What is a Blue Moon?

By Michael Myers


Modern Definition:

A Blue Moon is commonly the name given to the second full moon in a month. Since a full moon occurs every 29 1/2 days, if there is a full moon on the 1st or 2nd day of a month, there is a good chance there will be a second full or blue moon that month.

In 1999, there were two blue moons very close together. One on January 31st (after the full moon on Jan. 2nd) and the other on March 31st (after the full moon on March 2nd).

There was another in November 2001, but not again until July 2004.

There was a blue moon in May 2007 (and one in June 2007 for those in Europe and Asia), and December 2009. The next one was August 2012, then July 2015.

And we won't see two blue moons in one year again until 2018!

Sorry, it's not really blue : (

Older Definition:

More traditionally, a blue moon was referred to as the 4th full moon in a season. That is, each of the 4 seasons of the year has 3 months, and will usually have 3 full moons. Each of these 12 moons has a name like "Harvest Moon," "Hunter's Moon" and the like. But when a season occurs that contains 4 full moons, there is no name for this occasional moon and it was given the name, "Blue Moon."

Some further comments:

from a contributor named John:
"There are two definitions given for the blue moon.

The first definition (second full moon in a calender month) was invented by Sky and Telescope magazine in March 1946, apparently by a contributor who misinterpreted the Maine Farmer's Almanac. This definition was subsequently promulgated in other news media, and has become almost the "accepted" one, despite its modernity. In March 1999, Sky and Telescope magazine finally published a very belated correction/retraction.

The second definition (fourth full moon in a season) is closer to the mark, but alas is not quite right either. A traditional blue moon does occur only when there are four full moons in a season, but it is the _third_ of these which is the blue moon, not the fourth. Traditionally, names are given to the first, second, and last full moon of a season, so it is the third moon of a season with four which is "blue". This is the traditional definition, but its origins are a bit hazy."

Check the Old Farmer's Almanac at
which gives the North American names for the full moons, and explains the Blue Moon naming confusion.


Blue Moon Images

Here are some pictures of the January and March 1999 blue moons, and also the one in May, 2007:

Blue Moon 1/31/99

6:00 pm, rising behind the treetops.

9:30 pm

Blue Moon 3/31/99

A hazy evening in Northern Arizona

Blue Moon 5/31/07

This blue moon was actually red!


Blue Moon 8/30/2012:

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