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So You Think You Know the Leap Year Rule?


    The Leap Year Rule is as follows.  Every four years an extra day is added to February.  The purpose of doing this is to keep the calendar accurate with the seasons.  Every so often we hear of a second (like the smaller part of a minute) being added, but that makes no difference to most people.  But the extra day every four years is only part of the Leap Year Rule, plus there is an exception to the Rule.  That exception occurred in the year 2000 and won’t occur again until the year 2400.  And to find out what that exception is, and why the Leap Year Rule is covered in a church history article, read on.

    Before the year 500, not all people counted years with numbers.  Instead, when keeping track of an event the rules of kings, etc. would be used to show approximately when something happened.  An example of this exists in the Bible.  Luke 3:1-2 tells when John the Baptist began to preach and baptize.  It goes thus:  “In the fifteenth year of the reign of Tiberius Caesar—when Pontius Pilate was governor of Judea, Herod tetrarach of Galilee, his brother Philip tetrarch of Iturea and Traconitis, and Lysania tetrarch of Abilene—during the high priesthood of Annas and Caiaphas, the word of God came to John son of Zechariah in the desert.”  Why list all those rulers?  If you wanted to tell when some event occurred, you listed a number of current rulers.  That way people later on would have an idea of when the event occurred.

    But as you can see, it can be a bit long.  And there was a numbering system in place, that of the Romans.  According to this system, the year 1 was when Rome was founded (we would call this 753 BC).  At the beginning of the Middle Ages, a certain Dionysius Exiguus (died sometime before 544) changed that numbering system to one which reflected a Christian origin.

    He decided to create a numbering system based on the birth of Jesus.  He gave this year the number “1” and called it “Anno Domini 1,” or “in the year of the Lord 1.”   (That is why there is no year “0”.  The year before AD 1 was 1 BC.)

    This system eventually covered all Europe.  It was commonly used in Italy in the 500s, made it to Spain in the 600s, to England in the 700s, France, in the 800s, and eventually was universally accepted by the Catholic Church throughout Europe in the 900s. 

    This yearly numbering system worked well, but the number of days in a year didn’t:  by the middle of the 1500s, the calendar was off a bit:  10 days to be exact.  So, in 1582 Pope Gregory XIII decreed that the day after Oct 4, 1582 would be Oct 15, 1582.  The Catholic countries of France, Spain, Portugal, and Italy complied. England and the American colonies followed suit in 1752, but by then the calendar trailed by 11 days; therefore Sep 2, 1752 was followed by Sep 14, 1752.  Russia did not change until 1918, when Jan 31, 1918 was followed by Feb 14, 1918.

    And to ensure this change would not be needed again, the Leap Year Rule was created in 1582:  every fourth year will add an extra day, unless in ends in “00”, in which case it is not a leap year, unless it can be divided by 400, in which case it is a leap year.  So, 1900 cannot be divided evenly by 4 and so it was not a leap year.  2000 can be divided evenly by 4 and so it was a leap year.  The year 2400 is the next exception. 

    This system is so effective it will take 3300 years for another day-correction to be necessary.


©2005 Mark Nickens


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