The Full Moon, imaged by Tom Ruen.
In recent decades, the term blue moon has entered widespread usage to describe a second full moon which falls within a single calendar month. This usage first appeared in the March 1946 issue of Sky & Telescope magazine. The magazine incorrectly stated that this was an established tradition which was already followed by the Farmers' Almanac. In fact, it was an entirely new usage of the term. However, the article became widely cited, and so the term has entered common usage.
It is possible for two full moons to fall within the same calendar month since the Moon's phases cycle, on average, 12.37 times each year. So once every 2.8 years, a single year contains 13 lunar months rather than the usual 12. In such a year, one of the months must have two full moons.
This awkward ratio of 12.37 arises because the Earth's seasons are determined by the period of the Earth's orbit around the Sun, while the Moon's phases are determined by the period of the Moon's orbit around the Earth. The periods of these two cycles are 365.25 days and 29.53 days respectively, and are unrelated to each other.
This awkward ratio has long been familiar to cultures which use lunar calendars, in which the months follow the phases of the Moon. Such calendars need the flexibility for some years to have 13 months rather than the usual 12, if the months are to remain sychronised with the Earth's seasons. Many cultures have devised elaborate procedures for deciding when a thirteenth month should be added into a year – called an intercalary month. Such months are routinely added into the Jewish calendar, though they are spurned by the Muslim lunar calendar.
The term blue moon appears to have been coined by the Maine Farmers' Almanac in the 1930s. In its efforts to recreate the calendars of Native American peoples, this almanac divided the year into four seasons, separated by the equinoxes and solstices, and used a list of three names for the full moons which fell within each season.
However, once every 2.8 years, one of these seasons would have four full moons rather than the usual three. The name blue moon was given to the third of these. The three traditional names were applied to the first, second, and fourth full moon falling within the allotted period. This custom appears to have originated with the Maine Farmers' Almanac in the 1930s, with no earlier precedent.
It was in their efforts to understand the procedure being followed by the Maine Farmers' Almanac that the writers of a 1946 article in Sky & Telescope magazine incorrectly concluded that the term was applied to a second full moon falling within a single calendar month.
Writer Joe Rao once speculated that the term blue moon might have medieval origins as a corruption of an earlier term, betrayer moon, since the Old English words for betrayer and blue are very similar: belæwe and belewe respectively. While this speculation has been often repeated, there is absolutely no evidence that the term predates the 20th century, or that either of these Old English words was ever used applied to the Moon. Rao himself described his theory as "completely wrong" in 2007.
Curiously, though, the phrase "once in a blue moon" is much older: it was first noted by the Oxford English Dictionary in 1824.
List of blue moons 1950–2299
Even though the Moon reaches full phase at almost exactly the same moment for all observers across the Earth, any particular full moon may occur in a different month depending on the timezone of the observer. This may lead to differences between published lists of blue moons.
In the list below all times are calculated in universal time, also known as Greenwich Mean Time (GMT). According to that definition, the following full moons are the second to fall within a single calendar month:
List of old-style blue moons 1950–2299
The following full moons are the third of four falling within one of the three-month periods between the Earth's solstices and equinoxes. They are thus blue moons according to the original definition invented by the Maine Farmers' Almanac in the 1930s: