The average time it takes the earth to revolve once around the sun
(_one_ year) is 365 days, 5 hours, 48 minutes, 47.8 seconds, or about
365-1/4 days.
The change in the length of the mean sidereal day, _i.e._, of the time
of the earth’s rotation upon its axis, amounts to 0.01252 seconds in
2400 mean solar years.
Instead of reckoning this part of a day each year, it is disregarded,
and an addition is made when this amounts to one day, which is very
nearly every fourth year. This addition of one day is made to the month
of February. Since the part of a day that is disregarded when 365 days
are considered as a year, is a little less than one quarter of a day,
the addition of one day every fourth year is a little too much, and, to
correct this excess, addition is made to only every fourth centennial
year.