The Gregorian calendar that is in common use today was instituted
by Pope Gregory in 1582 AD because the Julian calendar, in use since
45 BC, introduced an error of one day in every 128 years that had
moved the date of the vernal equinox to March 11.
In the Julian calendar a common year contained 365 days. A year was
a leap year of 366 days if it was exactly divisible by 4. This is
true in the Gregorian calendar except that century years that are
not divisible by 400 are common years; that is, 1900 and 2100 are
not leap years while 2000 is a leap year.
new calendar restored the vernal equinox to March 21. The numerical
algorithm of the Gregorian calendar was devised by the Neapolitan
astronomer-physician Aloysius Lilius (Luigi Lilio Ghiraldi). It
took effect in Europe's Roman Catholic countries in October of 1582;
October 4, 1582 was followed by October 15, 1582. Protestant countries
continued to use the Julian calendar until the 1700's or later
Copyright © 2004, Stephen R. Schmitt