Blue Moon - four Full Moons in a season

According to an article in the May 1999 issue of Sky and Telescope, the traditional definition of a Blue Moon is the third Full Moon in a season which has four Full Moons. Compilers of almanacs such as the Maine Farmer's Almanac would use a coloured symbol to denote this third Full Moon, hence the name.

When do the seasons begin?

Astronomers define the start of each season according to the actual position of the Sun in its annual journey against the fixed stars. This leads to seasons of unequal length, since the Earth's orbit is not circular. The northern-hemisphere Spring and Summer are slightly longer than Autumn and Winter, so a traditional Blue Moon is more likely to occur in Spring or Summer.

The Maine Farmer's Almanac, by contrast, defined the start of each season according to a quantity known as the Right Ascension of the Mean Sun. As its name suggests, this is based on an idealised, fictional Sun which moves at a uniform speed, giving seasons of equal length.

Since there are two definitions of the start of the seasons, the dates of the traditional Blue Moons can sometimes be different, depending on whether you adopt the astronomical definition or follow the Maine Farmer's Almanac. In the tables below, we give the dates and times of the traditional Blue Moons using both definitions. If a date and time span both columns, then the two methods yield the same Blue Moon.

All dates and times are Greenwich Mean Time.

Traditional Blue Moons in the 20^th Century

Year	Astronomical Seasons	Maine Farmer's Almanac
1902	May 22 10:46
1905	Feb 19 18:52
1907	Aug 23 12:15
1910	Aug 20 19:14
1913	May 20 07:18	Feb 21 02:03
1915	Aug 24 21:40	Nov 21 17:36
1918	Aug 22 05:02
1921	May 21 20:15
1924	May 18 21:52	Feb 20 16:07
1926	Aug 23 12:38
1929	Aug 20 09:42	May 23 12:50
1932	May 20 05:09	Feb 22 02:07
1934	Aug 24 19:37	Nov 21 04:26
1937	Aug 22 00:47
1940	May 21 13:33
1943	May 19 21:13	Feb 20 05:45
1945	Aug 23 12:03	Nov 19 15:13
1948	Aug 19 17:32	May 23 00:37
1951	May 21 05:45
1953	Aug 24 20:21	Nov 20 23:12
1956	Aug 21 12:38
1959	May 22 12:56
1961	Nov 22 09:44	No Blue Moon
1962	May 19 14:32	Feb 19 13:18
1964	Aug 23 05:25	Nov 19 15:43
1967	Aug 20 02:27	May 23 20:22
1970	May 21 03:38
1972	Nov 20 23:07
1975	Aug 21 19:48
1978	May 22 13:17
1981	Feb 18 22:58
1983	Aug 23 14:59	Nov 20 12:29
1986	Aug 19 18:54
1989	May 20 18:16	Feb 20 15:32
1991	Nov 21 22:56
1994	Aug 21 06:47
1997	May 22 09:13
2000	Feb 19 16:27

The curious "discrepancy" of 1961 and 1962

Keen-eyed readers will have noticed that 1961 and 1962 both had traditional Blue Moons by the astronomical seasons. This leads to an interesting discrepancy between the number of traditional Blue Moons during the 20^th century: there is one more Blue Moon by the astronomical seasons than by the Maine Farmer's Almanac definition.

We thank Victor Engel and members of the CALNDR-L discussion group for drawing this curious fact to our attention.

The explanation is that Winter 1961-2 had only two Full Moons. Here are the dates and times, expressed in GMT:

Date Time Event

1961 Dec 22 00:42 Full Moon

1961 Dec 22 02:19 Winter Solstice

1962 Jan 20 18:16 Full Moon

1962 Feb 19 13:18 Full Moon

1962 Mar 21 02:30 Spring Equinox

1962 Mar 21 07:55 Full Moon

Date	Time	Event
1961 Dec 22	00:42	Full Moon
1961 Dec 22	02:19	Winter Solstice
1962 Jan 20	18:16	Full Moon
1962 Feb 19	13:18	Full Moon
1962 Mar 21	02:30	Spring Equinox
1962 Mar 21	07:55	Full Moon

Notice that the December Full Moon occurs less than two hours before the solstice, and that the March Full Moon falls only five and a half hours after the equinox.

This is possible because the length of the lunar month is slightly variable, following a cyclical pattern so that several "long" lunar months can follow in succession. This happened in the winter of 1961-2. Three long lunar months are slightly longer than the interval between the December solstice and the March equinox, and as a result, there were just two Full Moons during that period in 1961-2.

Traditional Blue Moons in the 21^st Century

Year	Astronomical Seasons	Maine Farmer's Almanac
2002	Aug 22 22:29	Nov 20 01:34
2005	Aug 19 17:53
2008	May 20 02:11	Feb 21 03:30
2010	Nov 21 17:27
2013	Aug 21 01:44
2016	May 21 21:14
2019	May 18 21:11	Feb 19 15:53
2021	Aug 22 12:02	Nov 19 08:57
2024	Aug 19 18:25
2027	May 20 10:59	Feb 20 23:23
2029	Aug 24 01:51	Nov 21 04:02
2032	Aug 21 01:46
2035	May 22 04:25
2038	May 18 18:23	Feb 19 16:09
2040	Aug 22 09:09	Nov 18 19:05
2043	Aug 20 15:04
2046	May 20 03:15
2048	Aug 23 18:06	Nov 20 11:19
2051	Aug 22 01:34
2054	May 21 15:16
2057	May 18 19:02	Feb 19 11:56
2059	Aug 23 09:41	Nov 19 13:09
2062	Aug 20 03:55
2065	May 20 02:05
2067	Nov 20 23:49
2070	Aug 21 19:53
2073	May 21 10:02
2076	May 18 17:38	Feb 19 23:48
2078	Aug 23 08:11	Nov 19 12:52
2081	Aug 19 11:15
2084	May 20 02:36
2086	Nov 20 20:12
2089	Aug 21 06:15
2092	May 21 10:00
2095	May 19 09:21	Feb 19 06:59
2097	Aug 22 23:52	Nov 19 13:03
2100	Aug 19 21:29

Acknowledgements

We thank Jeff Klarer for pointing out that our original list of traditional Blue Moons, based only on the astronomical seasons, differed from the list in Sky and Telescope which used the Maine Farmer's Almanac rule. His feedback prompted us to revise this web page.

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