February 30th 2000

[Pseudopath]

The Earth takes 365 days and just over 6 hours to complete one orbit of the Sun, so we have a leap day every four years to compensate for the cumulative effect. However, I remember reading that, under the original design of our Gregorian calendar, an additional day should be added to every year divisible by 400 to compensate for the remaining minutes. That being the case, why did we not observe this in the year 2000?

[WaldoMcFroog]

The Gregorian calendar year is intended to be of the same length as the cycle of the seasons. However, the cycle of the seasons, technically known as the tropical year, is approximately 365.2422 days. Since a calendar year consists of an integral number of whole days, a calendar year cannot exactly match the tropical year. If the calendar year always consisted of 365 days, it would be short of the tropical year by about 0.2422 days every year. Over a century, the calendar and the seasons would depart by about 24 days, so that the beginning of spring in the northern hemisphere would shift from March 20 to April 13. To synchronize the calendar and tropical years, leap days are periodically added to the calendar, forming leap years. If a leap day is added every fourth year, the average length of the calendar year is 365.25 days. This was the basis of the Julian calendar, introduced by Julius Caesar in 46 B.C. In this case the calendar year is longer than the tropical year by about 0.0078 days. Over a century this difference accumulates to a little over three quarters of a day. From the time of Julius Caesar to thesixteenth century A.D., the beginning of spring shifted from March 23 to March 11. When Pope Gregory XIII instituted the Gregorian calendar in 1582, the calendar was shifted to make the beginning of spring fall on March 21 and a new system of leap days was introduced. Instead of intercalating a leap day every fourth year, 97 leap days would be introduced every 400 years, according to the rule given above. Thus, the average Gregorian calendar year is 365.2425 days in length. This agrees to within a half a minute of the length of the tropical year. It will take about 3300 years before the Gregorian calendar is as much as one day out of step with the seasons. From: http://aa.usno.navy.mil/faq/docs/leap_years.html

[Andromeda]

Although Waldo's answer may be completely true, my brain hurts just thinking about it. The more easy explanation is that we did. The actual stipulation is that ONLY century years divisible by 400 should be leap years, that is 1900 and 2100 will not be leap years (despite being divisible by 4), and 2000 was. Dropping 1 in 25 (the century year) compensates for the third decimal place in the difference between the actual year length 365.24219 days and 365, and adding back 1 in 100 compensates for the next decimal place.

Andy

[Pseudopath]

Fantastic! That one was bugging me for ages...well, ever since 1st March 2000 anyway.