The Calendar


The calendar in use today in the United States is derived from the Julian calendar, named after Julius Caesar and invented by the ancient astronomer Sosigenes in about 46BC.  It is based upon the observation that one year contains about 365.25 days.  It was one of the first calendars to abandon any attempt to keep the months consistent with the phases of the moon.  Instead, it divided the year into 12 arbitrary months, which altogether contained 365 days for most years, but 366 days every fourth year - the extra day being added to the month of February.


The BC/AD conventions originated during some attempts to calculate the dates of Easter and Passover.  About 460AD the astronomer Victorius devised an effective algorithm that was widely adopted.  But in 530AD, the astronomer Dionysius Exiguus realized that Victorius's method did not work before the year we call 1BC.  Common usage erroneously dubbed this "before Christ", but in actuality the best guess is that Christ was born in 4BC.  The year after 1BC was 1AD.  "AD" does not mean "after died".  The best guess is that Christ died in 29AD.


Despite such confusions, the Julian calendar worked well for almost 1600 years.  But, a year is actually about 365.2422 days long.  This means that every 130 years or so the Julian calendar accumulated an extra day, which caused the solstices and equinoxes to occur earlier in the calendar year.  As Easter is tied to the spring equinox, this caused Easter to occur earlier and earlier in the calendar year - which concerned the Catholic Church.


In 1582, Pope Gregory XIII proclaimed a new calendar designed by Jesuit astronomer Clavius.  The new design called for not adding the extra 366th day on centennial years; but, to compensate for the fact that a year is not 365.24 days long, the extra day is added every 400 years.  In other words, 1700, 1800 and 1900 were not leap years, but 1600 and 2000 were.  The new calendar was named the Gregorian calendar.  At its adoption, to compensate for past errors, 10 days were dropped from the month of October 1582.


Great Britain was not a Catholic country in 1582 and did not convert to the new calendar until September 1752, at which time 11 days were dropped from that month.  Historical legend claims the event caused near riotous protests against a purported Papist plot to steal days from the peoples' lives; but primary evidence for such riots is lacking and such claims are now generally dismissed.


The Eastern and Russian Orthodox countries did not convert to the new calendar until the twentieth century.  This is the reason why the Russian Revolution is sometimes called the October Revolution, even though we date it in November.  It was October by the old Julian calendar used in Russia in 1917.


The Unix computer operating system includes a perpetual calendar program named "cal".  It is based upon the so-called "English Gregorian" calendar and appropriately shows:

Sept 1752


ŠJames S. Freeman, 2006  

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