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On Thursday Aug. 20, a relatively rare event will take place in the skies above you—it's called a blue moon.
Why it occurs is somewhat complicated. A full moon, when no shadow from Earth covers the lunar body, happens every 29.5 days. Normally this means that there will be 12 full moons a year and three per season. However, once every 2.7 years there will be an extra full moon in one of the four seasons. When this occurs, the third of the four full moons is called a "blue moon." This was devised by Native Americans who gave names to each full moon and then passed it on to the first European settlers.
People have been confused as to what a blue moon really is after an expert mistakenly told "Sky & Telescope" magazine in 1943 that the 1937 Maine Farmers Almanac revealed that a blue moon occurs when any particular month had two full moons. A widely read 1946 article in Sky & Telescope confimed this.
However, the magazine did not rest on its laurels. Editors examined more than 40 editions of the Maine Farmers Almanac from 1819 to 1962 and found references to several blue moons—and all were based on an extra full moon per season—not the two full moons in a month version.
BUT WAIT. Before you plan to watch the full moon, you'll probably be very disappointed. (continue)
The colors you'll see from the moon might be orange, red, and its normal yellow. The infrequency of blue moons originated the expression, "once in a blue moon."
Here's a photo of the 2004 blue moon, probably made with a blue filter. This year Photoshop programs will probably be going crazy.