anno domini

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 yera length that defines the repetition of the seasons. the length of the tropical year is 365.24219 days
are you still with me?
In 46bc 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.
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.
your eyes are glazing over. stick with me on this.
the change from the julian calendar to the gregorian calendar 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 were not leap-years whereas 2000 was.
The net effect is to make the adopted average length of the year 365.2425 days. the difference between this and the true length (365.24219) will not have a serious effect for many thousands of years. the error amounts to about 3 days in 10, 000 years.
so what is the point of all this science and history?
happy anniversary to rhonda and me! married 2/29/00 and still in love!