Once in a Blue Moon

When you hear someone says "Once in aBlue Moon…" you know what they mean: Rare, Seldom, or maybe even absurd. After all, when was the last time you saw the moon turn blue?

But what exactly is a Blue Moon? Is there a scientific meaning behind this phrase or does the moon really turns blue?

In astronomical terms, a 'blue moon' really doesn't have anything to do with color. Instead, it is the term used to denote the second full moon that occurs within a given calendar month. Because it takes the moon about 29 days to circle the Earth once in its orbit, it is possible that two full moons can occur within the same calendar month. On average, this takes place once every two and a half years. A second blue moon will appear this May, with the moon displaying its full-phase on the 1st and the 31st of May, 31st would be the “Blue Moon.”

Although a 'blue moon' doesn't really look blue, there have been times when the moon does seem to have a blue color. This can be caused by dust particles in the atmosphere, which scatter light. The effects of this dust on the light coming from the moon can cause it to appear bluish in color. Fine dust particles are ejected into the Earth's upper atmosphere after large volcanic eruptions, for example. The eruption of the Krakatoa volcano in 1883 gave us one such 'blue moon'. For about 24 months after this volcano exploded, the dust it spewed into the upper atmosphere caused the moon to appear green and blue when viewed from around the world. Even in a case like forest fire or due to large amount of smoke moon would look bluish.

There is little scientific significance to either an astronomical blue moon or a bluish-looking moon. But astronomers are interested in how the moon reflects different colors. The moon generates no light of its own, but reflects light from the Sun. By studying how the moon reflects this light, scientists can explore the chemical composition of the lunar surface.

Other Meanings

According to old folklore, some people say, the second full Moon in a calendar month is called a "Blue Moon." They go on to explain that this is the origin of the expression "once in a blue Moon." But the term "blue Moon" has been around a long time, well over 400 years, but its calendrical meaning has become widespread only in the last 20 years. 

Philip Hiscock, an author and archivist of Folklore & Language Archive, Memorial University of Newfoundland, camp up with seven meanings to the phrase “Blue Moon”.

Seven meanings of 'Blue Moon':

1. as an ABSURDITY. 'He would argue that the moon is blue!', as if to say, he would argue that black is white! Such expressions can be found in the literature of the 16th century.

2. NEVER. "I wouldn't be seen with him even if the moon turned blue." This use was apparently common in the 1900's.  

3. ACTUALLY IS BLUE. Seen against a blue sky in daytime or twilight, our eyes may see the moon as a bluish tint. Smoke or dust in the air can emphasize the blue or other color.

4. RARE. An actual blue moon color is fairly uncommon, so the term took on the meaning of rarity or, 'I only do this once in a blue moon'.

5. SADNESS, LONELINESS. Well, there is only one up there! It can and does console us when we are alone and blue.

6. There is a drink, made from Blue Curacao, called BLUE MOON

7. SECOND FULL MOON IN A CALENDAR MONTH. Because 11 of the twelve months have more days than the lunar cycle of 29 1/2 days, it is easily possible for a month to have two full moons. And this is the meaning we use nowadays.

Then according to Sri Lankan calendrical full moons, we call as “Poya days”. While for the 2nd full moon of the month we use the name “Adhi Poya”. Which I beilive must have a history than the western definition of the 2nd full moon of the month. If anyone has an earlier reference in published literature to the “Adhi Poya”, please write to me.

Since the next Blue Moon occurs in December of 2009. Don’t miss the opportunity to view the Blue Moon on 31st of May. The Moon will be visible from 6pm on wards and the Planet Jupiter will lies closer to moon on the date which will make it a fascinating site.



Thilina Heenatigala
Secretary
Astronomical Association of Lanka

Dedicated to Keshya Kumaranayagam.