Home PageAstronomy ArticlesStars PageAstronomy Links

Blue Moon & the Metonic Cycle

In 2010, the November Full Moon counts as a Blue Moon because it's the third of four Full Moons to occur in one season. A season represents the time period between an equinox and a solstice - or vice versa. Most of the time, only three Full Moons take place in one season, but if there is an extra Full Moon, the third one is called a Blue Moon. (By the way, we'll eventually get around to talking about the other type of Blue Moon - the second of two Full Moons in a calendar month.)

Is the November 21, 2010 Full Moon Truly A Blue Moon?

If the first Full Moon of the season falls very shortly after an equinox or a solstice, a fourth Full Moon may sneak in just before the change of seasons. In 2010, for instance, the first Full Moon falls about 6 hours after the September equinox, enabling to fourth Full Moon to slip in in the nick of time, or in less than one day before the December solstice.
Although the equinoxes and solstices fall on or near the same dates every year, the Moon does no such thing, as it follows the beat of a different drummer. Next year, in 2011, all the Full Moons will fall about 11 calendar days earlier than in 2010. The November 2011 Full Moon will occur on November 10, featuring the second of the season's three Full Moons between the September 2011 equinox and December 2011 solstice.

The Metonic Cycle

In periods of 19 years, the phases of the Moon recur on or near the same calendar dates, synchronizing the solar and lunar cyles. This is known as the Metonic cycle. So in 2029, we can expect the September Full Moon to fall on or near the September equinox once again. This time around, however, the September 2029 Full Moon happens about one hour before the September 2029 equinox, making the August 24, 2029 Full Moon the third of four Full Moons in between the June 2029 solstice and the September 2029 equinox.
Because 235 Full Moons take place during the 19-year Metonic cycle, that means 7 seasons must have four Full Moons in this 19-year period. Given that there are four seasons in one year, with each season harboring only three Full Moons, then you would have a grand total of 228 Full Moons (19x4x3=228) in each Metonic cycle. However, there are total of 235 Full Moons, not 228 Full Moons. Once again, that means 7 seasons must have an extra Full Moon (235-228=7).

Blue Moon Dates 2012-2029

Here are the Blue Moons - the third of the season's four Full Moons - in the upcoming Metonic cycle:
1) August 21, 2013
2) May 21, 2016
3) May 18, 2019
4) August 22, 2021
5) August 19, 2024
6) May 20, 2027
7) August 24, 2029

The Second Full Moon In A Calendar Month Blue Moon

As promised earlier, we now present to you to most popular kind of Blue Moon: the second of two Full Moons in one calendar month. It's the same idea. There are 235 Full Moons but only 228 calendar months in this 19-year (228-calendar month) Metonic Cycle.
Since there are an extra 7 Full Moons (235-228=7), that means 7 of these 19 years must have at least one month that harbors two Full Moons. Yet, if February has no Full Moon at all in this cycle - as is the case in February 2018 - that means one more calendar month has to sport a Blue Moon, giving a total of 8 calendar-month Blue Moons for the upcoming Metonic cycle:
1) August 31, 2012
2) July 31, 2015
3) January 31, 2018
4) March 31, 2018
5) October 31, 2020
6) August 31, 2023
7) May 31, 2026
8) December 31, 2028

copyright 2010 by Bruce McClure

Equinox and Solstice Dates

Phases of the Moon: 2001 to 2100