## A New Year and a New Moon

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.

photo credit: galaxywire.net

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!

Tags: , ,

## 7 thoughts on “A New Year and a New Moon”

1. I just realized that a New Moon occurs on Jan 1, 2014 and Googled “New Moon on New Year’s Day.” Thanks for the great article! Very informative!

2. In that case… 1 January year 0 was a new moon. 2014 is a multiple of 19 and so is 1995 thus its easy to know that year 0000 on January 1st is a new moon.

3. When I googled “New Moon on New Year’s Day” your article came up. Thanks for the great info! How cool to start a new year with a new moon. So much symbolism for me.
Happy New Year!

4. Eddy,

While 235 synodic months are almost exactly 19 years, there is a difference of about 2 hours, so over hundreds of years, moon phase dates can shift by a day or two.

However, there is an additional complication with moon phase dates prior to the adoption of our current calendar. With the change from the Julian to the Gregorian calendar in 1582, ten days were omitted from the month of October that year (October 4 was followed by October 15). This of course changed moon phase dates by ten days. So, moon phase dates on the Julian calendar, which includes the year 0 (or 1 BC), differ by ten calendar dates from what they would have been on our current calendar.

5. Just wanted to say thanks — this is a wonderfully informative article and helped settle a “that’s not true” debate in about 10 seconds flat!

6. Robert,
Thanks for the extra info. I remember a little of those facts but it is really interesting how events in the past made little things like this something to ponder about.

7. Great article. Thank you! I’m an oldster who has seen two previous occurrences, and both were highly significant years in very good ways. Perhaps this year will be the same.