018
2018March 2, 2018March 31, 2018
2020October 1, 2020October 31, 2020
2023August 1, 2023August 31, 2023
2026May 1, 2026May 31, 2026
2028December 2, 2028December 31, 2028
2031September 1, 2031September 30, 2031
2034July 1, 2034July 31, 2034
2037January 2, 2037January 31, 2037
2037March 2, 2037March 31, 2037
2039October 2, 2039October 31, 2039
2042August 1, 2042August 31, 2042
Blue Moon

Blue Moon

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