What is a Blue Moon?

Most people have no idea that the term blue moon has absolutely nothing to do with the color of the moon, but describes the second full moon in a calendar month. This month we will have a blue moon.

The phrase "Once in a Blue Moon" was first noted in 1824 and refers to occurrences that are uncommon, perhaps even rare. Yet, to have two full moons in the same month is not as uncommon as one might think. In fact, it occurs, on average, about every 32 months. And in the year 1999 it actually occurred twice in three months.

On past occasions, usually after forest fires or volcanic eruptions, the moon can indeed take on a bluish or lavender hue. Soot and ash particles, deposited high in Earth's atmosphere, can sometimes make the moon appear bluish. Such a situation was noted across eastern North America in late September 1950, due to smoke from widespread forest fire activity in western Canada. Also, in the aftermath of the massive eruption of Mount Pinatubo in the Philippines in June 1991 there were reports of blue moons worldwide.

MSNBC July 2, 2004

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