The Gregorian calendar was invented in 1582 to improve the Julian calendar and keep dates in sync with the solar cycle. It was created by an Italian physician named Aloysius Lilius and named after Pope Gregory XIII. The new system had twelve monthly divisions of similar, though unequal, length, with February being the only month to contain 28 days. Leap years exist once every four years, except for years ending in -00, which are only leap years if they can be divided by 400. The calendar was not accepted by any European nation until October 1582, and non-Catholic countries had reservations about accepting it. By 1929 most countries in the world had adopted the Gregorian calendar.
The Gregorian calendar is used throughout most of the modern world. Invented in 1582, the system used a cycle of leap years different from that employed by the Julian calendar. The new calendar was considered an improvement over the previous Julian model. In the Gregorian calendar, four years constitute a cycle, with an extra day or “leap year” added to the fourth year to keep dates and months in sync with the solar cycle.
Although the Gregorian calendar was named for Pope Gregory XIII, who sanctioned its use, it was created by an Italian physician named Aloysius Lilius. The Julian calendar, in use for centuries, did not include a system for making the dates of the equinoxes, and consequently the Catholic holidays associated with them, fall on the same day. In addition to losing ten days from the Julian system, the Gregorian calendar introduced different rules into the leap year system to establish consistent dates in relation to the equinox. This new approach allowed, for example, Christmas to fall on December 25th every year.
In the new system, there were twelve monthly divisions of similar, though unequal, length. February, the second month, was the only month to contain 28 days, so the day added in leap years was solidified as February 29th. Leap years exist once every four years and occur only in years divisible by four. There is one exception to the divisible by four rule, and that is if the year ends in -00. Even though years ending in -00 are divisible by four, it’s not a leap year. However, there is another exception to the exception. Years ending in -00 that can be divided by 400 are in fact leap years.
The Gregorian calendar was decreed valid by Pope Gregory XIII on February 24, 1582, but it was not accepted by any European nation until October of that year. The first countries to start enforcing the use of the calendar were Spain, Portugal, Italy and the Polish-Lithuanian Commonwealth. France and Holland quickly followed, both adopting the new method before the end of 1582.
Non-Catholic countries understandably had reservations about accepting a calendar specifically aimed at furthering the goals of the Roman church. Europe was heavily involved in the Protestant Reformation when the calendar was proposed, and anti-Catholic sentiment has long deferred the unification of Europe under a common calendar. Eventually, the benefits of a common date system became impossible to ignore, and while the calendar was created at the whim of the Catholic Pope, its scientific basis made a lot of sense.
It took several centuries, but by 1929 most countries in the world had started using the Gregorian calendar. China, the last nation to adopt the system, technically accepted it starting in 1912, but civil unrest left the calendar question undetermined until the country’s unification in 1929. Other countries, such as Japan, accepted the use of the calendar for relations with the Western world, but kept the local systems in force for centuries.
Typically, when discussing dates prior to the establishment of the 1582 calendar, scholars continue to use the Gregorian count retroactively. Occasionally, you may come across double dating, which uses both Julian and Gregorian years. This most often refers to the intervening time when the Gregorian calendar may not have been accepted yet, but was already in widespread use.
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