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| Joseph Justus Scaliger |
One of the fundamental concerns of civilized man is the study of the past, natural or human. For this, it is necessary to be able to fix the date on which each event took place. This is the goal of a science called chronology.
If the exact date of an
event is known, we can define it by giving the day, month and year in which it
took place. For example, we can say that the Second World War began on
September 1, 1939. We have no problem with the day and the month, but how are
the years numbered? Obviously we must take
an origin or starting point that everybody will agree to use.
This dating system causes a curious effect: the numbers assigned to the years after the origin grow towards the future, while the previous years grow towards the past. Thus, the year 2000 of our era came after the year 1000, but the year 2000 B.C.E. came before 1000 B.C.E. (see below the meaning of these acronyms). The years prior to the origin work as negative numbers.
If we could adopt the
date of the Big Bang as the
beginning of the count of years, the years prior to the origin would disappear,
because nothing is known to have happened before the beginning of the universe.
The problem, of course, is knowing the exact moment when this event took place.
The chronological system
of the Roman Empire counted years from the mythical date of the founding of the
city (753 B.C.), which allowed the Romans to dispense with negative numbers. A
certain date was named year 533 ab urbe condita (A.U.C.), which corresponds to
221 B.C.
Among the chronological
systems based on specific events we can mention the Islamic era, which counts
the years from the flight of Muhammad from Mecca to Medina (the hegira), which took place on
July 17, AD 622. This dating system has the peculiarity that we can't calculate
the current Islamic year by subtracting 622 from the international date. The
Islamic year is lunar and has 354 or 355 days. Therefore, the count of their
years grows faster than ours, so the year 1980 of the Christian era was the
year 1400 of the hegira rather than 1358, as would correspond with a count of
solar years.
The French
revolutionaries of the late 18th century also tried to impose a new
chronological system. In addition to changing the calendar, they adopted September
22, 1792 as the initial date of history, calling it the day one of vendémiaire of the year 1 of the
Republic. This chronology was short-lived, less than fourteen years, being abolished
by Napoleon Bonaparte in 1806.
The international
chronological system we use is the Christian era. After the fall of the Western
Roman Empire, the Roman era was used for a few more centuries, but in the 6th
century the theologian Dionysus Exiguus calculated the year of Christ's birth
as 754 A.U.C. (see this
post in my blog). This date was used later as the beginning of the count of
years and was called year 1 A.D. (Anno Domini, the Year of the Lord). Later
dates of the Christian era could be calculated by subtracting 753 years from
the Roman date, in force until then. Those prior to 754 A.U.C. corresponded to
negative numbers in the new era, and were obtained by subtracting the Roman
date from 754 and adding the acronym B.C. (Before Christ). For instance, the year 533 A.U.C.
corresponds to 221 B.C., as mentioned above.
Today the Christian era
has become the international system of chronology, used by commerce, history
and science to fix all historical dates between 5000 B.C. and today. But as the
West insists on renouncing the Christian values that gave it birth, other
acronyms are now used for the count of years. So we speak of the Common Era
(C.E.) and Before
the Common Era (B.C.E.). But since I am a Christian, I insist on
interpreting these new acronyms in a more traditional way, as Christian Era
and Before
the Christian Era.
A small discrepancy was
introduced for astronomical calculations. In the general use system there is no
year zero, since 1 B.C.E. immediately precedes 1 C.E. Astronomers, on the other
hand, do use zero. All dates since 1 C.E. are the same, but 1 B.C.E. is the
astronomical year zero, and any year before the beginning of the Christian era
loses one unit: thus, 300 B.C.E. is the astronomical year -299.
There is another method
of counting dates, sometimes used by historians and astronomers. They are the Julian days,
invented in 1582 by Joseph Justus Scaliger. Julian days have nothing to do with
the Julian calendar or with the Roman dictator Julius Caesar, as their author
gave them that name in honor of his father, Julius Caesar Scaliger. Dates are
defined by the number of days since January 1, 4713 B.C.E., which took place before
the beginning of history. A specific date is defined by a single number rather
than three (year, month, and day). This number is always positive for all
historical dates. To find out the number of days elapsed between two specific
dates, we must simply subtract their respective Julian days. Of course, it has
the disadvantage of working with very large numbers. For instance, January 1,
2001 (the first day of the 21st century and the third millennium of our era)
corresponds to Julian day 2,451,544.
The attached listing
shows a computer program written in the C language that calculates the Julian
day of any date.
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long JulianDay ( int day, int month, int year) { int f = (month<3) ? 1
: 0; int g =
year+4900-f; if (year<1582
|| (year==1582 && (month<10 || (month==10 && day<5))))
return day - 32114L + ((1461L*(g-100))/4) + ((367L*(month-2+12*f))/12)
; else if (year>1582
|| (year==1582 && (month>10 || (month==10 && day>14))))
return day - 32075L + ((1461L*(g-100))/4) + ((367L*(month-2+12*f))/12) -
(3*(g/100))/4; else printf (“That
date does not exist\n”); return 0; } |
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