On Leap Year
question: Why is it that: the year 1900 was not a leap year; the year 2000 will be a leap year; and the year 2100 will not be a leap year?
answer: A year that is evenly divisible by 100 is not a leap year unless it also is evenly divisible by 400.
explanation: The time required for Earth to make one lap around the sun -- to go from vernal equinox to vernal equinox -- is approximately 365.2422 days. Because the period of 365 days that our calendar assigns to the basic year falls approximately 0.2422 of a day short of the time it actually takes to orbit the sun, the calendar adds one day to every fourth year. This quadrennial "leap year" causes the average duration of a year to be approximately 365.2500 days. Because this quadrennial adjustment overshoots its goal by approximately 0.0078 of one day, the calendar "skips" the quadrennial adjustment at the turn of each century. This centennial "re-adjustment" causes the average duration of a year -- over the long run -- to be approximately 365.2400 days. Because this centennial readjustment overshoots its goal by approximately 0.0022 of one day, the calendar "un-skips" the centennial re-adjustment once every four centuries. This quadricentennial "re-re-adjustment" causes the average duration of a year -- over the very long run -- to be approximately 365.2425 days. All of this has the effect of reducing the difference between our calendar and our actual orbit around the sun to approximately 0.0003 of a day -- about 26 seconds.
(IHUB (WAVX-32153), 2/29/04)
