The first new moon of 2014 occurs on January 1. How often does a new moon occur on New Year’s Day? After consulting a multi-year “Phases of the Moon” table, I find this last happened in 1995. It will happen again in 2033. Some quick subtraction reveals that each of these New Year’s Day new moons are 19 years apart. This is not a coincidence. It turns out that lunar phases repeat on the same calendar dates every 19 years. This 19 year cycle was known to Babylonian astronomers. It is called the Metonic cycle after the Athenian astronomer Meton who introduced a calendar based on it in 432 BC.
What is the reason for this 19 year cycle? One complete moon phase cycle takes about 29.5 days. This is called a synodic month. Twelve synodic months are about 354 days, 11 days short of a 365 day year. So, after one year the moon phase on a given date will differ from the previous year. The moon phase on January 1, 2015 will be four days past first quarter, which is 11 days after new moon. In order to have a particular moon phase repeat on the same calendar date we need an interval of time that contains both a whole number of synodic months and a whole number of years. It turns out that 235 synodic months are almost exactly 19 years, hence the 19-year Metonic cycle.
In addition to being that last year with a New Year’s Day new moon, 1995 is also the year I began working at the Planetarium (it was Hansen Planetarium in those days). So, later this year I will complete one Metonic cycle at the Planetarium. Incidentally, the moon phase on my hire date (as it will be on my upcoming anniversary) was three days past last quarter.
Here’s to a wonderful holiday season and a happy new year!