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Seasonal Blue Moon on August 21, 2013

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?

Ambiguity of Definition gave rise to monthly Blue Moon

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.

Seasonal Blue Moons for the next 19 years:

May 21, 2016

May 18, 2019

August 22, 2021

August 19, 2024

May 20, 2027

August 24, 2029

August 21, 2032

Phases of the Moon for 2013 and 2032

Phases of the Moon, courtesy of the NASA Eclipse Web Site

copyright 2013 by Bruce McClure

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

July 2013 Feature * Sept 2013 Feature