Blue Moon / You saw me standing alone / Without a dream in my heart …

Actually, I do have a dream …  Blue Moon / I’ll be standing with you / Without a cloud in the sky …

For many people, December 31st will mark the last day of another year, just as it ends every year of the Julian Calendar.  But, for sky enthusiasts, this December 31st has poignant meaning, marking the end of a very exciting year of astronomy outreach – the International Year of Astronomy 2009. So, it seems most appropriate that this wonderful year of sharing the night sky should end with a Blue Moon!

But, what is a Blue Moon?  We often use the term to reference a rare event – “once in a blue moon” – but is the Moon ever really blue?

Actually, on rare occasions, the Moon does “appear” blue due to specific atmospheric conditions, thus that reference for rare events.  But, more often, the term “Blue Moon” defines an “extra” Full Moon within a given calendar, an event that can be credited to the Julian/Christian calendar, farming cycles, and the tradition of naming Full Moons, rather than any cosmic phenomenon.

Of course, there never really is any “extra” moon.  The Moon orbits our planet, phasing through New to Full with such consistency that it occasionally clashes with our less-precise, seasonal timekeeping.  This leaves us with an “extra” moon about every 2.7 years – and we account for it with the name “Blue Moon”.  For yearly timekeeping, this Blue Moon occurs as the thirteenth of a twelve-moon calendar; for seasonal timekeeping, it is the fourth of a three-moon season; and more recently, due to an interpretation error in the mid-twentieth century, a Blue Moon is the second Full Moon in a one-month period.

This year’s Blue Moon just happens to fall on December 31 – the last day of the month, the last day of this year, and the final day of a decade.  On that day, I’ll be there to watch it, rising on the east horizon at sunset – big, bright, and full of promise – as I reflect on the successes of a year of star-filled outreach.  And then I’ll return at sunrise, to watch as that same Moon – the last Moon of this century’s first decade – sinks into the west horizon, bringing forth a New Year under the Milky Way and another decade of new discoveries.

*For a real treat, pop over to Rob Self-Pierson’s Moonwalking site and check out his beautiful #BlueMoonWalk video!