What is a Blue Moon

Most people will have heard of the phrase “once in a blue moon”, meaning something that occurs infrequently. It is a phrase that comes about directly from the cycle of the lunar moon, and today it relates to the appearance of a second full moon in a period of time, though it does have nothing to do with the actual colour blue.

Coloured moons are of course nothing new and red moons indeed are very common sites, the redness caused by sand particles being spread through the atmosphere. In a similar way the moon could look blue depending upon local atmospheric conditions. The eruption of a major volcano and its release of ash, with a recent example of Mount St Helens in the United States in 1980, being enough to give the moon a blue tinge. Equally it is a phenomenon that can be caused by massive forest fires.

Whilst dismissing the fact that Blue Moons are not named because of the visual colour of the moon, there is no real certainty about why blue is used in connection with infrequent events.

Today a Blue Moon refers to the occurrence when a full moon occurs twice in a calendar month, something that will happen every two and a half years, or more accurately forty one times in a century.  It is an occurrence that happens due to the inconsistencies between the lunar cycle and the days in a calendar month. The lunar cycle lasts for 29.5 days, whilst months last for between 28 and 31 days. The twelve months of the year add up to an average of 365.24, whilst twelve lunar cycles last for only 354 days.   The missing eleven and a bit days will eventually accumulate to allow for thirteen full moons to occur in a year, and therefore a second, Blue Moon, to occur in a single month.

To allow a month to have two full moons, the first will have to occur at the start of the month, with the Blue Moon squeezing in at the end. Thus months with 31 days in are more likely to have a Blue Moon. The last Blue Moon occurred on the 31st December (19:11 GMT), whilst the next will occur on the 31st August 2012 (13:56 GMT).

Most people will think that other than being an interesting quirk of astronomy there is no real point to considerations about a Blue Moon. Historically though the Church and Agriculture would often make use of the lunar cycle to date future events, they though were more interested in a fourth full moon occurring in a quarter. A fourth new moon could throw out the planting or reaping of crops, or the date of a religious festival such as Easter. It was indeed a sixteenth century manuscript relating to the placement of Easter that gave up the sentence, “if they say the moon is blue, we must believe that it is true”. It might be a mistranslation but it was a phrase that soon stuck in the English language.   

Whilst counting the fourth moon in a season has no impact upon the frequency of the Blue Moon in a century, it will alter the date of the Blue Moon in a particular year.

The change from seasonal count to a monthly one comes about die to a mistake in a 1946 addition of the Sky and Telescope magazine. An assumption was made in an article in that addition. It was an assumption that Blue Moons quoted in the 1937 Maine Farmer’s Almanac referred to a second monthly full moon, which of course it didn’t, as it referred to seasonal occurrences.

Most people are today more aware of the phrase in regards to the Blue Moon rather than in the lunar occurrence. It is interesting to consider though that once every two and a half year is not as infrequent as when “once in a Blue Moon” is used as a phrase.

Source – http://www.obliquity.com/astro/bluemoon.html