The modern calendar is known as the Gregorian Calendar. It is named after Pope Gregory XIII who favored it over the Julian Calendar. But Gregory XIII only approved the calendar that bears his name. The actual inventor was Alosyius Lilius.
Alosyius Lilius Biography
Lilius was an Italian astronomer, physician and chronologist. He was from Ziro in Calabrio, Italy. Lilius’ chosen profession was medicine. He studied in Naples to become a doctor and served under the earl Carafa, a prominent nobleman. Some time later, Lilius began to teach in the Perugia University in 1552.
Aside from his interest in medicine, Alosyius Lilius was also fascinated by time keeping and calendars. He noted problems with the Julian Calendar, the one that was being used in his time. So he wrote a paper proposing a new calendar system. But it did not get attention until years after his death in 1576. After Alosyius died, his brother Antonio turned over his manuscript to Pope Gregory XIII. The latter put it for review before a council. It was approved after some changes made by another astronomer, Christopher Clavius. In 1582, Pope Gregory XIII issued a papal bull superseding the Julian with the Gregorian Calendar. It was slowly adopted by Christian countries one after the other. Now it is the system used by virtually everyone in the world.
Why the Gregorian Calendar Replaced the Julian Calendar
Julius Caesar invented the Julian system. All months had either 30 or 31 days, except for February. February or Februalia had 29 or 30 days depending on whether it was a leap year or not. Caesar named one month after himself, July. When Octavian became Roman emperor, he also renamed a month to August, after his new imperial name Augustus. He further borrowed another day from February so his month would have 31 days as well.
This calendar worked fine until later on in the Christian world. The problem was there was too much of a difference between the Julian system and the astronomical seasons. The astronomical equinoxes and solstices were moving ahead of this calendar by 11 minutes a year. Easter was moving out of sync with the vernal equinox. The Julian calendar grew one extra day every 134 years. By 1582 when the Gregorian was adopted, it had already gained 10 days.
The Gregorian Calendar reforms the Julian system. It amends the four year leap cycle in this way: Except for years that are divisible by 100, a year that can be divided into four is a leap year. The centurial years divisible by 400 are leap years.
The Christian pope and his council believed that this adjustment would let people celebrate Easter at the correct time. That is, on the date planed at the Council of Nicea in year 225 AD.