The Maine Farmers Almanac, under the editorship of Henry Porter Trefethen from 1932 to 1957, defined a Blue Moon as the third of four full Moons to take place in one season. A season is defined as the period of time between a solstice and an equinox - or vice versa.
Normally a season only has three full Moons. Now and again - or once in a Blue Moon - a season can have four full Moons. In 2013, for instance, four full Moons fall in between the June 2013 solstice and the September 2013 equinox. The third of the bunch is called a Blue Moon:
Solstice: June 21, 2013
Full Moon: June 23
Full Moon: July 22
Blue Full Moon: August 21
Full Moon: September 19
Equinox: September 22, 2013
Photo credit: Tim Geers
Seven times in 19 years, you can count on a season to harbor four full Moons. Because the Maine Farmers Almanac named full Moons relative to the solstices and equinoxes, it was less disruptive to full Moon nomenclature to call the third (rather than the fourth) full Moon of the season a Blue Moon.
Can you tell me full moon names?
Possible for only two full Moons in one season?
The seasonal definition of Blue Moon is not to be confused with the monthly definition of Blue Moon: the second of two full Moons to occur in one calendar month. Although the monthly definition is the better-known one nowadays, the origin of the monthly Blue Moon finds roots in an article by James Hugh Pruett in the March 1946 issue of Sky & Telescope magazine, whereby the author's misinterpretation of the seasonal Blue Moon gave rise to the monthly Blue Moon.
The Maine Farmers Almanac asserted that 7 times in 19 years, there are 13 full Moons in the span of one year, the odd one out being a Blue Moon. However, the Maine Farmers Almanac was referring to the tropical year - the period of time between successive December solstices, not the calendar year. The ambiguity of the Blue Moon definition led James Hugh Pruett to think a Blue Moon took place whenever 13 full Moons fell in one calendar year.
Had Pruett looked up the date of the August 1937 full Moon, he might have known better. But that wondrous "mistake" may well be one of the greatest legacies of Sky & Telescope magazine and StarDate, the popular radio program initiated by Deborah Byrd!
Origin of Once in a Blue Moon in folklore and history
Seven Times in 19 Years
The phases of the Moon recur on (or near) the same dates every 19 years. In 19 years, there are 76 seasons (19 x 4 = 76). If three full Moons occurred in every season, then there would be a total of 228 (76 x 3 = 228) full Moons in 19 years. Yet, 235 full Moons occur in 19 years, so it's inevitable that 7 of these extra full Moons (235 - 228 = 7) should fall within the bounds of 7 different seasons.
May 21, 2016
May 18, 2019
August 22, 2021
August 19, 2024
May 20, 2027
August 24, 2029
August 21, 2032
Seasonal Blue Moons - 20th and 21st Centuries
Solstices & Equinoxes from 1900 to 2099
Phases of the Moon: 1901-2000
Phases of the Moon: 2001-2100