Ah, cosmological karma. All for Neil.
Contrary to our casual phraseology, a "Blue Moon" refers less to color of our lunar satellite than to lunar cycles, and has had many definitions throughout history. Simply put, a blue moon is a FULL MOON that is not timed to the regular monthly pattern – a "full moon" being when the Moon is on the opposite side of the Earth from the Sun, thus fully illuminated and appearing voluminously round.
Most solar years have one full moon per month, but each calendar year contains about eleven excess days in addition to those twelve cycles. These days accumulate, so that every 2.7 years, there is an extra full moon, commonly referred to as a Blue Moon.
Prior to this modern definition, blue moons referred to an extra full moon in a three-month season, when folkloric names for each monthly moon of the Gregorian Calendar followed ecclesiastical rules. Seasonal names were assigned relative to solstices and equinoxes, largely for determining the dates of Easter and Lent. Any "extra" moon (early or late) was referred to as a blue moon, though they did not always fall in the "same month."
Are actual BLUE moons ever reported? Sure. When the moon appears to an Earthling viewer as curiously bluish in terms of tint, look around for a forest fire or recently erupted volcano. Such phenomena have been known to disperse smoke or dust particles into the atmosphere, in which cases short-wavelength light transmits blue rays into human eyes.
Of course, I'm simplifying a complex process here, so if you're really interested in the full origins and history, there’s a fantastic article over at Sky & Telescope Magazine. The explanation of a blue moon simply being the second full moon in one month is often considered a "trendy mistake" derived from an almanac published in the 1930s, but most astronomers don’t seem too terribly offended by it.
Tonight's Blue Moon seems all the more beautiful and timely, considering that today is Neil Armstrong's private memorial in his home state of Ohio. Such a fitting cosmic coincidence for our dearly department Moonwalker.
Please also consider donating to the Neil Armstrong Scholarship Fund, founded by the American Institute of Aeronautics & Astronautics (AAIA). There are options for snail-mail, telephone, internet, credit card and Paypal.
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