The more I read about leap years the more I found out.
1. I knew that the calendar had been reformed in the Middle Ages and that the British had been late to adopt this but I didn't know the reason for the reform. It has to do with the correct calculation of the date for Easter, which was calculated from the Spring Equinox.
2. The reason that this was out of sync was that a year lasts 365.2422 days not 356.25 days. The calendar was slipping out of sync with the sky by about a day every 200 years. I knew that the solution was to drop the leap year day at the turn of the century but that didn't happen on the Millennium. You drop 3 leap years every 400 years.
3. The year is called a "tropical year" https://en.wikipedia.org/wiki/Tropical_year. The definition involves precession so that will take a bit of thinking about.
4. There is an astronomical year numbering system that involves giving a Year 0 to the usual western calendar dates. https://en.wikipedia.org/wiki/Astronomical_year_numbering
