Blue Moon of 21 august 2013

Blue Moon of 21 august 2013, a photo by menteblu61 on Flickr.

Original Blue Moon Definition

The modern definition of a Blue Moon was derived from an earlier idea of what a Blue Moon was. This earlier definition says a Blue Moon is when there are 4 Full Moons in a season rather than the usual 3. The Blue moon is the 3rd full moon out of the 4. This definition gets a bit complicated and it's origins are murky. One school of thought has to do with the naming of the full moons. Many cultures named the Full Moons each month to reflected the times for planting, harvesting or seasonal conditions. When an extra full moon was thrown in it was referred to as a Blue Moon to keep the Full Moon names constant throughout the year.

Another origin could be from the Christian ecclesiastical calendar. This one gets even more tricky but basically has to do with the idea that there are usually 12 full moons in a year. The full moons on this calendar were important markers for determining curtain dates such as Easter. When a 13th Full moon was thrown into the year it made things messy so giving it a name allowed the calendar to stay on track.

The idea of a Blue Moon being the extra full moon in a season (or when there were 13 in a year) was widely used in 19th and early 20th center Farmers Almanacs and the more modern version seems to have come from an article written in the 1930's that misinterpreted the Farmers Almanac definition. The article was names "Once in a Blue Moon" and from that point on the term became part of popular culture.