blue moon

What is a Blue Moon???

Popular understanding says a Blue Moon is when two Full Moons land in the same calendar month. ...Apparently our current understanding of a Blue Moon comes from a 1940’s article that appeared in astronomy magazine suggesting the definition of a Blue Moon was the second Full Moon in a calendar month. This event only happens every two and a half to three years.

In a Sky and Telescope article we learn the original definition of a Blue Moon is referring to 4 Full Moons in one season. In this instance a season is defined as the time between a Solstice and Equinox or an Equinox and Solstice. (See Table below) There are four seasons in a year and generally three Full Moons in each season or 12 Full Moons per year. Occasionally we find there are 4 Full Moons in one season and 13 Full Moons in a year marking the annual solar and lunar cycle from December Solstice to Solstice.

from an article by Cayelin K Castell on celestial timings website

first fullmoon of december 09, another one coming up on newyear's eve, which will make it, by one definition, a BLUE MOON !

blue moons coming up...

Dec 2009	Two Full Moons	December 2 and 31
Aug 2012	Two Full Moons	Aug 2 and 31
Jul 2015	Two Full Moons	July 2 and 31
Jan 2018	Two Full Moons	Jannuary 2 and 31 with no Full Moon in February
Oct 2020	Two Full Moons	October 1 and 31

...and by the other definition...

Dec 2010	Fourth Full Moon in a Season	Sep 23, Oct 23, Nov 21, Dec 21
Sep 2013	Fourth Full Moon in a Season	Jun 23, Jul 22, Aug 19, Sep 19
Jun 2016	Fourth Full Moon in a Season	Mar 23, Apr 22, May 21, Jun 20
Jun 2019	Fourth Full Moon in a Season	Mar 21, Apr 19, May 18, Jun 17
Jun 2024	Fourth Full Moon in a Season	Jun 22, Jul 21, Aug 19, Sep 18
Jun 2027	Fourth Full Moon in a Season	Mar 22, Apr 20, May 19, Jun 18
Dec 2029	Fourth Full Moon in a Season	Sep 22, Oct 22, Nov 21, Dec 20
Jun 2036	Fourth Full Moon in a Season	Mar 23, Apr 22, May 22, Jun 20
Jun 2038	Fourth Full Moon in a Season	Mar 21, Apr 29, May 18, Jun 17 +eclipse
Sep 2040	Fourth Full Moon in a Season	Jun 26 + eclipse, Jul 24, Aug 22, Sep 20

once in a blue moon

uh-oh, a blue moon ends the 00s