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Gregorian Calendar: Julian Calendars
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The Gregorian Calendar was devised both because the lunar calendar had grown conspicuously wrong, and the mean Julian Calendar year is slightly too long, so that the vernal equinox slowly drifts backwards through Julian calendar years. This caused problems in computing the date of Easter.
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The Gregorian Calendar was introduced as the correction to the previously universal Julian calendar. The Julian calendar year consisted of 365.25 days, a little too long since the correct value for the tropical year is 365.242199 days [3]. This error of 11 minutes 14 seconds per year amounted to almost one and a half days in two centuries, and seven days in 1,000 years. The problem was placed before church councils, but no action was taken because the astronomers who were consulted didn't have enough precise information concerning the exact value of the tropical year.
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From a seasonal point of view, a the difference between the Gregorian and the Julian calendar wasn't yet really all that pressing a matter, only 10 days in 1582, only 14 days in the twentieth century. The logical question ... becomes, why did it matter? The short answer is religion, which is, in most cultures, very concerned with keeping time. For Christianity, the fundamental chronological problem was the calculation of Easter, and this difficulty drove the reform. Recall that Easter is supposed to be the first full moon after the spring equinox. But as Easter was calculated by rule, not astronomical observation, the slip of the equinox meant that the rule was badly in error.
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The last country of Eastern Europe to adopt the Gregorian calendar was Greece in 1923. However, these were all civil adoptions — none of the national churches accepted it. Instead, a Revised Julian calendar was proposed in May 1923 which dropped 13 days in 1923 and adopted a different leap year rule that resulted in no difference between the two calendars until 2800. The Orthodox churches of Greece, Bulgaria, Romania, Poland, and a few others around the Eastern Mediterranean (Constantinople, Alexandria, Antioch, and Cyprus) adopted the Revised Julian calendar, so these New calendarists will celebrate the Nativity along with the Western churches on 25 December in the Gregorian calendar until 2800. The Orthodox churches of Russia, Serbia, Jerusalem, and a few bishops in Greece did not accept the Revised Julian calendar. These Old Calendarists will continue to celebrate the Nativity on 25 December in the Julian calendar, which is 7 January in the Gregorian calendar until 2100. All of the other Eastern churches that are not Orthodox churches, like the Coptic, Ethiopic, Nestorian, Jacobite, and Armenian, continue to use their own calendars, which usually result in fixed dates being celebrated in accordance with the Julian calendar.
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The Gregorian calendar is the calendar that is used throughout the most of the world. It began to be used from 1582. It replaced the previous Julian calendar because the Julian Calendar had an error: it added a leap year (with an extra day every four years) with no exceptions. The length of the Julian year was exactly 365.25 days, but the actual time it takes for the Earth to go around the Sun once is closer to 365.2425 days. This difference is just over ten minutes each year.
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The Gregorian calendar resulted from a perceived need to reform the method of calculating dates of Easter. Under the Julian calendar the dating of Easter had become standardized, using March 21 as the date of the equinox and the Metonic cycle as the basis for calculating lunar phases. By the thirteenth century it was realized that the true equinox had regressed from March 21 (its supposed date at the time of the Council of Nicea, +325) to a date earlier in the month. As a result, Easter was drifting away from its springtime position and was losing its relation with the Jewish Passover. Over the next four centuries, scholars debated the "correct" time for celebrating Easter and the means of regulating this time calendrically. The Church made intermittent attempts to solve the Easter question, without reaching a consensus.
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Julian Calendars