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Once in a Blue Moon
The moon has often served as a heavenly body of wonder, the way it floats in the darkness, casting its silver glow. It has frequently been thought that the moon may be made of green cheese and we have also used the moon to refer to a rarity. For example, it is said that one finds a blue lobster "once in a blue moon" as blue lobsters are quite unusual. From a 1528 saying, we have one of the earliest references to the blue moon: "If they say the moon is blue / We must believe that it is true."
Though we often use the phrase to describe an absurdity, the astronomical definition describes a blue moon as the second full moon in one month. This is actually not too rare of an event but fairly common and regular: blue moons occur approximately every three years.
Now, blue moons typically are not blue, but it is entirely possible. This happens during volcanic eruptions or wildfires, when small particles of ash and smoke are thrown into the atmosphere, scattering the light from the moon. Depending on the size of the particles, the moon can appear red or blue. If most of the particles happen to be about one micron, or one millionth of a meter, red light will be scattered while the blue light passes through the particles and the moon appears blue. However, this is quite infrequent because the particles tend to be less than one micron, scattering the blue light and making the moon look red. So, a blue blue moon is indeed a rarity.
December of 2009 offers us two full moons, one on the second and one of the thirty-first. We will not see another blue moon until August of 2012--and there will be two blue moons in 2018. So, this New Year's Eve, we have a sort of astronomical grand finale to the International Year of Astronomy with a blue moon rising in the east. In the words of poet Billy Collins, "…tonight would be the night to carry some tiny creature outside and introduce him to the moon."
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