Once in a Blue Moon

Four Full Moons in a season

According to an article in the May 1999 issue of Sky & 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 Farmers' 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 Farmers' 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 Farmers' 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 20th Century

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

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

DateTimeEvent
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.

We would like to thank Victor Engel and members of the CALNDR-L discussion group for drawing the curious Full Moon discrepency of 1961-2 to our attention.

Easter Blue Moons

The Maine Farmers' Almanac actually assigns the Easter Full Moon as the first Full Moon of Spring, rather than the Full Moon which falls after the mean Spring Equinox, defined as the moment when the Right Ascension of the Mean Sun is zero. This ensures that the Lenten Moon—the Full Moon preceding the Easter Full Moon—is never considered blue.

The Easter Full Moon is not an astronomical Full Moon. It is calculated using a method which was devised in the late 16th century, before the development of calculus and celestial mechanics enabled astronomers to calculate the Moon's orbit accurately. Instead, the 19-year Metonic Cycle is used to approximate the phases of the Moon, and the Spring Equinox is defined to fall on 21 March. The date of the Easter Full Moon can differ from the astronomical Full Moon by a day or two. The interested reader can explore this fascinating subject in more detail at Obliquity's sister web site The Date of Easter, De-mystified.

In most years, this does not affect the date of the Blue Moon in the Maine Farmers' Almanac. In the 20th and 21st centuries, there are only two exceptions: 1913 and 2008. In both years, the astronomical Full Moon in March falls just before the mean Equinox but just after the true (astronomical) Equinox. As a result, the Blue Moon in the Maine Farmers' Almanac falls in May, in agreement with the astronomical seaons, and not in February.

We would like to thank Ernest T. Wright of NASA's Scientific Visualization Studio at the Goddard Space Flight Center for drawing our attention to the role of the Easter Full Moon in calculating the dates of Blue Moons in the Maine Farmers' Almanac.

Traditional Blue Moons in the 21st Century

Year Astronomical Seasons Maine Farmers' Almanac
2002 Aug 22 at 22:29 Nov 20 at 01:34
2005 Aug 19 at 17:53
2008 May 20 at 02:11 May 20 at 02:11
(see "Easter Blue Moons" above)
2010 Nov 21 at 17:27
2013 Aug 21 at 01:44
2016 May 21 at 21:14
2019 May 18 at 21:11 Feb 19 at 15:53
2021 Aug 22 at 12:02 Nov 19 at 08:57
2024 Aug 19 at 18:25
2027 May 20 at 10:59 Feb 20 at 23:23
2029 Aug 24 at 01:51 Nov 21 at 04:02
2032 Aug 21 at 01:46
2035 May 22 at 04:25
2038 May 18 at 18:23 Feb 19 at 16:09
2040 Aug 22 at 09:09 Nov 18 at 19:05
2043 Aug 20 at 15:04
2046 May 20 at 03:15
2048 Aug 23 at 18:06 Nov 20 at 11:19
2051 Aug 22 at 01:34
2054 May 21 at 15:16
2057 May 18 at 19:02 Feb 19 at 11:56
2059 Aug 23 at 09:41 Nov 19 at 13:09
2062 Aug 20 at 03:55
2065 May 20 at 02:05
2067 Nov 20 at 23:49
2070 Aug 21 at 19:53
2073 May 21 at 10:02
2076 May 18 at 17:38 Feb 19 at 23:48
2078 Aug 23 at 08:11 Nov 19 at 12:52
2081 Aug 19 at 11:15
2084 May 20 at 02:36
2086 Nov 20 at 20:12
2089 Aug 21 at 06:15
2092 May 21 at 10:00
2095 May 19 at 09:21 Feb 19 at 06:59
2097 Aug 22 at 23:52 Nov 19 at 13:03
2100 Aug 19 at 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 & Telescope which used the Maine Farmers' Almanac rule. His feedback prompted us to revise this web page.

Further Reading

Once in a Blue Moon Modern Blue Moons Who wrote this?

Copyright © 2006-2024 by David Harper and L.M. Stockman
All Rights Reserved
Designed and maintained by Obliquity
Last modified: 29 September 2024
http://www.obliquity.com/astro/blue-seasonal.html