What is a Blue Moon?

Published by Misty Wood
December 29, 2009, Category: Astronomy

The Earth will experience a Blue Moon on 31st August 2012. A Blue Moon is the second full moon in a month. These don’t occur very often as there is a discrepancy between the lunar cycle and a calendar month.

A blue moon is a full moon that does not fit into the regular monthly pattern. The majority of calendar years have twelve full moons which occur roughly on a monthly basis. But, in addition to the twelve full lunar cycles, each calendar year contains approximately an extra eleven days. The extra days add up, so that every two or three years (this averages to be every 2.7154 yrs), there is an extra full moon. The extra moon is called a “blue moon.” Different definitions place the “extra” moon at different times.

The Clergy identify the Lent Moon and use this to work out the dates for Lent and Easter. If the moon’s timing was too early, it is historically thought, that the Clergy named the earlier moon as a “betrayer moon” (belewe moon), so the Lent moon came at its expected time.
Folklore gives each moon a name according to the time of year that it occurs. A moon that came too early had no folk name – and was then called a blue moon – which meant that the subsequent moons had the correct seasonal timings.
The Farmers’ Almanac defined blue moon as an extra full moon that occurred in any one season; one season normally has three full moons. If a season had four full moons, then the third full moon was named a blue moon.
Recent usage has defined a blue moon as the second full moon in a calendar month, this has come from an interpretation error made in 1946.
The term “blue moon” is often used metaphorically to describe a very rare event, as in the idiomatic expression, “once in a blue moon.”
The phrase “blue moon” is also said to be because the extra full moon in a calendar month was often printed in blue.