Once in a Blue Moon

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 20^th Century

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:

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

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.

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

Further Reading

• "What's a Blue Moon?" by Donald W. Olson, Richard Tresch Feinberg and Roger Sinnott, Sky & Telescope, May 1999 (http://www.skyandtelescope.com/observing/celestial-objects-to-watch/what-is-a-blue-moon/)

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

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