Full Moon on Christmas Day

Here it is, the first full Moon on Christmas Day since 1977, as seen somewhere out on the North Atlantic. I wondered why it had taken so long and what factors affect how often any given date gets a full Moon. It turns out that there is more than one way to measure a complete orbit of the Moon around the Earth. One is to measure a complete 360 degree rotation, but you have to do this against the background of the stars and it turns out to take 27.3 days. It is called the sidereal month. My next task will be to figure out how to observe this. More logical is the month from New Moon to New Moon ie measuring the time it takes between occasions on which the Moon and the Sun line up. Here it rotates more than 360 degrees round the Earth because the Earth itself has moved on round the Sun. It takes about 29.5 days but varies during the year because the Earth's orbit is not round. This is the synodic month. 12 lots of 29.5 days comes out at 354 days, so each full Moon will slip by 11 days in a regular year but 12 days in a leap year. I started trying it out on a calendar but gave up. Anyway, next full Moon on Christmas Day is something like 2034 so it will be a while.