What Is The Metonic Cycle?

Did you know this week's Moon phase will be the same in 19 years? Lean more about the fascinating patterns in the Moon's cycle.

Are you fascinated by patterns as well as the phases of the Moon? Here are some interesting facts: All are cyclical, the most noteworthy being the so-called Metonic Cycle that was independently discovered by the Greek astronomer Meton (born about 460 B.C.). This is a 19-year cycle in which the phases of the Moon are repeated on the same days of the year, or approximately so.

For instance, in 2017, there was a last-quarter Moon on Wednesday, September 13th. In nineteen years from that date, in 2036, there’ll be another last-quarter Moon on September 13. Another interesting pattern emerges: after 2 years, the preceding lunar phase occurs at nearly the same calendar date. Thus, in 2019, a full Moon will occur on September 13. After 8 years, the same lunar phases repeat but occurring one or two days later in the year. The Greeks called this 8-year cycle the octaeteris.

In 2025, a last-quarter Moon occurs on September 14. Finally, in our Gregorian Calendar, 372 years provides an excellent long period cycle for the recurrence of a particular phase on a given date.

Thus, we know with absolute certainty that the same last quarter Moon that shines down on us on Wednesday will also be shining on September 13 in the year 2389!

Be sure to bookmark the Farmers’ Almanac’s Moon Phase Calendar! It’s our most popular page!

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Joe Rao is an expert astronomer.
Joe Rao

Joe Rao is an esteemed astronomer who writes for Space.com, Sky & Telescope, and Natural History Magazine. Mr. Rao is a regular contributor to the Farmers' Almanac and serves as an associate lecturer for the Hayden Planetarium in New York City.

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George Lee

I have been fascinated with the patterns and the fact the Moon goes through cycles.
We know a single orbit is a cycle obviously, but that there’s more than just a simple cycle.
Each seems connected with another.

Did you know there’s also the triaeteris? it’s 3 years and it’s when a moon phase jumps back 3 to 4 days before the day it previously fell on 3 years ago.
It’s not very useful as a standalone but it’s noteworthy when you put it on top of an octaeteris, so you get a hendecaeteris (11 years), so for example, a full/new moon repeats itself but a day or 2 before the day it previously fell on after 11 years.
In 2020 there was a full moon on Halloween, so 1 octaeteris later, there will be a full moon between the 1st and 2nd day of November in 2028 and in another triaeteris, there will be a full moon the day before Halloween in 2031 (with a penumbral lunar eclipse as a bonus), so Halloween 2031 will mark 1 hendecaeteris after Halloween 2020 and 1 octaeteris before Halloween 2039, which’s 1 Metonic cycle after Halloween 2020.

Same with a full moon happening between Christmas Eve and Day in 1996 (keep in mind of the timezones), and a full moon on Boxing Day 1 octaeteris later before a Christmas Eve full moon in 2007 1 triaeteris after Boxing Day’s in 2004 (December 26), 1 hendecaeteris after Christmas Eve/Day’s in 1996 and 1 more octaeteris before Christmas Day’s in 2015.
In 2023, there will be another Boxing Day full moon, 1 Metonic cycle after Boxing Day’s in 2004 and 1 octaeteris after Christmas Day’s in 2015 and another hendecaeteris before Christmas Day’s (which will also be a full supermoon) in 2034 (1 Metonic cycle after 2015’s).
Yet 1 octaeteris before Christmas Day’s in 2034, there will be a full moon when it’s Christmas Eve for part of the world in 2026 (also coinciding with a full supermoon), 1 Metonic cycle after Christmas Eve’s in 2007 (in which the Moon passed by Mars).

I haven’t heard anyone else talk about the triaeteris or hendecaeteris to this day, but it’s true what I know them for.

Yet we knew all about the general and eclipse cycles long before we had the internet, showing us we can understand patterns with the moon.

Last edited 1 year ago by George Lee
George Lee

Though it wasn’t Christmas Eve in my timezone (EST) in 2007 during the full moon and it’ll also be before Christmas Eve for half of the world and Christmas Eve in the other half during the last full moon of 2026 (1 Metonic cycle apart).
Yet the total lunar eclipse of December 21, 2010 will be preceded by a total lunar eclipse the day before the northern winter solstice (in my area but on the day of solstice in Asia, Australia and New Zealand) in 2029 (after 1 Metonic cycle) 1 triaeteris after 2026 (where Christmas Eve full moon falls on).
Too bad we have to wait until 2094 when another total lunar eclipse falls on the 1st day of northern winter in my area because that’ll mark almost a century since I was born (yet it won’t even be in the same Metonic series).

Last edited 1 year ago by George Lee
George Lee

Though I do mention the triaeteris is still noteworthy for achieving an integer number of weeks, meaning the same phase such cycle apart falls on/near the same day of the week.

For example, January’s full moon in 2011 (depending on what timezones you’re in) fell between the 19th and 20th (between Wednesday and Thursday), and a triaeteris later, it fell between the 15th and 16th of January (also Wednesday to Thursday) in 2014, then another triaeteris later, it was between the 12th and 13th (Thursday and Friday) of January in 2017, yet another triaeteris later, it was between the 10th and 11th of January in 2020 (Friday and Saturday), yet another triaeteris later, was between the 6th and 7th of January (Saturday and Sunday) in 2023, and another triaeteris after that, it will be between the 2nd and 3rd of January (Friday and Saturday) in 2026, and finally a triaeteris afterwards, it’ll be between New Year’s Eve 2028 (Friday) and New Year’s Day (Saturday) 2029 (with a total lunar eclipse).

Though I did say near, as a triaeteris doesn’t always catch up with the same weekday in the long term, yet leap years worsen the synchronization with the weeks, so it’s not perfect.


The Celts were so very advanced in artistic and scientific (metallurgical, mathematical and astronomical) knowledge and clearly not the ‘barbarians’ the romans reported. This gold ceremonial hat was discovered in Switzerland the home of the ancient Helvetians. The gold hat is incised with advanced astronomical and mathematical information. The Germans purchased this hat and their researchers found that this beaten gold conical hat, manufactured between 1,000 and 800 B.C., has 1739 sun and half-moon symbols decorating its surface. This makes up a scientific code which corresponds almost exactly to the Metonic Cycle. We are told that this cycle was “discovered” by the Greek astronomer Meton in 432 B.C. Meton had obviously learned it from the Helvetian sources who were already well acquainted with it. It explains the relationship between moon and sun years and the symbols on the cone-hat are, in fact, a logarithmic table which enables the movements of the sun and the moon to be calculated in advance, and allows long-term, empirical astronomical observations.


Farmers Alamac has the best information to go by. The best tips and answers to everything I want to know.


very good article

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