|Perpetual Gregorian Calendar
by Stephen R. Schmitt
About the Gregorian Calendar
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.
The 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.