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Once in a blue Moon!

The phrase 'once in a blue Moon' is a familiar one meaning once in a very long interval of time. The phrase goes back to at least 1824 when an explanation of its meaning appears as a footnote attached to its use. It is not absolutely clear what is the origin of this phrase but the rare blue colouring of the moon is the most likely. A possible astronomical explanation also is given below.

Two Full Moons in one month
The use of the phrase 'blue Moon' when there are two Full Moons in one calendar month appears to be spreading. The author has no knowledge of the origin of this usage but suspects that it has been 'back-derived' from the phrase denoting rarity with a rather misconceived idea that it is an uncommon thing for there to be two Full Moons in any one month.

For example, between 1984 and 2000 there were two full moons in the following months:

Jul 1985 May 1988 Dec 1990 Sep 1993 Jul 1996 Jan 1999 Mar 1999

It can be seen that in 1999 there were two such months with only February between them. It could well be said this is not rare enough a phenomenon to qualify for the normal use of the phrase 'once in a blue Moon'.

The phrase 'once in a blue Moon' may be used often enough, yet it’s true meaning has recently been revealed. A blue Moon, by folklore definition is said to be the second Full Moon of the month. However, researchers at Southwest Texas State University have been using historical documents to prove that a blue Moon was really a term that was used by the Marine Farmers’ Almanac as a calendrical meaning to indicate the presence of a 13th Full Moon in a tropical year which usually had 12. A tropical year is a measurement of time from one winter solstice to the next.

When the Moon appears blue

There are rare occasions when the disk of the Moon appears to take on a bluish tint. These should not be confused with contrast effects in which the Moon appears blue because the viewer's eye has been `fooled' by a bright red light. Such an appearance can result from the Moon being seen next to a cloud which is strongly illuminated by the red setting Sun. By contrast, the Moon appears to be green but after the Sun's light has gone the Moon's appearance returns to normal.

A similar effect can be seen by campers sitting around the red light of a camp-fire. Their eyes become attuned to the bright red light of the fire and the Moon then appears greenish by contrast. To anyone whose eyes are not affected by the red firelight, however, the Moon appears normal.

On very rare occasions, however, the Moon does appear to have a bluish hue. This occurs after a violent volcanic eruption or, occasionally, after a big forest fire. The particles in the smoke, or in the volcanic dust, can be of just the right size to scatter the light preferentially in the red so as to leave an excess of blue light in the Moon's image. These rare events give rise to the origin of the phrase.

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