The month of May, 2007, in India has two full moons. A Blue Moon is commonly the name given to the second full moon in a month.
A more traditional explanation of 'Blue Moon' refers to the 4th full moon in a season. That is, each of the 4 seasons of the year has 3 months, and will usually have 3 full moons. Each of these 12 moons has a name like "Harvest Moon", or "Hunter's Moon" etc. However, when a season occurs that contains 4 full moons, there is no name for this occasional moon and it was given the name, "Blue Moon".
Although the expression Blue Moon nowadays is taken to mean the second Full Moon in a single calendar month, the Moon doesn't actually change colour, despite the name! However, there have been occasions when the Moon has appeared to be blue in hue. The phenomenon is caused by dust or smoke high in the Earth's atmosphere which is thrown up by a major volcanic eruption or smoke from a large forest fire. Tiny particles have a strange effect on moonlight passing through them. They scatter the light in every direction, but red light is scattered more strongly than blue light, so that less red light passes directly through the dust or smoke. Thus the Moon has a blue tinge.
The expression 'Blue Moon' as a way of saying 'not very often' was first used about 150 years ago. By that time it was clear that visibly blue Moons, though rare, did happen from time to time. So the phrase "once in a blue Moon", originated to mean 'an infrequent event'.
To view the calendar of all upcoming full moons at Arunachala until December, 2009, check link.
