Gregory XIII aligns the equinox

Pope Gregory XIII issues a papal bull on February 24 1582 outlining his calendar reforms. The old Julian calendar had an error rate of one day in every 128 years. The Julian calendar, proposed by Julius Caesar, took effect on January 1 45BC. The Julian year is on average 365.25 days long, consequently drifting over time with respect to the tropical (solar) year (365.24217 days) and gains about three days every four centuries compared to observed equinox times and the seasons. Gregory adjusts the calendar so that the first vernal equinox (the two moments in the year when the sun is exactly above the Equator and day and night are of equal length) is near March 21, where it was during the Council of Nicaea (May 20 – August 25 325AD). This requires removing 10 days of drift. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, year numbers evenly divisible by 100 are not leap years, except for those evenly divisible by 400. Even then, the Gregorian calendar diverges from astronomical observations by one day in 3,030 years. When the Gregorian calendar is enacted in 1582, October 4 is followed by October 15 (the 10-day cut). It is immediately adopted by the major Catholic powers in Europe, but the Protestant countries refuse to follow suit until the 18th century. The adjustment is made by Eastern European countries only after World War 1. It is now used in most countries