There are two basic sources for calendars presently in use: the monthly motion
of the Moon (Lunar calendars) and the yearly motion of the Sun
Calendars). Examples of Lunar calendars still in
use are the traditional Jewish and Chinese calendars. The difficulty with Lunar
calendars is that the seasons are correlated with the Sun, not the Moon. Thus,
Lunar calendars require elaborate adjustments or translations to relate to the
seasons. That calendars correlate with seasons is now primarily a matter of
convenience, but in more ancient cultures keeping track of the seasons was
serious business: it could be a matter of survival to know things like
the proper time to plant to ensure a bountiful harvest.
The Roman Lunar Calendar
Our present calendar (called the Gregorian Calendar) is a basically
solar calendar that grew from what was originally a Lunar calendar used by the
Romans. The original calendar contained 10 months of length 29 or 30 days.
This was later modified to a 12 month calendar, but 12 months of average length
29.5 days gives only 354 days in the year, whereas the orbital period of the
Earth is 365.242199 days. Thus, at the end of each year this calendar was
11 days out of step with the seasons and at the end of 3 years it was almost a
month out of step. This was initially corrected in an arbitrary way by adding
13th months, but this was used for various political purposes and soon threw
the calendar into severe confusion.
The Julian Calendar
In 46 B.C., Julius Caesar reformed the calendar by ordering the year to be 365
days in length and to contain 12 months. This forced some days to be added to
some of the months to bring the total from 354 up to 365 days, so the months
now were out of phase with the cycles of the Moon: although the Julian
Calendar retained monthly divisions, it was no longer a Lunar calendar.
The Julian Calendar improved things tremendously, but there was still about a
quarter day difference between the true length of the year and the 365 days
assumed for the Julian Calendar. Thus, February was given an additional day
every 4 years (leap years) and the average length of the Julian
year with leap years added was 365.25 days.
The Gregorian Calendar
However, the Julian year still differs
from the true year of 365.242199 days by 11 minutes and
14 seconds each year, and over a period of 128 years even the Julian Calendar
was in error by one day with respect to the seasons. By 1582 this error had
accumulated to 10 days and Pope Gregory XIII ordered another reform: 10 days
were dropped from the year 1582, so that October 4, 1582, was followed by
October 15, 1582. In addition, to guard against further accumulation of error,
in the new Gregorian Calendar it was decreed that century years not
divisible by 400 were not to be considered leap years. Thus, 1600 was a leap
year but 1700 was not. This made the average length of the year sufficiently
close to the actual year that it would take 3322 years for the error to
accumulate to 1 day.
modification to the Gregorian Calendar has been suggested:
years evenly divisible by 4000 are not leap years.
This would reduce
the error between the Gregorian Calendar Year and the true year to
1 day in 20,000 years. However,
this last proposed change has not been officially adopted; there is plenty of
time to consider it, since it would not have an effect until the year 4000.
Adoption of the Gregorian Calendar
An interesting historical sidelight on the Gregorian Calendar
is that not all countries adopted it immediately. In particular, it was
adopted uniformly in Catholic countries, but Protestant countries often still
used the Julian Calendar. Thus, the date could change by 10 days simply by
crossing certain country borders! England and its American colonies
did not adopt the Gregorian Calendar
until 1752, when 11 days were removed from the calendar,
and Russia resisted this change
until after the 1917 Revolution. One conseqence of the
British adoption of the Gregorian Calendar in 1752 is that George Washington
was born on February 11, 1731, according to the calendar in use on his
birthday, but we now celebrate his date of birth as February 22, 1731 (actually,
even that is no longer true with the advent of Presidents Day).