What is a Blue Moon?
Contrary to our casual phraseology, it 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" of course being when the Moon is on the opposite side of the Earth from the Sun. From Earth, the near side of the moon is thus fully illuminated by the Sun and appears 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."
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 where astronomers can explain it much better than I can.
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.
Are actual blue moons ever reported? Oh 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.
As we accept the twice-in-a-month definition today, a Blue Moon to ring in the new year is exceedingly rare! In December 2009, we’ve already had a regular full moon on December 2nd, and the next will occur on December 31st. The last was in 1990; the next one won't come again until 2028.
The New Year's Eve Blue Moon will be visible in the Americas, Canada, Africa and Europe. Revelers in Asia or Australia will see theirs on New Year’s Day, making January their "blue moon" month.
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