What is a Blue Moon

The term Blue Moon can be used in several contexts, right from its use in English literature to the actual bluish appearance of the Moon. In order to understand its meaning, we need to get into the details of each of these contexts, wherein the use of Blue Moon is cited. Though it is most often used in the phrase once in a blue moon, to refer to 'a rare event', there also exist some scientific explanations for this term.

What is a Blue Moon and How Often Does it Occur?

Usually there are 12 full Moons in a year, each of which roughly occur on a monthly basis. These 12 full Moons are divided into 3 full Moons each season. The lunar year, based on the 12 lunations, is around 11 days shorter than the solar calendar year. Owing to these extra days in the solar year, an extra full Moon occurs once in 2-3 years. This means that there are 13 full Moons once in every 2-3 years. This in turn means that one of the seasons has 4 full Moons for that particular season, contrary to the actual 3 full Moons per season. During such a year, the third full Moon in the season of four is referred to as the Blue Moon.

According to some definitions, the second full Moon in a particular calendar month is referred to as the Blue Moon. This also works on the same principle as above with the extra days of the solar calendar contributing to the 13th full Moon every once in a while. This phenomenon typically characterized by the occurrence of the second full Moon in a calendar month generally happens once in every 2 and a half years. In 2009, December had two full Moons - first on December 2nd and the second on December 31st.

Blue Moon: A Visual Illusion

At times, the Moon is referred to as the Blue Moon when its color appears to be unusually bluish. This bluish appearance of the Moon can be attributed to the Tyndall effect - an optical phenomenon, wherein light gets scattered by small particles in its path. This rare event can be caused as a result of a forest fire, volcanic eruptions or basically any phenomenon which releases smoke and dust particles in the atmosphere. These particles have to be slightly wider than the wavelength of red light, so that they facilitate the scattering of light, which in turn gives the Moon its blue color. This rare event has been reported from various parts of the world over the last century. Most popular examples being the sighting of Blue Moon in Canada in 1951, caused due to smoke from a forest fire, and in Mexico, due to smoke from El Chichon volcano in 1983.

It may be disheartening for many to know that the color of a Blue Moon is not actually blue, instead it is the visual illusion which gives it the bluish appearance.