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Julian Calendar: Gregorian Calendars
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The Julian calendar was in general use in Europe and Northern Africa from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian Calendar. Reform was required because too many leap days are added with respect to the astronomical seasons on the Julian scheme. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian year, causing the calendar to gain a day about every 134 years. While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value, it was evidently felt to be of little importance at the time of the Julian reform. However, it accumulated significantly over time.
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The Julian calendar is used for all dates up to 1582 Oct 04. After that date, the Gregorian calendar is used. Due to the Gregorian Calendar reform, the day after 1582 Oct 04 (Julian calendar) is 1582 Oct 15 (Gregorian calendar). Note that Great Britain did not adopt the Gregorian calendar until 1752. For more information, see Calendars.
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The Julian calendar system is calendar system that was widely used from about 4CE to 1582CE. It was replaced by the Gregorian calendar on 1582-10-15. The Julian calendar defines a leap year as once every four years. This becomes inaccurate over time, to such an extent that by 1582, 10 days had to be 'lost' to correct the calendar.
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NOTE: By order of Pope Gregory XIII, the Julian calendar was replaced by the Gregorian calendar in late 1582, so that the calendar day following Oct. 4, 1582 was Oct. 15, 1582. Most (but not all) historical dates prior to 1582 are JULIAN calendar dates. To make matters worse, the protestant nations of Europe were slow to adopt the Catholic calendar reform (Great Britain did not adopt the Gregorian calendar until 1752). As if that weren't bad enough, when Julius Caesar, on the advice of the astronomer Sosigenes, introduced the leap year to create the Julian calendar, the keepers of the calendar mistakenly recorded leap years every THREE years, instead of every FOUR (as was, of course, intended). Unfortunately, Sosigenes was dead at the hands of the Egyptians and Caesar was dead at the hands of the imperial senate, so the mistake persisted until corrected by Augustus Caesar in 8 B.C.
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The Julian calendar was introduced by Julius Caesar in 45 BC. It was in common use until the 1500s, when countries started changing to the Gregorian Calendar. However, some countries (for example, Greece and Russia) used it into this century, and the Orthodox church in Russia still uses it, as do some other Orthodox churches.
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The Julian calendar is late one day every 128 years in relation to the real astronomic time. Therefore, since the contrast between the real Equinox and the one that could have been expected by the calendar became apparent and visible, Pope Gregory XIII decided to introduce the Gregorian calendar (that was named by him). This calendar is today the regular civic calendar throughout the world and is used by the Roman Catholic Church, Old Catholic Church and all the Protestant Christian Churches. In the Gregorian calendar as well, every fourth year is a leap one, but this ... goes for the secular years (ones divisible by 100), and thus years 1700, 1800 and 1900 were not leap, but year 2000 is. This calendar is much more precise and is late one day every 14.400 years compared to the real astronomic time. The difference between the Gregorian and Julian calendar at this moment equals 13 days.
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Gregorian Calendars