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Blue Moon Charlie Taverna ColoradoJake Blue Moon Myths
Second full moon in a month, or third full moon in a single season?


Although the full moon occurring this Saturday, February 19th, will look like an ordinary full moon, it will actually be a bit extraordinary—because it is a Blue Moon.
What is a Blue Moon?

Some confusion exists as to what exactly constitutes a Blue Moon. Most people believe that the Blue Moon is the second full moon in a calendar month, a phenomenon that occurred twice in 1999, as there were two full moons in both January and March (and none in February). This is a fairly recent definition, however, that has become popular only in the last couple of decades.

An older rule for the Blue Moon comes from the Maine Farmer's Almanac and dates back to the early 19th century. According to this definition, the Blue Moon is the third full moon in a season that has four full moons.

This year there are indeed four full moons between the 1999 winter solstice and the 2000 spring equinox. The third full moon falls on February 19th.

Winter Season 2000
Dec. 22 (8 am EST) Winter solstice
Dec. 22 (1 pm EST) Full moon
Jan. 21 Full moon
Feb. 19 Full moon—BLUE MOON
Mar. 20 (12 am EST) Full moon
Mar. 20 (3 am EST) Spring Equinox

Why is the third full moon the Blue Moon?

The third full moon, and not the fourth, is called the Blue Moon because the Almanac rule was influenced by the ecclesiastical rules for determining the dates of Easter and Lent. Easter is observed on the first Sunday after the first full moon of spring, known as the Egg Moon or Easter Moon. Lent begins 46 days before Easter, and must contain the last full moon of winter, known as the Lenten Moon. Only when the third full moon of the winter season is the Blue Moon do the names for the other moons fall into place.

How often does a Blue Moon occur?

Occurrences of Blue Moons are not all that rare, especially given that there are two definitions for the term. Over the next twenty years there will be a total of 17 Blue Moons, with an almost equal number of both types occurring. No Blue Moon will occur at all in the years 2003, 2006, 2011, 2014, and 2017.

The more recent phenomenon, where the Blue Moon is considered to be the second full moon in a calendar month, last occurred in March 1999. The next time it happens will be in November 2001. Two full moons in one month may occur in any month out of the year except for February, which is shorter than the lunar cycle.

The other, older Blue Moon event, which happens when there are four full moons in a season, occurs this Saturday and then not again until November of 2002. Since this type of Blue Moon is reckoned according to the seasons, it can only occur in February, May, August, or November, about a month before the equinox or the solstice.

Once in a Blue Moon

Since Blue Moons seem to occur relatively often, where does the expression once in a blue moon, meaning "rarely," come from?

Actually, "blue moon" appears to have been a colloquial expression long before it developed its calendrical senses. According to the Oxford English Dictionary, the first reference to a blue moon comes from a proverb recorded in 1528:
If they say the moon is blue,
We must believe that it is true.

Saying the moon was blue was equivalent to saying the moon was made of green (or cream) cheese; it indicated an obvious absurdity.

In the 19th century, the phrase until a blue moon developed, meaning "never." At about this point "blue moon" appears to have been used to describe the first lunar phenomenon, which only seldom occurred. It is perhaps through association with the lunar event that the phrase once in a blue moon today has come to mean "every now and then" or "rarely."

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