In a wonderful quirk of coincidence December 31st, 2009 is not just New Year’s Eve. It’s not just the end of a decade. It’s also a night of a blue moon… in partial eclipse.

A blue moon occurs when there are 13 full moons during a calendar year rather than 12. This is the result of the fact that the lunar cycle is slightly shorter than our calendar months, meaning there is an “extra” full moon every 2.7 years.

Exactly which full moon during a year should be classed as the blue moon is open to debate. It’s generally referred to as the second full moon of a calendar month, though this is though to be a misinterpretation of a definition used by farmers which classed the blue moon as the fourth full moon of a season. (Under this system, the first full moon of a season is the one closest to the relevant solstice or equinox.)

That means that whether next week brings a blue moon depends on your definition. The farmers’ method would have the next blue moon in November next year. And thanks to time zones, the more common calendar method means that anyone from the Indian and eastern time zone (UTC +5 and -5 respectively) will consider the forthcoming full moon to be the first of January, thus making the “blue moon” a few weeks later.

Still, that’s all human construct. What certainly isn’t is the partial lunar eclipse. That’s when the Earth is placed in between the sun and the moon, such that the sun’s rays are blocked and the moon appears to be in a shadow. It usually happens twice a year, and for it to happen with a blue moon is particularly rare: depending on your location and definition, it happened just four times in 20th century.

The peak of the shadow this time round will be at 19:22:39 UTC (that’s also known as Greenwich Mean Time or UK time) and will be visible in most of Europe, Africa, Asia and Australasia. So as a very generalized guide:

• North and South America: Blue moon, but no visible eclipse.
• Europe, Africa and Middle East: Blue moon, visible eclipse
• Asia and Australasia: Full moon (not blue), visible eclipse

And as if the decade’s end/blue moon/partial eclipse wasn’t freaky enough, get this: it occurs on the final day of the International Year of Astronomy.