Did you know that most years have 12 full moons? A calendar month is actually longer that a lunar month, so every two or three years there is an extra full moon. This extra full moon is called a blue moon. Different definitions and cultures place the extra moon at different times of the year.

The Farmer’s Almanac used to define a blue moon as the third full moon in a season (every three months)  that has four full moons instead of three. More recently, due to misinterpretation, a blue moon came to mean a second full moon in a month. This kind of blue moon will happen on New Year’s Eve this year, December 31, 2009.  However, in Australia and most of Asia, it will occur not in December but in January 2010 due to date differences between time zones.

The origins of the name blue moon are unclear. The expression “once in a blue moon”, dating from the 19th century, means something that happens very rarely. Another explanation connects it with the word belewe from the Old English, meaning, “to betray”, since it skewed the calculations for the date of Easter in church calendars, based on moon cycles. Want more information?  Read more about Full Moon Names here.