Happy Blue Moon!

Today there will be a full moon, and it will be the second time this month, therefore we call it a blue moon.

Astronomically, a blue moon isn't special. It's just another time when the side of the moon facing Earth is lit by the sun. The fact that there are two in a month is just an accident of our calendar system. It reminds us, though, that although our calendar is based on astronomical events like moon cycles, it doesn't match up very well.

Also interesting:

*A "blue moon" isn't any more likely to look blue than any other moon, but micron-sized particles of smoke or ash in the atmosphere can make the moon look blue.

*While we in the Americas see a blue moon tonight, the full moon will occur after midnight in Europe. Their full moon will be in June, not May, so they won't see a blue moon until the next full moon at the end of June. (In fact, New Zealand won't see a blue moon until the end of July!)

*Since there are a little more than 12 full moons per year, there is about one per month, but sometimes two. Twelve per year can also be seen as three per season, and traditionally the first, second and last full moons of a season had names (like "harvest moon" or "egg moon"). Since there are more than 12 full moons some years, some seasons have four full moons. The first, second, and last would still have names, but the third was just called the blue moon. This was the traditional definition of the blue moon. In 1946, Sky and Telescope published an article, using the twice-in-a-month definition, based on a misunderstanding of the traditional blue moon usage in the Farmers Almanac.* Since most people know when the months are, but don't know when the egg moon is, the new definition has stuck.

_{*As a farm girl at heart, I find it quite amusing that Sky and Telescope was outsmarted by the Farmer's Almanac.}