Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!aramis.rutgers.edu!athos.rutgers.edu!christian From: firth@sei.cmu.edu Newsgroups: soc.religion.christian Subject: Re: Calendars (was Re: Christmas) Message-ID: Date: 11 Sep 89 05:23:34 GMT Sender: hedrick@athos.rutgers.edu Organization: Software Engineering Institute, Pittsburgh, PA Lines: 24 Approved: christian@aramis.rutgers.edu The difference between the Julian and Gregorian calendars is that the former prescibes that every fourth year be a leap year, while the latter requires a century year (xx00) to be a leap year only every fourth century. Accordingly, 1900 was a leap year in the Julian calendar but not in the Gregorian, and so the two calendars diverged by a further day and are now 13 days apart. Since 2000 is divisible by 400, both calendars consider it to be a leap year, so the next one-day change will be in 2100, unless by then we have yet again revised the calendar. Clearly, the average rate of divergence is 3 days in 400 years, or about 1 day every 133 years. The length of the mean solar year is approx 365.2422 days, so the Julian calendar (365.25) falls behind the sun by 0.78 days per century, and the Gregorian (365.2425) falls behind by 0.03 days per century. That is why 10 days [*] were dropped when the changeover occurred: the accumulated drift of about 1400 years had to be corrected. [*] if you changed in +XV, that is. When Britain changed the difference was 11 days; Russia in 1923 had to drop 13.