I wrote an article recently on a rare calendric event of the Jewish Calendar. The article, "
Myths and Maths of the Blessing of Sun" discusses some aspects of the Julian calendar and the Gregorian reform in the following terms:
Because the mean year-length of the Julian calendar is longer than a tropical year, each season begins on progressively earlier dates in the calendar. Their start dates regress in the calendar at the rate of 7.8 days per 1000 years.
This was a problem for the Church, because it caused Easter to drift ever closer toward summer. ... Easter is a northern spring festival and must occur shortly after the equinox. In 532, the council of Nicaea had irreversibly linked Easter not to the equinox itself, but to its presumed date, March 20. However by 1582 it was occurring on March 10. To correct this, Pope Gregory 13th reformed the calendar. As a one-off adjustment he dropped 10 days from that year, and ... However, on checking the maths, I find that it doesn't quite add up. The above was based on an estimate of the mean
tropical year as 365.24219 days.
Accordingly, the actual regression of the seasons in the Julian calendar is 7.81 days per 1000 years. The calculation is: 365.25 - 365.24219 = 0.00781 days/year. From 532 (the year of the council of Nicaea) to 1582 (the year of Pope Gregory's reform) is 1,050 years, and 1050 years x 0.00781 days/year = 8.2005 days. So why did Gregory drop 10 days rather than 8 days?
