The Fascinating History of Leap Years Explained
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Chapter 1: Understanding Leap Years
Leap Day, which falls on February 29, is a unique day that appears only once every four years during a Leap Year, also known as an intercalary year. However, not every year that is a multiple of four qualifies as a Leap Year. For instance, a year ending in "00," such as 1900, is not considered a Leap Year unless it is also divisible by 400. Hence, while 2008 is a Leap Year (being divisible by 4), 1900 fails to meet the criteria. A notable case is the year 2000; despite being a year ending in "00," it was indeed a Leap Year, as it is divisible by 400.
Section 1.1: The Calculation Conundrum
The complexities of Leap Year calculations stem from a cumulative rounding error when reconciling the Julian calendar, instituted by Julius Caesar in 46 B.C., with the tropical calendar. The Julian calendar set a year at 365 days for three consecutive years, with an additional day added every fourth year, averaging 365.25 days. However, the tropical calendar indicates a year lasts approximately 365.24219 days.
Subsection 1.1.1: Defining Time
A Leap Second is defined as 1/31,556,925.9747 of the tropical year based on the ephemeris time from January 0, 1900. This definition has evolved since 1960, when the second was determined by the radiation frequencies of the Cesium 133 atom.
It Adds Up
Section 1.2: The Role of St. Bede
In A.D. 730, St. Bede, an Anglo-Saxon monk, identified that the Julian year was approximately 11 minutes and 14 seconds longer than it should be, leading to a discrepancy of about one day every 128 years. Despite this crucial observation, no corrective measures were taken for centuries.
Chapter 2: The Gregorian Reform
In A.D. 1582, Pope Gregory XIII addressed the accumulated error, which had reached about 10 days. He declared that the day after October 4 would be October 15, a significant shift that the Catholic world adopted. However, the British Empire, including America, did not implement this change until 1752, when September 2 was followed by September 14, and New Year’s Day was moved from March 25 to January 1.
To correct for future errors, it was decided that years ending in "00" would generally not be Leap Years, except for those divisible by 400.
Bill Petro, your friendly neighborhood historian
www.billpetro.com
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