Re: Why isn't there a leap year every 100 years

Hi Kate, Like many things that are passed as lore, there are a couple details about leap year that are not common knowledge. I found a couple online sources.

Here is information on leap that I quote from http://www.usinfo.com/staff/rheil/LEAPYEAR.HTM (link defunct as of 7/24/2006).

The year is defined as being the interval between two successive passages of the Sun through the vernal equinox. Of course, what is really occurring is that the Earth is going around the Sun but it is easier to understand what is happening by considering the apparent motion of the Sun in the sky.

The vernal equinox is the instant when the Sun is above the Earth's equator while going from the south to the north. It is the time which astronomers take as the definition of the beginning of Spring. The year as defined above is called the tropical year and it is the year length that defines the repetition of the seasons. The length of the tropical year is 365.24219 days. In 46 BC Julius Caesar established the Julian calendar which was used in the west until 1582. In the Julian calendar each year contained 12 months and there were an average of 365.25 days in a year. This was achieved by having three years containing 365 days and one year containing 366 days. (In fact the leap years were not correctly inserted until 8 AD). The discrepancy between the actual length of the year, 365.24219 days, and the adopted length, 365.25 days, may not seem important but over hundreds of years the difference becomes obvious. The reason for this is that the seasons, which depend on the date in the tropical year, were getting progressively out of kilter with the calendar date. Pope Gregory XIII, in 1582, instituted the Gregorian calendar, which has been used since then. The change from the Julian calendar to the Gregorian involved the change of the simple rule for leap-years to the more complex one in which century years should only be leap-years if they were divisible by 400. For example, 1700, 1800 and 1900 are not leap-years whereas 2000 will be. The net effect is to make the adopted average length of the year 365.2425 days. The difference between this and the true length will not have a serious effect for many thousands of years. (The error amounts to about 3 days in 10,000 years.) The adoption of the Gregorian calendar was made in Catholic countries in 1582 with the elimination of 10 days, October the 4th being followed by October 15th. The Gregorian calendar also stipulated that the year should start on January 1. In non-Catholic countries the change was made later; Britain and her colonies made the change in 1752 when September 2nd was followed by September 14 and New Year's Day was changed from March 25 to January 1.

Produced by the Information Services Department of the Royal Greenwich Observatory.

More about Leap Year

The Jesuit astronomer Christopher Clavius helped Pope Gregory XIII to introduce what is now called the Gregorian calendar.

The Julian leap-year rule created 3 leap years too many in every period of 385 years. As a result, the actual occurrence of the equinoxes and solstices slowly moved away from their calendar dates. The date of the spring equinox determines the date of Easter so the church began to press for reform.

Clavius proposed that Wednesday, Oct. 4, 1582 (Julian) should be followed by Thursday, Oct. 15, 1582 (Gregorian). He proposed that leap years occur in years exactly divisible by four, except that years ending in 00 must be divisible by 400 to be leap years. This rule is still used today and is so accurate that no further reform of the calendar will be necessary for many centuries.

Vičte did not like Clavius's calendar and the people of Frankfurt rioted against mathematicians and the Pope who, they believed had conspired together to rob them of 11 days.

Although Clavius produced little mathematics of his own he did more than any other German scholar of the 16C to promote a knowledge of mathematics. He was the first, however to use the decimal point.

He was a gifted teacher and writer of textbooks. His arithmetic books were used by many mathematicians including Leibniz and Descartes.

Sincerely,

Tom "Naught Plus Naught Is Naught" Cull