This would dictate 969 leap years every 4000 years, rather than the 970 leap years mandated by the Gregorian calendar.This could be achieved by dropping one leap year from the Gregorian calendar every 4000 years, which would make years divisible by 4000 non-leap years.(Exercise for the reader: Why is the error in the 1600s the same as in the 1500s.) The following list contains the dates for changes in a number of countries.
Changes in the 1700s required 11 days to be dropped.This rule has, however, not been officially adopted.When the Orthodox church in Greece finally decided to switch to the Gregorian calendar in the 1920s, they tried to improve on the Gregorian leap year rules, replacing the “divisible by 400” rule by the following: Every year which when divided by 900 leaves a remainder of 200 or 600 is a leap year.It was proposed by Aloysius Lilius, a physician from Naples, and adopted by Pope Gregory XIII (pictured above) in accordance with instructions from the Council of Trent (1545-1563) to correct for errors in the older Julian Calendar.It was decreed by Pope Gregory XIII in a papal bull on 24 February 1582.
In 1958, de Vries showed that baffling anomalies in the carbon-14 dates, observed by Willard Frank Libby for Egyptological samples, were in fact systematic anomalies on a global scale, represented in the carbon-14 dates of tree rings.