The Calendar                    

            Everybody knows that a year has 12 months.  But does anybody know why?  Do you know where the words day, month, and year come from? 

          The Babylonians named each of the days.  The word "month" is derived from the word "moon." The Month is taken from the cycles of the Moon phases, which takes about 29.5 days.  If the Synodic months (lunar) are used to count a year, 12 synodic months equal 354.36706 days.  The year is the period for one orbit of the Earth around the Sun. 

In the year 46 BC the calendar was hopelessly confused. So Julius Caesar initiated a reform of the entire system. He appointed the Alexandrian astronomer Sosigenes to undertake the revision of the Calendar.  Sosigenes did away with the lunar system and replaced it with the (tropical) solar year of 365.25 days. Pope Gregory III issued a "papal bull" and the Jesuit astronomer Christopher Clavius went to work on this problem. The length of the year was redefined as 365.2422 days a difference of 0.0078 days per year from the Julian calendar. (We now have here a Gregorian calendar). This changed amount of error to 3.12 days every 400 years. Clavius had allowed for such discrepancy and suggested that three out of every four centennial years, which would ordinarily be leap years, should instead be regarded as common years. This lead to the practice that no centennial year could be a leap year unless it was divisible by 400. Following this rule 1700, 1800, and 1900 were common years, but the year 2000 would be a leap year. This Gregorian reform gives us an extremely accurate calendar system.

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