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.
|Year||First Full Moon of month||Second Full Moon of month|
|2012||August 2, 2012||August 31, 2012|
|2015||July 2, 2015||July 31, 2015|
|2018||January 2, 2018||January 31, 2018|
|2018||March 2, 2018||March 31, 2018|
|2020||October 1, 2020||October 31, 2020|
|2023||August 1, 2023||August 31, 2023|
|2026||May 1, 2026||May 31, 2026|
|2028||December 2, 2028||December 31, 2028|
|2031||September 1, 2031||September 30, 2031|
|2034||July 1, 2034||July 31, 2034|
|2037||January 2, 2037||January 31, 2037|
|2037||March 2, 2037||March 31, 2037|
|2039||October 2, 2039||October 31, 2039|
|2042||August 1, 2042||August 31, 2042|