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The Gregorian Reform The Julian calendar is phased out
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.
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 Education, Inc. All rights reserved.
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