This year February has an extra day, according to a leap year rule that has caused a lot of
confusion. This is the first time a leap year day has been added to a century year in the United
States. This is the story.

When Julius Caesar imposed his Julian calendar in 46 B.C., the year was considered to be exactly
365.25 days long. That extra quarter day was saved for three years and added all at once as a
whole day. (Caesar had it added to February, not because it was the shortest month, but because
it was then the last month of the year. The names of the last four months of the year come from
their positions then. For example, September is Latin “sept” for seven.) For the next 16 centuries,
people faithfully added a leap year day to every fourth February.

The year, however, is actually 365.2422 days long. The difference between 0.2422 and 0.25 day is
11 minutes, and that error added up. People added too many leap year days, and the calendar
slowly fell out of step with the seasons. The error had accumulated to ten days by the late sixteenth
century. The spring equinox, which fell on March 21 by the calendar, was in reality occurring on March
11.

This caused a problem for church authorities responsible for fixing holy days. The date of Easter is
determined by the date of the spring equinox and the phase of the moon, and because the equinox
was wrong, people were celebrating Easter on the wrong day. In 1582, Pope
Gregory XIII decreed that a one-time adjustment be made and
immediately dropped 11 days. October 4 that year was abruptly
followed by October 15, and that brought the calendar back into synchronization with the sky. Pope
Gregory’s second decree modified the rule for adding leap year days to keep the calendar
synchronized with the sky. The new rule calls for there to be three fewer leap year days every 400
years. Prior to 1582, all years divisible by four were leap years. Henceforth, beginning with the year
1600, century years are not leap years unless they are divisible by 400. Thus, 1700, 1800, and 1900
were not leap years, but 2000 is. 2100 will not be. This is the second century leap year since
Gregory’s reform of 1582.

The pope’s calendar reform was adopted immediately in Catholic
countries, but Protestant countries resisted. The Gregorian calendar was adopted in Britain and her
American colonies in 1752, in Japan in 1873, and in Russia not until 1918.

The term “leap year” comes from the way the dates cycle through the days of the week during the
years. A common year of 365 days contains exactly 52 weeks of seven days plus one day. The
same date falls one day later in the week each successive year. March 1, for example, fell on
Saturday in 1997, on Sunday in 1998, and on Monday in 1999. This year it “leaps over” Tuesday and
falls on a Wednesday.

Pope Gregory’s Gregorian Calendar will not need another adjustment for 3000 years – and we’ll
worry about it then.

For additional leap year day information go to these web sites:

http://www.rog.nmm.ac.uk/leaflets/leapyear/leapyear.html (Royal
Observatory of Greenwich)

http://aa.usno.navy.mil/AA/faq/docs/leap_years.html (U.S. Naval Observatory)


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