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August 2005 Feature: "Old-Style" Blue Moon


There are several definitions for "blue moon." Most commonly, a blue moon refers to the second of two full moons in a calendar month. A blue moon by this definition last appeared on July 31, 2004, with the first of these two July full moons falling on July 2. More on the July 2004 blue moon can be found here.

Yet there's another definition. A blue moon is also the third of four full moons in one season. Normally, there are only three full moons in a season, a season being the period of time between a solstice and an upcoming equinox, or from an equinox to the following solstice. This month's August 19th full moon qualifies as a blue moon because it's the third of four full moons between the June solstice and the September equinox. The dates and times below are for Eastern Daylight Time.


June solsticeJune 21, 20052:46 a.m.

June full moonJune 22, 200512:14 a.m.
July full moonJuly 21, 20057:00 a.m.
August blue full moonAugust 19, 20051:53 p.m.
September full moonSeptember 17, 200510:01 p.m.

September equinoxSeptember 22, 20056:23 p.m.


The much more fashionable definition for a blue moon -- the second of two full moons in one calendar month -- is thought to have originated from a misinterpretation of the other definition -- the third of four full moons in one season. It's a remarkable tale, full of twists and turns, and "stranger than fiction." Read all about it in this Sky & Telescope article -- a humorous case of mistaken identity, reminiscent of Shakespeare's A Midsummer Night's Dream.

One fascinating thing about the moon is that its phases realign themselves on (or near) the same calendar dates every 19 years. Yes, we'll have another "old-style" blue moon (third of four full moons in a season) on this same date in 2024, which, of course, is August 19.

In 19 years, there are a total of 228 calendar months (19 x 12 = 228) and 235 lunar months (a lunar month being the period of time between successive full moons). This 19-year synchronization between lunar and seasonal cycles is called the Metonic cycle. The lunar month, incidentally, averages a bit more than 29.5 days in length while the average calendar month is nearly 30.5 days long.

Since there are 235 full moons but only 228 calendar months in this 19-year period, that leaves a remainder of 7 full moons (235-228 = 7) that have to lodge with another full moon in the same calendar month. So that's why 7 times in 19 years, we can expect two full moons to appear in a calendar month -- the second full moon, of course, being called a blue moon.

This reasoning applies to "old-style" blue moons as well, though it might not be as intuitively obvious. There are always a minimum of three full moons per season, but 7 of these seasons have to absorb an extra full moon. That means 7 seasons in this 19-year period must have four full moons. Looking 19 years into the future, let's see if our rule holds water. . .

Year 12006No blue moon
Year 22007May 31 "new-style" blue moon
Year 32008May 19 "old-style" blue moon
Year 42009December 31 "new-style" blue moon
Year 52010November 21 "old-style" blue moon
Year 62011No blue moon
Year 72012August 31 "new-style" blue moon
Year 82013August 21 "old-style" blue moon
Year 92014No blue moon
Year 102015July 31 "new-style" blue moon
Year 112016May 21 "old-style" blue moon
Year 122017No blue moon
Year 132018January 31 & March 31 "new-style" blue moons
Year 142019May 18 "old-style" blue moon
Year 152020October 31 "new-style" blue moon
Year 162021August 22 "old-style" blue moon
Year 172022No blue moon
Year 182023August 30 "new-style" blue moon
Year 192024August 19 "old-style" blue moom

Adding everything up, it looks like we have a working formula: 7 years with a "new-style" blue moon, 7 years with an "old-style" blue moon, and 5 years with no blue moon at all. The only hitch comes in 2018, a year which actually features TWO "new-style" blue moons -- giving us a total of 8 rather than 7 "new-style" blue moons for this 19-year cycle.

That's because February in 2018 has NO FULL MOON whatsoever, so this left-over full moon has to fall somewhere -- into the lap of March. February, which is shorter than the 29.5-day lunar month, is the only month where it's possible to have no full moon.

Maine Farmer's Almanac

Actually, I simplified things a bit, neglecting to mention that the above old-style blue moons don't quite match the format laid out by the Maine Farmer's Almanac -- this almanac apparently being the original source of our blue moon saga. It's my understanding that the Maine Farmer's Almanac uses fictitious "mean" seasons rather than true astronomical seasons to calculate the third of the season's four full moons. You can read about this "traditional" blue moon in this article, which lists dates for blue moons as determined by both astronomical seasons and the Maine Farmer's Almanac.

Whichever method you choose, the August 19, 2005 full moon counts as the third of the season's four full moons -- a bona fide blue moon by either reckoning.


copyright 2005 by Bruce McClure


Equinoxes & Solstices -- dates & times

Moon Phases by Fred Espenak

Moon Phases -- U.S. Naval Observatory

July 2005 Feature * Sept 2005 Feature