Thursday, August 07, 2014

What a difference a (quarter) day makes

We are taught in school that the year is 365 days long, yet we also know that it is not always the case, because we have a leap year every 4 years in which one month is longer by one day (in our calendar February has 29 instead of 28 days).  That means that the leap year is in fact 366 days long.  If you average it out that means that every year is in fact 365.25 days.  How was it discovered that there is this discrepancy of an extra quarter of a day?  Robert Wolfe, a brilliant historian, Professor of History at New York University, has written an (unpublished) book entitled "Revolution in Time: The case for a new calendar" that describes the history of the calendar and how the extra quarter day was discovered and what were the consequences.
This is a complex and esoteric history, that involves Romans, Greeks, and Egyptians.  Originally the calendars developed by most early civilizations, in the middle East (Jewish, Arab), in the far East (Chinese, Vietnamese) and in America (Inca, Aztec) were mainly lunar calendars, since the phases of the moon were easier to see and classify.  But, some also developed a solar calendar, notably the Aztecs and the ancient Egyptians, often alongside and mixed with a lunar calendar.  The lunar calendars are not straighforward and also require the insertion of days, weeks or months (as in the Jewish calendar) to make them consistent each year.  Otherwise you have the situation where it is impossible to predict when a certain sacred day will arise, for example Ramadan in the Muslim calendar can be in the winter or the summer or any time.  The solar calendar allows one to avoid this, as long as one knows that the year is neither 365 nor 366 days long. 
It seems to have been the Egyptians who originally realized this and in order to obtain a consistent calendar, i.e. one that could reliably predict the occurence of sacred days, necessary for religion, agriculture and the governing elite, would require a fractional day each year or a leap day.  How did this information become incorporated into our current universal calendar.  The Greek dynasty of the Ptolemys conquered Egypt in 323 bce and in order to be accepted by the Egyptians adopted their religion and their calendar.  But, they realized that there was this discrepancy and decided to modify the calendar.  This modification was taken up by the Greeks themselves, and so it came to Rome.
Rome was the center of the great Roman Empire, that had a complex calendar consisting of a solar calendar of 365 days with a lunar calendar imposed upon it.  Julius Caesar was not only dictator of Rome, but also the Pontifex Maximus or Head Priest ("Pope") and in 56 bce he reformed the Roman calendar, into what became the Julian Calendar named after him, with its leap day every 4 years so that it takes account of the extra 0.25 days per year.  He also changed the calendar by adding two months so that instead of the year beginning in March it began on January 1.  But, he was assassinated only two years later in 54 bce on the Ides of March by a group of Republican zealots, who were subsequently defeated by Caesar's followers.  So his revised calendar remained that used by Rome.  Caesar intended to restart numbering the years from the time of his ascension, but many years after Rome became Christian the numbering of the years was re-dated from the supposed date of birth of Jesus Christ, although they got it wrong.  Most analysts agree that Jesus Christ was born sometime around 6 bce, and unfortunately they numbered the beginning of the calendar as year 1 and not zero, so its quite inaccurate. 
Finally, we have the Gregorian revision of the Julian Calendar by Pope Gregory XIII in 1582.  On the basis of the new knowledge that had been aquired by then, following Copernicus, Gregory realized that after 1,500 years the Julian calendar was already 10 days out of line.  He therefore introduced a further adjustment taking into account that the extra "quarter day" is actually 5.52 hrs in length.  So he added 10 days and changed the leap years so that they could not fall on years divisible by 100 (such as 1500) but only those divisible by 400 (such as 1600).  This took care of the problem, and the Gregorian Calendar has become the standard universal calendar throughout the world.  Despite all its quirks, its pagan Roman origins and its strange nomenclature for days and months (see next blog), it is what we have and it works.


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