Pope Gregory XIII slightly revised the calendar in 1582, and implemented/introduced this on the day after October 4th, but it was very much the same as the previous (Julian) calendar, i.e. it had the same number of days in the year and days in the months, and the months had the same names.
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Note: The Gregorian calendar was introduced on the day after October 4th, which officially became October 15th. In this way the year 1582 would end 10 days earlier that it would otherwise have done. This dramatic step immediately aligned the calendar year more closely with the solar year.
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DETAILS
By Gregory's day, about a millennium and a half after the Julian calendar had been introduced, the calendar year had moved ahead of the solar year, and was now about 10 days ahead the solar year, and moving even further ahead each year. This was mainly due to the fact that the existing leap year arrangement hadn't adequately accounted for the earth's cycle around the sun.
To correct this anomaly two things were done:
- ten days were removed from the calendar with immediate effect, and
- the 4-year 'leap year' system was modified slightly: years that are divisible by 100 are not leap years, except for years divisible by 400 which are leap years.
Although these change to the calendar were done under the authority of Pope Gregory, the main calculations and proposals for correction had been made earlier by various astronomers and mathematicians, some of whom had died before their ideas were implemented by Gregory.
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JULIAN CALENDAR
As an example of an earlier but significant change to the calendar was when Julius Caesar introduced the 'Julian' calendar in 46 BCE. He decided that the year would begin on January 1st!
With hindsight we can say that this seems like a very good idea!
Furthermore, prior to the introduction of the Julian calendar, the year was about three months adrift from the solar year! Julius Caesar introduced dramatic changes to correct the matter.
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INTRODUCING THE GREGORIAN CALENDAR
Britain introduced the Gregorian calendar in 1752.
Alaska introduced the Gregorian calendar in 1857
The Chinese introduced the Gregorian calendar to their own culture in 1912.
Russia introduced the Gregorian calendar in 1917, but the Russian Orthodox church still uses the Julian calendar for its holy days and celebrations.
Greece introduced the Gregorian calendar in 1923, by which time their calendar was running 13 days ahead of the solar year.
GREGORIAN CALENDAR NOT INTRODUCED
Some countries and cultures use their own calendars, e.g. The Islamic, Hindu and Hebrew calendars.
Many Orthodox religions still use the Julian calendar for their 'church' calendars, a consequence of which is that common religious events are celebrated on entirely different days in different parts of the world.
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YEAR NUMBERING IN THE GREGORIAN CALENDAR
The present year numbering system which, by extension back, started in the year 1 AD (anno domini), was already in wide use, and had been so for many centuries, long before Pope Gregory authorised the 1582 calendar adjustments. He retained the 'AD' year numbering system.
The 'AD' year dating system was actually computed by Dionysius Exiguus in… 525 AD. i.e. in the year that was calculated by Exiguus to be the year '525'! Previously (and elsewhere) people had been (and were) using a different starting point for the year numbers.
The starting point for Exiguus' year numbering system was based on the year he had calculated to be the year of the birth of Jesus Christ, with 1 AD being the first year of his life, i.e. 'the first year of our Lord, Jesus Christ', and the year 525 AD was therefore 524 years later.
However, we have a problem here, because Exiguus did not provide cross references to other calendars or epochs, so we do not know if he intended year 1 AD to be the actual year of Jesus birth, or if it was intended to represent the year after Jesus birth, in which case Jesus would have been born in 1 BC, which might sound very strange!
Even so, more people gradually began using Exiguus' year numbering system. A couple of centuries later it was officially adopted by Roman Emperor Charlemagne (742-814 AD) sometime during his reign, and Exiguus' system (which has a starting point of '1 AD') consequently became extensively used throughout Charlemagne's empire.
The AD 'year numbering system' has become the most widely adopted calendar system, now being the most common system, even in lands where the birth of Jesus Christ would seem to be an irrelevance. Consequently many historians and academic works now prefer to use the neutral term 'Common Era' (C.E.) for years which were formerly designated AD, thus 325 AD becomes 325 CE.
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