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Blue Moon Rises Tonight


July 31, 2004

Ketchikan, Alaska - When the moon rises in the sky tonight, you can watch a phenomena that rarely occurs. According to modern folklore although tonight's full moon will look like an ordinary full moon, it will actually be a bit extraordinary - a Blue Moon. But will it really be blue? Probably not. The moon tonight as viewed in Ketchikan should be pearly-gray, as usual.

photo moon Ketchikan, Alaska

The moon over mountains around Ketchikan appears almost full Thursday evening.
Photograph by Carl Thompson

According to NASA, the key to the moon actually appearing the color blue is having lots of particles in the air slightly wider than the wavelength of red light (0.7 micron) - and no other sizes present. This is rare, but volcanoes sometimes spit out such clouds, as do forest fires. With wildfires burning on July 31st in the western U.S. and Alaska, if any of those fires produce ash or oily-smoke containing lots of 1-micron particles, the Blue Moon could be blue.

For a Blue Moon to occur, the first of the two full moons that fall within a month must appear at or near the beginning of the month so that the second will fall within the same month. Since there are roughly 29.5 days between full moons, it is unusual for two full moons to occur in a 30 or 31 day month. The first full moon in July was on the 2nd and the second full moon - the Blue Moon - will be tonight, July 31st.

The last Blue Moon happened in November 2001. In 1999, there were two blue moon occurrences - January 31st and March 31st. The next blue moon will appear on June 30th of 2007. On average, there will be 41 months that have two full moons in every century, so you could say that "once in a Blue Moon" actually means once every two-and-a-half years.



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