Once in a Blue Moon
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Right: Artist Duane Hilton's portrayal of the March 1999 "Blue Moon" as viewed between White Bark pine trees in California's Eastern Sierra.
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. Such was the case in January 1999, when the moon was full on the 2nd and the 31st, making the full moon on the 31st a 'blue moon.' On average, this takes place once every two and a half years. This time, however, we don't have to wait over 2 years for another blue moon. A second blue moon will appear this March, with the moon displaying its full-phase on the 2nd and the 31st of March.
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Studying the moon can be tricky, because the moon is too bright to be photographed with large, highly sensitive telescopes on the ground or with the Hubble Space Telescope. The moon's brightness can potentially damage such sensitive optical instruments. Less sensitive telescopes on the ground and on satellites, however, have given us some stunning images of the full moon. The moon can also be photographed using different light wavelengths, such as ultraviolet.
Above: An Ultraviolet Moon. One of the more interesting space-based images obtained of the moon was taken from the Astro-2 observatory, lofted aboard the space shuttle in 1995. Although the moon appears blue in both images, the left-hand image is actually taken in the ultraviolet, which is higher in energy than blue or violet light.
The oddity of having two blue moons within three months is not of much interest to scientists, but it is a notable event to many others. Ever charming and compelling, the blue moon will likely continue to serve as inspiration for songwriters and poets for generations to come.
The Nine Planets: the Moon -- from SEDS
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