http://www.evolvingdoor.ca/miscarticles/bluemoon.php
<<The current popular definition of a Blue Moon is “the second Full Moon in the same calendar month.” But just figuring out when any Full Moon takes place can be tricky business. Time zones must be taken into account, which could change the day and even the month that it falls on. My Canadian calendar for 2004 says there is a Full Moon on July 31 at 2:04 PM, Eastern Daylight Time, but a calendar sold in Tokyo, for instance, would show it on August 1 at 3:05 AM. We know there are also Full Moons on July 2 and August 29 (or 30, in the far east time zones), so depending on where you live the second Moon will either be on July 31 or August 30. Does this really mean the “Blue Moon” is different for different parts of the world? Well, yes and no.
The deeper problem is that the popular definition of “Blue Moon”—the second Full Moon in a calendar month—is based on misinterpreted information that was published in a Sky and Telescope magazine article in 1946! Ironically, it was a 1999 article in the same magazine that discovered and corrected the mistake, but by that time the media, Internet, and even the Trivial Pursuit game, had proliferated the incorrect definition and now most of the world understands an incorrect, if persistent, definition of “Blue Moon.”
So Just What Is A "Blue Moon?"
An older traditional meaning of “Blue Moon” goes back to the 19th century and means the third Full Moon in a season which has four Full Moons. So what, you say? Well, each season consists of 3 months, typically with one Full Moon per month, so having four Full Moons land in a 3-month season is something that happens, quite literally, only once in a Blue Moon. The Maine Farmers’ Almanac (which was incorrectly referenced in the now-infamous 1946 article) reportedly marked this third of four Full Moons in blue, in honour of an even older use of the term.
What—another definition?? Yup. The earliest reference to anything about a “blue Moon” comes from a rhyme going back to 1528: “If they say the Moon is blue, We must believe that it is true.” Saying the Moon was blue was like saying the Moon was made of green cheese—in other words, it was an unquestioned impossibility.
Or is it impossible?? There is yet another explanation of “Blue Moon” which refers to the actual colour of the Moon to the naked eye. Now: I’ve seen brilliant white Moons and warm yellowish Moons, orange and blood red Moons during lunar eclipses, but I’ve never seen a blue Moon. However, there are times throughout history when the Moon actually had a bluish tinge, after forest fires or volcanic eruptions, caused by refracted light in Earth’s atmosphere. (By the way, the Moon could be in any visible lunar phase for this to happen, not just a Full Moon.)
The actual phrase “once in a blue Moon” apparently dates back to the mid-19th century. By this time it was reasonably well known that occasionally the Moon really did appear blue under certain atmospheric conditions, so the phrase took on the revised meaning of “once in a while,” rather than “never” or “gimme a break!”
Odd Moon Out
But wait a second—how did we get from a silly cultural expression to the third Full Moon out of four being marked in the Maine Farmers’ Almanac? And more to the point, why would anyone care how many Full Moons there are in a season?
The main reason for identifying the seasonal Full Moons was to calculate Christian holidays. Easter is deemed to be the Sunday after the first Full Moon following the vernal (spring) equinox (called the Paschal or “Passover” Moon in the Jewish calendar—Jesus being Jewish, and the timing of Easter being linked to Passover). Since many Christian holidays are timed in relation to Easter, it became extremely important to be able to determine an accurate date for it.
Full Moons are given special names and meanings in many cultural traditions, as any good Pagan knows. Full Moons are spaced 29.5 days apart, so there is typically one per month—12 months, 12 Moon names, 12 Full Moons. So far so good. The problem comes when we occasionally get 13 Full Moons in the span of a year (which happens about every 2-3 years). With only 12 Moon designations, what to do with the 13th Moon? The Maine Farmers’ Almanac claimed this caused the early Christian monks such distress when calculating their calendars that it is the reason why the number 13 became cursed as being unlucky. At some point this extra Moon became known as the “Blue Moon,” which was deemed to be the third Full Moon in a season that had an extra—fourth—Full Moon.
The Well-Seasoned Moon
But wait a minute—the third Full Moon? Why not the fourth, which would seem logical as the “extra” Moon in a season normally populated by three? For this we must go back to the Easter-related Christian holidays. The period of Lent, which begins precisely 46 days before Easter, must contain the Lenten Moon which is considered to be the last Full Moon of the winter season which ends at the vernal equinox.
Ahhh, now we start to see the need to count the number of Moons per season! Since the last Moon of a season can be special (e.g., Lenten Moon) and the first Moon can be special (e.g., Paschal Moon), the “extra” position falls to the second or third Moon in a season that happens to contain four. Why the third is designated as the “extra” rather than the second remains a mystery—no one seems to know where the Maine Farmers’ Almanac got their “Blue Moon” rule from. One website I found speculates that Full Moons were simply counted as the “first,” “second” and “last” of a season, so that the extra defaulted to the third.
So now all that’s left is to figure out the beginning and end of the seasons, which is straightforward—right? Well, again, not exactly. Seasons are defined by the solstices (times of maximum or minimum daylight, in June and December) and the equinoxes (times of equal day and night, in March and September). But anyone born near the cusp between two zodiac signs knows that the position of the Sun varies slightly from year to year. The Sun actually reaches the vernal equinox position anywhere from the evening of March 19 to the early morning of March 22, depending on the year and which time zone you happen to be in at the time.
And then there’s whether you calculate the equinox by the Sun’s actual position or by averaging its position (like the Maine Almanac did), or just using a fixed date (like the Roman Catholic Church does). Just as time zones can complicate the date of Blue Moons rendered by the “monthly” method, your method of calculating the equinoxes and solstices can change which season winds up saddled with the 13th Full Moon. You can start to sympathise with those poor Christian monks!
Blue Moon? You Choose!
So at the end of the day (or month...or season) just when is the next Blue Moon, and how often does it happen? By the current popular definition (two Moons in a month) a Blue Moon happens about every 2½ years, and by the older definition (third Moon in a season of four) it is about the same frequency, but the formulae yield totally different dates and even different years. The next Blue Moon by the “monthly” definition is July 31, 2004...or August 30 if you happen to be reading this while basking on a beach in the Fiji Islands. By the older “seasonal” calculation, there hasn’t been a Blue Moon since 2002 (in either August or November, depending on how you partition your seasons) and there won’t be one again until August 2005.
© 2004, Wendy Guy. All rights reserved. Originally published in the Summer 2004 issue of Cauldron and Quill Magazine.
Blue Moon Eclipse
I missed the description again... 
