May 31, 2020

AstroEvent: Would the REAL Blue Moon Please Rise?

This week, we here at Astroguyz seek to re-ignite the controversy (or do you say non-troversy?) That swirls in some of the more obsessive astronomical circles; just what is a Blue Moon? Modern vernacular states this as simply the “second Full Moon of a calendar month,” but as researchers first point pointed out in a Sky & Telescope article in the March 1999 edition, the history behind the term is much more convoluted. The “two per month” definition itself arose from a misinterpretation by Sky & Tel in the 1940’s of an obscure almanac. As per ye ole Maine Farmer’s Almanac circa 1937, a Blue Moon is the third Full Moon is a calendar season containing four, (i.e. between a given solstice-to-equinox) span. Such an obscure event occurs with the Full Beaver Moon on Sunday, November 21st at 12:27PM EST. This Full Moon is number three since the autumnal equinox, and number four will squeak in next lunation just before the winter solstice, with a total lunar eclipse to boot! This brand of Blue Moon occurs every three years, with the last on May 2008 and the next on August 2013. What the shade of blue has to do with it is somewhat of a historical mystery. Is it the fact that almanac printers used to denote this extra moon of the season with blue printing ink, or does it just speak to the fact that such an occurrence is indeed rare? Whatever the cause, be sure to raise a glass or three of Blue Moon beer (they have a slammin’ new Winter Abbey Ale) or try out a Blue Moon cocktail… cheers!

This week’s astro-word is Callipic cycle. An obscure event deserves an equally obscure word, right? Like eclipses, calendar month Blue Moons of both confusing varieties repeat roughly every 235 lunations or 19 years. Known as a Metonic cycle, this period over-shoots the matched values by about 30 minutes per one Metonic cycle. In 334 BC, astronomer Callipus of Cyzius introduced what became known as the Callipic cycle, a period of four Metonic cycles or 76 years/ 940 lunations. This has the added benefit of a reduced level of error of only 11 minutes. The Callipic cycle fell out of favor in the Middle Ages as the Golden Number (year+1/19) came into general use to calculate the all important date of Easter… numerologists take note that November 2029 is indeed another 3rd Full Moon of a 4 Moon season!





  1. [...] above Boston’s on the Beach, an outstanding restaurant complete with one of our astronomical faves, Blue Moon beer. One of our first astronomical exploits occurred when we journeyed down to the beach early Saturday [...]

Speak Your Mind