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Leap Year and Julius Caesar

2,057 years ago the first leap year was celebrated.  It was the year 45 B.C. (or  608 A.U.C.) and Julius Caesar had been named Pontifex Maximus.  As the high priest of Rome he gained control over the Roman calendar; a very powerful office indeed as the High Priest had the power to interpret the omens and decide on which days public meetings, votes, and even declarations of war could be held.  The Roman calendar by this time, however, had some major problems.  The seasons were off from their assigned months.  The original calendar had been created by Romulus and was based on the phases of the moon and a ten month agricultural cycle.  Later rulers had added two more months, but by Caesar’s day they did not fall in the proper seasons.  Caesar hired Sosigenes, an Egyptian astronomer, to help him create a new solar calendar for Rome and for the world.  Caesar moved the first day of the year from March to January.  He balanced the lengths of the months with alternating days of 30 or 31, but left February with only 29.  He then gave February the Bissextile, which was the first leap day.  The bissextile came once every four years.  In these leap years, February had 30 days.  This gave the new Julian Calendar, as he called it after himself, 365.25 days.  The seventh month, the month of Caesar’s birth, was renamed July in his honor. After his adopted son and heir, soon to be known as Augustus, took control of the empire he decided to follow the lead of his predecessor.  Augustus renamed his own birth month August.  At that time, however, August had only 30 days.  It was not fitting for the emperor’s month to be shorter than July, so he took another day from February, leaving it with only 28 and giving August 31.

The calendar has remained relatively unchanged from Caesar’s day until this.  February is still shortchanged, with a little redemption only once every fourth year.  There has, in fact, been only one significant change since Caesar’s leap year decree.  In the 16th century A.D. the Church began to realize that something was terribly amiss with the calendar.  The Easter Holiday, whose date is determined by the equinox, was off by ten days. (1600 years later and only a ten day error!)  At the Council of Trent in A.D. 1545 Pope Gregory XIII commissioned the astronomers Christopher Clavis and Luigi Lilio to determine the problem and reform the calendar.  They discovered that the Julian Calendar was off by 11 minutes and 14 seconds.  That is a difference of just .0078 days!  Amazing.  In order to better align the calendar with the rotations of the heavens, they created the Century Leap Year Rule.  Three out of every four centennial years ( a year divisible by 100) are “common.”  That is, they are a leap year.  No centennial year, however, can be a leap year if divisible by 400.  For that reason the year A.D. 2000 was not a leap year.  Did you notice?  That centennial rule will not come into play again until A.D. 2400, unless another ruler decides to reform the calendar again before then.  For now, Pope Gregory’s mandate to tweak the Julian Calendar earned him the right to rename our calendar after himself.  But remember, while the modern Calendar may be called Gregorian, it has Roman written all over it.


Nota Bene:  If you find this post interesting check out the other calendar post titled “Happy New Year!

Latin Alive Book 1 offers a lesson and activity on the Julian Calendar in the appendix for the Teacher’s Edition.


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