Look Up! A Blue Moon in May

Reported May 2007

CAMBRIDGE, Mass. (Ivanhoe Broadcast News) -- The term "once in a blue moon" means a rare occurrence, but what does it really mean? On May 31st, there will be a real blue moon in the sky, but many people still don't know what it means.

Kelly Beatty, the executive editor of Sky & Telescope Magazine, offered some insight to Ivanhoe. Beatty said, "A blue moon is when you have two full moons in the same month." So, a blue moon really means an extra full moon. It takes 29 and a half days to go from one full moon to another. But most months have either 30 or 31 days, so those extra days add up and eventually, about once every three years, we get a second full moon in the same month.

Beatty explains why we see the moon as full, "A full moon is that point in the moon's orbit when it's on the back side of the earth, as seen from the sun. So as we look behind us into the night, we see the moon fully illuminated by sunlight."

The expression "once in a blue moon" is not a reflection of what a blue moon really is. Beatty explained it is possible, with certain atmospheric conditions, to have a moon that may be tinged blue, but the expression is just a popular phrase.

The American Astronomical Society and the American Meteorological Society contributed to the information contained in the TV portion of this report.

Click here to Go Inside This Science or contact:

Sky Publishing
Cambridge, MA 02140 U.S.A.
617-864-7360 or 800-253-0245
http://skytonight.com/

American Astronomical Society
Washington, DC 20009-1231
202-328-2010
http://www.aas.org
aas@aas.org

American Meteorological Society

Boston, MA 02108-3693
617-227-2425
http://www.ametsoc.org

You've heard the expression and now on May 31st you can see a real blue moon.

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