Blue Moon (Calendar)

Blue Moon is commonly used in the English-speaking world to refer to a second full moon within a month in the Gregorian calendar . In colloquial language, the phrase Once in a blue moon means something very rare. In an astronomical definition, this means the third full moon within a season with four full moons.

The origin

The term blue moon originally referred to a very rare event that only occurs sporadically in the English-speaking world. It was first recorded in writing in 1528 in the small piece Rede Me and Be Not Wroth:

"Yf they say the mone is blewe
We must believe that it is true."

"Ife sī say the moons are blāw
[Sō] we must believe that it is true."

-

The blue moon in the 19th century

At the beginning of the 19th century in the USA it was associated with astronomical events through several intermediate stages , probably due to the fact that the moon sometimes really shimmers bluish due to volcanic eruptions or the like. In addition, the phases of the moon played a special role in agriculture at that time . The full moons were numbered as the first, second and last full moon of the respective season. Due to the difference between the synodic month (about 29.5 days) and the calendar month , it happens that four full moons occur in one season. In the Maine Farmers' Almanac from 1819, the third full moon of such a season was designated as a blue moon for the first time .

The blue moon in the 20th century

In common (English) parlance, knowledge of the definition of the blue moon was lost at the beginning of the 20th century . From a misinterpretation of the Maine Farmers' Almanac by the amateur astronomer James Hugh Pruett (1886–1955) in his article Once in a Blue Moon in the Sky & Telescope magazine from March 1946, the now more common definition arose that a blue moon is the second full moon is in a calendar month. This happens about 41 times a century or 412 times a millennium - on average around every 2.4 years, naturally most frequently in the months with 31 calendar days, while it can happen that in February there is no full moon at all, such as in the Years 1999, 2018 and 2037, with such a Blue Moon both in the previous January and in the following March.

additional

The evergreen Blue Moon , created in 1934, uses the phrase for something very rare, in this case love. The song by Richard Rodgers (music) and Lorenz Hart (text) has been interpreted many times - including by Billie Holiday (1952), Elvis Presley (1954), Frank Sinatra (1961), Bob Dylan (1970) and Rod Stewart (2004) - and contributed decisively to the popularization of the term.

On the search string once in a blue moon, the Google search engine delivers the result once in a blue moon = 1.16699016 × 10 ⁻⁸ hertz as a so-called Easter egg . This value means a distance of 2.71542689 years for the occurrence of two full moons within one month. Blue moon events, however, have a mean distance of 2.4278 years, in both the Julian and Gregorian calendars.

A full moon on New Year's Eve is - like every full moon on the 31st of a month - always a blue moon and occurs on average every 29.53058912 years. The number is identical to the length of a synodic moon orbit of 29.53058912 days. On December 31, 2009 there was also a partial lunar eclipse . On December 31, 2028 at around 5 p.m. UTC there will be a similar constellation with a total lunar eclipse.

In the August 2011 film adaptation of the Smurfs , the blue moon is said to have magical powers, which among other things serves as a basis for the magic of the Smurfs and is intended to help them return to their world. The film shows the English version of this Wikipedia article.

The private memorial service for Neil Armstrong , the first man on the moon, took place six days after his death on the day of a Blue Moon . The double meaning of blue (English for both “blue” and “sad, gloomy”) also plays a role here.

In the Wicca religion , the Blue Moon is a special festival day ( Esbat ).

Frequencies

Average constant lengths for day (earth's rotation), month (lunar orbit) and year (earth's orbit), corresponding to the values for the year 2000
Calendar switching according to the rules of the corresponding calendar
synodic period: ${\ displaystyle T = 29 {,} 53058912 {\ rm {\, \, days}}}$
the probability of blue moons is:
- for months with a length L shorter than T: 0
- for months with a length L between T and 2 · T: L / T - 1
- for months with a length L longer than 2 · T: 1
Blue moon frequency for years with four 30-day months and seven 31-day months and any number of months with 29 or fewer days: ${\ displaystyle H = 4 \ cdot ((30 / T) -1) +7 \ cdot ((31 / T) -1) = 0 {,} 41189559843 {\ rm {\, \, year ^ {- 1} }}}$
mean distance between two blue moons: ${\ displaystyle Y = H ^ {- 1} = 2 {,} 427799675 {\ rm {\, \, years}}}$
Blue-moon frequency of the Gregorian calendar: ${\ displaystyle f = {\ frac {H} {365 {,} 2425 \ cdot 24 \ cdot 60 \ cdot 60 {\ rm {\, \, s / year}}}} = 13 {,} 05245191 {\ rm {\, \, nHz}}}$

Frequencies of months without two full moons or with two full moons
event	Frequency per century		Average distance in years
event	Gregorian calendar	Julian calendar	Gregorian calendar	Julian calendar
a second full moon	41,189 560		2,427 800
... in a specific month with 31 days	4,975 894		20.096 890
... in a specific month with 30 days	1.589 575		62.909 895
no full moon in February	4.361 881	4,336483	22,925 890	23.060159
... with 29 days	1.796 744		55.656 228
... with 28 days	5,183,063		19.293 610
No full moon in February and second full moon
both in January and in March	4.048 020	4.025 727	24,703 437	24,840 232
either in January or March	0.156 931	0.155 378	637.224 550	643,589 998
neither in January nor in March	does not occur

Web links

Christian Pinter: Symbol of loneliness, Wiener Zeitung, November 16, 2001
Blue Moon ?, wend.de, June 30, 2007
To the definition of the Blue Moon at infoplease.com (English)
To calculate the Blue Moon at obliquity.com (English)
What is a Blue Moon , description and table 2018-2039 (engl.)

Individual evidence

↑ Blue moon, in: Online Etymology Dictionary (English).
↑ Overview 2001–2104, jgiesen.de (English)
↑ Google search: Once in a blue moon. Retrieved December 31, 2009 .
^ For the first time in decades, "Blue Moon" on New Year's Eve ( memento from January 2, 2010 in the Internet Archive ), Zeit.de, accessed on December 31, 2009.
↑ Blue Moon and Eclipse - Astronomy Picture of the Day of January 2, 2010.

[1] Blue moon, in: Online Etymology Dictionary (English).

[2] Overview 2001–2104, jgiesen.de (English)

[3] Google search: Once in a blue moon. Retrieved December 31, 2009 .

[4] For the first time in decades, "Blue Moon" on New Year's Eve ( memento from January 2, 2010 in the Internet Archive ), Zeit.de, accessed on December 31, 2009.

[apod-5] Blue Moon and Eclipse - Astronomy Picture of the Day of January 2, 2010.