The old adage “once in a blue moon” gives a sense of how rare an event a blue moon actually is. The name “blue” goes back to folklore, from the old English word belewe – “betrayer” – perhaps because of its unexpected showing. A blue moon is the second full moon in a month (typically only one full moon occurs monthly), so the appearance of a second one can throw off lunar calendar keeping; perhaps this was the trickiness belewe referred to. The event itself is not so rare – it happens once every two and a half years on average – but when combined with other rare astronomical phenomenon, particularly on the last day of the last year of the decade it is thought to be a good omen.

On this day, December 31, in 2009, a Blue moon occurred at the same time as a lunar eclipse.

Somehow, instead of considering the convergence of two such rare events to be another sign of the world’s impending doom, many astrologers went on record saying they considered the event a release of cosmic energy that will move metaphorical obstacles out of people’s lives. Had they lived a century before, in 1883, they might have gotten to see an actual blue colored moon: the volcano Krakatoa erupted, spewing plumes of ash into the atmosphere and filtering out the red color wavelengths. The ash stayed in the air for years, maybe even during the rise of a Betrayer Moon.