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History of the Calendar
The purpose of the calendar is to reckon past or future time, to show
how many days until a certain event takes place—the harvest or a
religious festival—or how long since something important happened.
The earliest calendars must have been strongly influenced by the
geographical location of the people who made them. In colder countries,
the concept of the year was determined by the seasons, specifically by the
end of winter. But in warmer countries, where the seasons are less
pronounced, the Moon became the basic unit for time reckoning; an old
Jewish book says that “the Moon was created for the counting of the
days.”
Most of the oldest calendars were lunar calendars, based on the time
interval from one new moon to the next—a so-called lunation. But
even in a warm climate there are annual events that pay no attention to
the phases of the Moon. In some areas it was a rainy season; in Egypt it
was the annual flooding of the Nile River. The calendar had to account for
these yearly events as well.
The Egyptian Calendar
The ancient Egyptians used a calendar with 12 months of 30 days each,
for a total of 360 days per year. About 4000 B.C. they added five extra days at the end of every
year to bring it more into line with the solar year.1 These five days became a festival because it was
thought to be unlucky to work during that time.
The Egyptians had calculated that the solar year was actually closer to
3651/4 days, but instead of having a single leap day every four
years to account for the fractional day (the way we do now), they let the
one-quarter day accumulate. After 1,460 solar years, or four periods of
365 years, 1,461 Egyptian years had passed. This means that as the years
passed, the Egyptian months fell out of sync with the seasons, so that the
summer months eventually fell during winter. Only once every 1,460 years
did their calendar year coincide precisely with the solar year.
In addition to the civic calendar, the Egyptians also had a religious
calendar that was based on the 291/2-day lunar cycle and was
more closely linked with agricultural cycles and the movements of the
stars.
1. The correct figures are lunation: 29 d,
12 h, 44 min, 2.8 sec (29.530585 d); solar year: 365 d, 5 h, 48 min, 46
sec (365.242216 d); 12 lunations: 354 d, 8 h, 48 min, 34 sec (354.3671
d).
Lunar Calendars
During antiquity the lunar calendar that best approximated a solar-year
calendar was based on a 19-year period, with 7 of these 19 years having 13
months. In all, the period contained 235 months. Still using the lunation
value of 291/2 days, this made a total of 6,9321/2
days, while 19 solar years added up to 6,939.7 days, a difference of just
one week per period and about five weeks per century.
Even the 19-year period required adjustment, but it became the basis of
the calendars of the ancient Chinese, Babylonians, Greeks, and Jews. This
same calendar was also used by the Arabs, but Muhammad later forbade
shifting from 12 months to 13 months, so that the Islamic calendar now has
a lunar year of about 354 days. As a result, the months of the Islamic
calendar, as well as the Islamic religious festivals, migrate through all
the seasons of the year.
The Roman Calendar
When Rome emerged as a world power, the difficulties of making a
calendar were well known, but the Romans complicated their lives because
of their superstition that even numbers were unlucky. Hence their months
were 29 or 31 days long, with the exception of February, which had 28
days. However, four months of 31 days, seven months of 29 days, and one
month of 28 days added up to only 355 days. Therefore the Romans invented
an extra month called Mercedonius of 22 or 23 days. It was added every
second year.
Even with Mercedonius, the Roman calendar eventually became so far off
that Julius Caesar, advised by the astronomer Sosigenes, ordered a
sweeping reform. 46 B.C. was made 445 days long
by imperial decree, bringing the calendar back in step with the seasons.
Then the solar year (with the value of 365 days and 6 hours) was made the
basis of the calendar. The months were 30 or 31 days in length, and to
take care of the 6 hours, every fourth year was made a 366-day year.
Moreover, Caesar decreed the year began with the first of January, not
with the vernal equinox in late March.
This calendar was named the Julian calendar, after Julius
Caesar, and it continues to be used by Eastern Orthodox churches for
holiday calculations to this day. However, despite the correction, the
Julian calendar is still 111/2 minutes longer than the actual
solar year, and after a number of centuries, even 111/2 minutes
adds up.
The Gregorian Reform
By the 15th century the Julian calendar had drifted behind the solar
calendar by about a week, so that the vernal equinox was falling around
March 12 instead of around March 20. Pope Sixtus IV (who reigned from 1471
to 1484) decided that another reform was needed and called the German
astronomer Regiomontanus to Rome to advise him. Regiomontanus arrived in
1475, but unfortunately he died shortly afterward, and the pope's plans
for reform died with him.
Then in 1545, the Council of Trent authorized Pope Paul III to reform
the calendar once more. Most of the mathematical and astronomical work was
done by Father Christopher Clavius, S.J. The immediate correction, advised
by Father Clavius and ordered by Pope Gregory XIII, was that Thursday,
Oct. 4, 1582, was to be the last day of the Julian calendar. The next day
would be Friday, Oct. 15. For long-range accuracy, a formula suggested by
the Vatican librarian Aloysius Giglio was adopted: every fourth year is a
leap year unless it is a century year like 1700 or 1800. Century
years can be leap years only when they are divisible by 400 (e.g.,
1600 and 2000). This rule eliminates three leap years in four centuries,
making the calendar sufficiently accurate.
In spite of the revised leap year rule, an average calendar year is
still about 26 seconds longer than the Earth's orbital period. But this
discrepancy will need 3,323 years to build up to a single day.
Reform Adopted Gradually
The Gregorian reform was not adopted throughout the West immediately.
Most Catholic countries quickly changed to the pope's new calendar in
1582. But Europe's Protestant princes chose to ignore the papal bull and
continued with the Julian calendar. It was not until 1700 that the
Protestant rulers of Germany and the Netherlands changed to the new
calendar. In Great Britain (and its colonies) the shift did not take place
until 1752, and in Russia a revolution was needed to introduce the
Gregorian calendar in 1918. In Turkey, the Islamic calendar was used until
1926.
Adoption of the Gregorian
Calendar
| Year |
Country |
| 1582 |
Catholic states of Italy, Portugal,
Spain, Belgium, Holland, and Poland |
| 1584 |
German and Swiss Catholic states |
| 1587 |
Hungary |
| 1700 |
German, Swiss, and Dutch Protestant
States, Denmark, and Norway |
| 1752 |
Great Britain and its possessions
(including the American colonies) |
| 1873 |
Japan |
| 1875 |
Egypt |
| 1918 |
Russia |
| 1924 |
Greece |
| 1926 |
Turkey |
| 1949 |
China |
A Better Calendar?
Despite its widespread use, the Gregorian calendar has a number of
weaknesses. It cannot be divided into equal halves or quarters; the number
of days per month is haphazard; and months and years may begin on any day
of the week. Holidays pegged to specific dates may also fall on any day of
the week, and few Americans can predict when Thanksgiving will occur next
year. Since Gregory XIII, many other proposals for calendar reform have
been made, but none has been permanently adopted. In the meantime, the
Gregorian calendar keeps the calendar dates in reasonable unison with
astronomical events.
Information Please® Database, © 2007 Pearson
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