APOD: On a Blue Moon (2012 Sep 01)

[Chris Peterson]

While language may evolve, definitions should not.

Those assertions directly contradict one another.

[neufer]

Very disappointed that you would continue a myth about what a blue moon is.

What myth? The current usage of "Blue Moon" may be based on a 50-year old misunderstanding of earlier (but similar) uses, but language evolves as often through mistakes or misunderstandings as it does by any other mechanism. The point is, that is the current usage, and there's nothing wrong with it. There is certainly no myth. By modern definition, a Blue Moon is just what this APOD says it is: the second full Moon in a calendar month.

I am also very disappointed to see the incorrect use of the term "Blue Moon"here on APOD. While language may evolve, definitions should not. A Blue Moon is the third Full Moon in a season that has four full moons. Period. Otherwise, they aren't rare - for example; both January and March of 2018 have two full moons. Hardly a rarity.

Click to play embedded YouTube video.

<<In the modern system of "traditional" full moon names tied to the solstice and equinox points, a supernumerary full moon in such a period is called a blue moon. The term "blue moon" used in this sense may date to as early as the 16th century, but it became well known in the United States due to the Farmers' Almanac (published since 1818).

According to the Farmers' Almanac, a "blue moon" is the third full moon in any period between either solstice and equinox, or between equinox and solstice, (calculated using the mean tropical year), which contains four full moons. These seasons are equal in length, unlike the astronomical ones, which vary in length depending on the Earth's speed in its elliptical orbit round the sun. To compare, in 1983 the equal length seasons began at 1.48 AM on 23 March, 9.15 AM on 22 June, 4.42 PM on 21 September and 12.10 AM on 22 December, while the astronomical seasons began at 4.39 AM on 21 March, 11.09 PM on 21 June, 2.42 PM on 23 September and 10.30 AM on 22 December (all times GMT). Due to a misinterpretation of this definition in the March 1946 Sky & Telescope magazine, "blue moon" has also been used in the sense of "the second full moon in any month which contains two full moons (this usage has been noted as "erroneous" by Sky & Telescope in 1999). According to either definition, "blue moons" occur with the average frequency of intercalary months, seven times in 19 years, the Farmers' Almanac system of "full moon names" effectively defining a lunisolar calendar.>>