Saturday, March 17, 2012

A note on calendars

Curiosities from my last subject...
Civilian life is regulated by the tropical* year because the seasons follow each other starting from the passage of Sun through the point. However, it is not convenient to use a year made up of 365 integer days + a fraction. Julius Cesar (around 46 AC) reformed the calendar by imposing a year of 365 days and adding one day to the shortest month every four years (i.e., February), thus introducing the leap year. So, on average, the Julian year is made up of 365.25 (mean solar) days. However, the Julian calendar gets behind by approximately 8 days every 1000 years (365.25 - 365.2422 = 0.0078 days in one year, hence 7.8 days every 1000 years). For this reason, Pope Gregory XIII (1582) corrected the calendar by skipping ten days (in the year 1582 the calendar jumps from Thursday, October 4 to Friday, October 15) and deciding that the beginning of century not muliples of 400 were not leap (so there are 97 leap years every 400 years). Hence, the Gregorian year, averaged over 400 years, is made up of 365.2425 mean solar days. It slowly goas ahead with respect to the Sun by approximately 26 seconds per year. The error of one day that will be accumulated every 3226 years will be corrected by considering normal (instead of leap) the year 4000.
*tropical year = time interval between two successive passages of the Sun over 365.2422 mean solar days

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