History of Leap year

	Revision 31 . . November 26, 2001 8:07 am by (logged).179.252.xxx [Simon Cassidy. Vernal equinox year is the reference year-length for the Gregorian calendar (not the mean tropical).]
	Revision 30 . . (edit) November 19, 2001 10:14 pm by Karl Palmen [correct link to lunar phase (move plural s out and point to moon phase)]
	Revision 29 . . November 13, 2001 5:49 am by Lee Daniel Crocker [Put back "about"...see Talk]

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Difference (from prior major revision) (no other diffs)

Changed: 11c11

By adding a day every four years, an average year is adjusted to 365.25 days. However, this still causes a discrepancy with the actual 365.2422 days period. To make the average year more accurate, a leap year is cancelled in each century. This removes 0.01 days to bring the average to 365.24 days.

By adding a day every four years, an average year is adjusted to 365.25 days. However, this still causes a discrepancy with the vernal equinox tropical year. To make the average year more accurate, a leap year is cancelled in each century. This removes 0.01 days to bring the average to 365.24 days.

Changed: 14c14

The adjusted average is still 0.0003 day ahead of the actual period. As a result, the Gregorian Calendar will still run a day ahead every 3333 years. A proposal has been made to add an additional rule: that years divisible by 4000 are not leap years. This has not been accepted, since the change of the length of the tropical year over a period of four millennia will overwhelm any small gain in accuracy by this rule.

The adjusted average is still about 0.0001 days ahead of the actual mean interval between vernal equinoxes (365.242375 days). As a result, the Gregorian Calendar will still run about a half-day ahead in 4000 years. Proposals have been made to add an additional rule: e.g. that years divisible by 4000 are not leap years. Such proposals have not been accepted, since the aim of the Gregorian calendar reform was to keep the vernal equinox steady in the calendar and the Gregorian calendar achieves this long-term goal well enough when considering the changing length of the vernal equinox year in the foreseeable future.