The average length of a year in the Julian Calendar is 365.25 days (plus one additional day being added every four years). This is significantly longer than the "real" length of the solar year, 365.2422 days. Tthis error accumulates so that after about 131 years the calendar is out of sync with the equinoxes and solstices by one day. Pope Paul III recruited several astronomers, principally the Jesuit Christopher Clavius (1537-1612), to come up with a solution. They built upon calendar reform proposals by the astronomer and physician Luigi Lilio (d. 1576). When Pope Gregory XIII was elected he found various proposals for calendar reform before him, and decided in favor of that of Clavius. On 1582-02-24 he issued a papal bull, Inter Gravissimas, establishing what is now called the Gregorian Calendar reform. The Gregorian reform consisted of the following: -Ten days were omitted from the calendar, and it was decreed that the day following (Thursday) October 4, 1582 (which is October 5, 1582, in the old calendar) would thenceforth be known as (Friday) October 15, 1582. -The rule for leap years was changed. In the Julian Calendar a year is a leap year if it is divisible by 4. In the Gregorian Calendar a year is a leap year if either it is divisible by 4 but not by 100, or it is divisible by 400. In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not. -New rules for the determination of the date of Easter were adopted. -The position of the extra day in a leap year was moved from the day before February 25th to the day following February 28th.