Perpetual Chinese Calendar

Blue Moon

	Copyright: Vic Winter www.icstars.com (used with permission)
The above picture of the Moon taken was taken in a dark blue morning sky. The bright crescent is the only part directly exposed to sunlight - the rest of the Moon glows from sunlight reflected from the Earth. The planet Jupiter is also visible along with its four of its largest moons. (Text adapted from NASA, for complete article visit: NASA )

Common Usage

". . .  once in a Blue Moon . . ."

This is a fairly common usage of the term Blue Moon. There is no such thing as a Blue Moon - but certain atmospheric conditions can cause the Moon to appear blue.

Sometimes it DOES Appears Blue

The moon turned blue in 1883, 1927 and 1951. The Indonesian volcano Krakotoa exploded in 1883 throwing up huge clouds of dust and fine debris, which the wind carried to various parts of the world. The dust particles caused scattering of the reflected sunlight causing a bluish glow from the Moon.

A similar dusty condition in India in 1927 was caused by the late arrival of the Indian monsoons. The unusually hot and dry weather threw up enough dust to cause a blue moon. The latest blue moon was witnessed in 1951, due to forest fires in western Canada. The smoke particles also caused the occurrence of a blue moon.

Blue Moon Defined

Blue Moon (What's a Blue Moon?, Sky & Telescope) is a term describing an astronomical event. Due to the Moon's synodic period of 29.53 days, there will be certain months, although rare, in the Gregorian calendar where there are two full moons. The 2nd of such full moon occurring within the same Gregorian month is a Blue Moon. This is the widely understood definition of Blue Moon until the following.....

Blue Moon Redefined

In the article "Blue Moon" Blooper, March 25, 1999, Sky & Telescope magazine, said that the earlier definition above is incorrect due to an error in interpretation 53 years ago! (Sky & Telescope's March 1946 issue). Recently, it was discovered that the 1937 edition of the Maine Farmers' Almanac, which reportedly mentioned the occurrence of a Blue Moon in August of that year but it was not the 2nd full moon. After much research, Olson Sinnott and Richard Tresch Fienberg concluded that Maine almanac meant the third full Moon in a season with four is the Blue Moon. They also noted that the definition is valid only if you mark the beginnings of the seasons using an outmoded rule based on the dates of Easter and Lent.

So whilst we finally put to rest the definition of Blue Moons, the search for such Blue Moons is a little more involved. Quite a number of astronomers prefers to stick with the earlier definition - the 2^nd full moon in the Gregorian month and so shall I.

Recent Blue Moons

The most recent Blue Moons occurred in January and March of 1999. There were two full moons in January - 2nd & 31st and again in March 2nd and 31st. January 31st and March 31st are Blue Moons (2nd full moon definition).

It is not that blue after all! This year we shall again witness a Blue Moon on November 30th. (Greenwich) while our Asian friends near the Equator shall witness it on December 30th. (Kuala Lumpur local time). This is correct, since the Blue Moon that occurs on 20:50 November 30th UT actually falls on 4:50 December 1st (KL local time).

The next Blue Moon will occur on 06:06 July 31st, 2004 UT.

Once in a Blue Moon!

Hence, the term "Once is a Blue Moon" is a reference to a rare infrequent event. Various discussions of Blue Moons have been taken to extremes which I shall not address.