Blue Moon in December 2009 by Danny Thomas

Most years have twelve full moons, which occur once in each month, but the cycle doesn’t take a complete year and there is about eleven days left over.   The extra days accumulate, so that every two or three years there is a thirteenth extra full moon which is known as a “blue moon.”

As 2009 is the International Year of Astronomy, it is almost fitting that we will see out the year with two full moon’s in December with the blue moon on New Year’s Eve. On the same evening, the blue moon will be partially eclipsed with the moon entering the Earth’s shadow at 18.51, with the greatest shadow at 19.22 and ending at 19.53.

There are several interpretations of the origin of the term blue moon.

One comes from the Old English word “belewe” which can mean “blue” or “betrayer.”

The first recorded use was in 1528 in a pamphlet attacking the English clergy which used the word in the context of “if they say the moon is blue, we must believe that it is true”.

An alternative explanation is that the dates of Lent and Easter are calculated based on the full moon.   Lent starts at the beginning of the Lent moon cycle (late winter moon) which is followed by the “egg moon” in early spring.   Easter usually falls on the first Sunday after the full egg moon.  A blue Moon means that that the Lent and egg moons would be too early and so the church would have had to tell people the extra moon was a false one.   This could have lead to the term “betrayer moon”.

In rare circumstances theMoon can appear bluish to an observers.  This is usually cause by smoke or dust particles in the atmosphere from forest sires or volcanoes.  If the particles in the atmosphere are the right size, the red light with longer wavelengths is scattered out of the line of sight and the shorter wavelengths, which appear blue are seen by the observer.