Back to the Astronomy Cafe - 365 of the most popular questions since 1996. Over 1 million page requests!
I know that these articles will dismiss the naming of the 'second full moon of the month' as a big misunderstanding, but as an astronomer, I will continue to define a 'Blue Moon' that way because otherwise this second full moon of a month will go unnamed, unlike the second new moon which is called the Black Moon, the Finders Moon etc.!! (Dr. Sten Odenwald, May 2000)
According to Sky and Telescope magazine, this term has a very complicated and surprisingly recent ontology. The expression 'once in a Blue Moon' appears to have a centuries-old history by most accounts. It is supposed to refer to a very rare event. But, its more exact definition as the second full moon in a single month, seems to be only a few decades old, and is due to a mistake made in an article published in Sky and Telescope back in the 1940's. This definition has only really become popular since the 1980's.
The second full moon in a month begs to be called something, and to call it a Blue Moon seems just as good as any other name I can come up with. There are 29.53 days between any two full moons. There are 365.24 days in the year, which equals 12.37 lunar months. The little bit that is left over (0.37) means that there can be 13 full moons during years that are about 1/0.37 = 2.7 years apart.
American folk lore has names for each of the Full Moons seen during the year. A Blue Moon is the name of the second Full Moon that happens in any given month, at least that was what many of us believed until recently. As it turns out, this calendar definition is itself not the one used by people earlier than the 1940's. It was introduced by mistake by an author of Sky and Telescope magazine, and since the 1980's has become the common 'technical' definition. Even many astronomers prefer to use this 'second full moon of the month' definition, otherwise this second full moon goes without any name, unlike the second new moon ( Black Moon, Spinner Moon etc).
There is a book, in German, called "Die Welt des Mondes" by R. Oldenbourg published in 1957, which includes an article apparently written by Patrick Moore in 'Guide to the Moon' printed in 1953 by Eyre and Spottiswoode. Steffens remarks that at his institute there are witnesses ( a Professor Isserstedt) who actually saw a Blue Moon in the year 1954. It was BLUE, not bluish or powder blue, but BLUE. Patrick Moore's article described several sightings of Blue Moons in 1944 in America, in 1949 in Queensland, and in England on September 26, 1950. According to Moore who witnessed the 1950 event,
"The moon was in a slightly misty sky and had a kind of lovely blue color comparable to the electric glow discharge. I never saw something similar before"
Apparently this phenomenon has been reported by many people all over the world. It is believed that dust in the atmosphere at very high altitudes causes it, and the event in 1950 seen in England may have been produced by an unusually heavy season of forest fires in Canada around the same time. The actual cause may have to do with selective absorption of moonlight by soot particles of the right size, rather than by scattering which accounts for why the sky is blue in the daytime. The term 'blue moon' may indeed have something to do with rare atmospheric conditions perhaps caused by distant forest fires or other 'soot like' particle sources which may be effective in preferentially scattering red light rather than blue light (the normal situation during a sunset).
Does a Blue Moon happen at the same time everywhere on the Earth?
No. As the June/July 1996 Blue Moon illustrates, a slight difference in time zone can spell the difference between whether the exact moment of Full Moon happens on one day or the next. Astronomers define the moment of Full Moon in terms of Universal Time which is the local time relevant to citizens of Great Britain. If you live at a longitude 90 degrees west of this, you clock is set to a time zone that is 6 hours earlier than Universal Time. This means that if a phenomenon occurred at 3AM on July 1, you would observe it at 9 PM on June 30. This isn't a relativistic effect, just a consequence of living on a BIG planet!
Can you have two Blue Moons in a year?
The Synodical Month is 29.53 days long between any two Full Moons. There are 365.24 days in the year which equals 12.37 Lunar Months. The little bit that is left over means that there can be 13 Full Moons during years that are about 1/.37 = 2.7 years apart or so. The following list is the list of Blue Moons for the next 54 years, and a double Blue Moon will happen in 1999 and 2018!:
Date UT January 31 1999 16:07 March 31 1999 22:49 November 30 2001 20:50 July 31 2004 18:06 June 30 2007 13:49 December 31 2009 19:13 August 31 2012 13:59 July 31 2015 10:44 January 31 2018 13:27 March 31 2018 12:37 October 31 2020 14:50 August 31 2022 1:36 May 31 2026 8:46 December 31 2028 16:49 September 30 2031 18:59 July 31 2034 5:55 January 31 2037 14:05 October 31 2039 22:37 August 31 2042 2:03 May 30 2045 17:53 January 31 2048 0:15 September 30 2050 17:33We see that the only months with 30 or 31 days can have a second Full Moon ( A Blue Moon in April will not occur during this particular series). In this series, there also happen to be more Blue Moons in January, July and August than in the other months, but this is due to the statistics of sampling over only a limited time span.
When was the last Blue Moon in February, and when will the next one be?
Don't hold your breath!
The lunar month is 29.53 days long. The largest number of days in February is 29.00000000 days. This means that even if the first Full Moon of February was at midnight on February 1 of a Leap Year, the very next Full Moon would be 0.5 x 24 = 12 hours after midnight on February 29, or in other words on the first day of March! The last time there was a Full Moon on February 1 was in 1980 at 2:22 UT. This was a Leap Year so the very next Full Moon happened on March 1 at 21:00 UT. The next time we get a Full Moon on February 1 will be in the year 2026 at 22:10, which is not a Leap Year. The Full Moon after that will happen on March 3 at 11:39.
There will be Blue Moons on the 31st day of July 2004, December 2009, August 2012, July 2015, January 2018, March 2018, October 2020, August 2023, May 2026 and December 2028. There will also be a Blue Moon on June 30, 2007. The next year after 1999 that will have two Blue Moons will be 2018.
RETURN to the Question and Answer Index.