Metonic Moon

27 June 2014

The Copernican World, from the *Harmonica Macrocosmica*, 1660. Andreas Cellarius
The Copernican World, from the Harmonica Macrocosmica, 1660.

Today the Moon is in its New Moon phase. This won’t happen again for another 19 years. Actually that isn’t quite true. It takes about 29.5 days for the Moon to go from one new moon phase to the next, so we’ll have a new moon roughly once a month, just like always, but there is a periodicity to moon phases that spans 19 years, and it is known as the Metonic cycle.

The time from one new moon to the next is known as a synodic month, or sometimes just a lunar month. Because a synodic month is 29.5 days long, there are about 12 of them in a given year. But since 12 synodic months is only 354 days, it is a bit short of a full year. Since today (June 27) is a new moon, next year in June the new moon will be on June 16. So there is a drift of the lunar phases relative to the solar year. But it turns out that the phases come back into alignment over the course of 19 years. That’s because 235 synodic months is equal to 19 solar years.

Actually, 19 years is 234.997 months, so the cycle isn’t exact, but it is accurate to within two hours. This means if you use the Metonic cycle to calculate lunar phases, you will be accurate to within a day over 220 years. Because of this accuracy, many calendars work under the assumption that 19 years is 235 lunar months. It is used in the calculation of Easter, for example.

So it is true that today’s new moon on June 27 won’t happen for another 19 years, but that is true of every lunar phase for a particular date. It turns out that every moon is a Metonic moon.