HELIOLITH

So tonight is, by common modern definition, a blue moon. Research about what exactly a blue moon is led to this richly informative article in Sky and Telescope.

To summarize based upon my own understanding, the actual definition was created in conjunction with Christian ecclesiastical naming of the moons of the year. Due to the church’s need for all the moons to keep their correct names, (especially around Easter) they would assign the name “blue moon” to the “extra” moon that would fall within a season. They gave that name to the third moon between any given solstice and equinox (as determined by the average path of the sun, not its actual path–a fudge factor) only when there were four moons in that season. This older definition is somewhat closer to a natural or mathematical definition of the phenomenon of an extra full moon occurring during one year (13 instead of 12). Its naming (and the decision about which of the 4 moons is the extra one) is based on a cultural/religious system, so it’s more of a measure of how the moon refuses to fit into the calendrical systems we create to measure nature, than a pure measure of an astronomical phenomenon.

The article referenced above explains extensively the math and logic behind the older definition, as well as a thoroughly researched explanation of exactly how the modern definition of 2 full moons per month was based on one man’s erroneous reading of a 1937 Farmers’ Almanac from Maine.

To find out when the sun & moon rise and set in your town, check out this tool from the naval database.

other blogs talking about the “blue moon” today