The Gregorian calendar is the world’s most widely used calendar today. A calendar is a system of organizing units of time and of reckoning time in advance in order to satisfy the needs of a specific society. It serves practical purposes, providing the basis for recording and calculating dates over extended periods, and for planning and maintaining cycles of civil and religious events. Therefore, a calendar can provide a sense of understanding and controlling time, linking humankind and the cosmos. Because the principles on which a calendar is based are shared by the whole society that adopts it, it can be considered a specific form of social contract, a source of social order and cultural identity.
A calendar is constructed on astronomical cycles: The principal ones are the year (which is based on the revolution of the earth around the sun), the month (which is based on the revolution of the moon around the earth), and the day (which is based on the rotation of the earth on its axis). Yet these cycles do not synchronize perfectly: The average periods of sun and moon do not comprise an integral number of days, nor are they exactly commensurable with each other and with the length of a day. Moreover, the irrational ratios between the fundamental time units (year, month, day) are not constant in the long run.
Therefore, calendar years and months, which have an integral number of days, cannot be synchronized exactly with respect to the astronomical year and to the cycle of lunar phases. Thus, each calendar tries to surmount the problems that arise from this incommensurability through rational approximations. For instance, the Julian calendar, which reformed the Roman calendar in 45 BCE, added a leap day to February every 4 years. In this way the average length of the calendar year became quite congruous with the length of the astronomical year.
Furthermore, each calendar fixes a specific date for beginning the year, and most calendars choose an initial epoch for counting years: a historical or legendary event is adopted as starting point of the year count in order to maintain a consistent chronology. For instance, the foundation of Rome was adopted as the initial epoch of the ancient Roman calendar, the birth of Christ as the initial epoch of the Christian calendar.
History of the Gregorian Calendar
The Gregorian calendar was developed by the end of the 16th century by the astronomers Aloysius Lilius and Christophorus Clavius in order to resolve the problems arising from the desynchronization between the Julian calendar and the astronomical data. As the mean Julian calendar was slightly too long, the astronomical equinox, which should be on March 21, had regressed to a date earlier in the month. Consequently, the calendrical equinox drifted forward, causing difficulties in determining the correct date of Easter. Indeed, in 325 the Council of Nicaea had determined to celebrate Easter on the first Sunday after the ecclesiastical full moon, which falls on or shortly after March 21, the date calculated for the vernal equinox. Over the centuries, the shifting forward of the calendrical equinox caused Easter to drift away from its correct date, losing its position in springtime and its relation with the Jewish Passover.
As early as the 13th century, scholars had realized that the method of calculating the date of Easter had to be reformed. Several attempts were made to find a solution to this question, but no consensus was reached. By the 16th century the astronomical equinox was occurring 10 days before the calendrical equinox, and astronomical full moons were occurring 4 days before ecclesiastical full moons. Therefore, in 1563, the Council of Trent established that the date of the vernal equinox had to be restored to March 21 and required the outlining of a new calendar in order to correct present errors and prevent future ones. Consequently, Pope Pius V introduced corrections to the lunar tables and the leap-year system (1568-1570).
In 1572 Pope Gregory XIII succeeded Pope Pius V; aiming to optimize the measures taken by his predecessor, he commissioned a study for a general reform of the calendar. The astronomer Aloysius Lilius designed a project for calendar restoration; it was worked out in detail by his colleague Christophorus Clavius. The project was forwarded by Pope Gregory XIII to many Christian princes and university professors, gaining consensus and agreement.
On February 24, 1582, Pope Gregory XIII published the bull Inter gravissimas, whose specific aim was the reform of the calendar. This reform corrected the length of the solar year, changing the previous value of 365.25 days to the new value of 365.2425 days. In order to obtain this average length, the number of leap years was corrected from 100 in 400 years to 97 in 400 years. To achieve this result, the leap year system of the Julian calendar, which fixed every 4th year as leap year, was maintained, making corrections only at the turn of centuries: The bull determined that secular years divisible by 100 but not by 400 should be considered common years. Moreover, the error accumulated over the centuries since the Council of Nicaea was corrected by the removal of 10 days: the day after October 4, 1582, was designated October 15, 1582.
The order of the months and number of days per month of the Julian calendar were confirmed: January 31 days, February 28 (in leap years 29), March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, and December 31.
Although Pope Gregory XIII recommended in his bull to defer by 10 days the expiration of any payment, many people strongly opposed the adoption of the new calendar, fearing it could conceal a deception. Nevertheless, the Gregorian calendar was adopted in 1582 in Spain, Portugal, and their colonies, in the Polish-Lithuanian Commonwealth, and by most of Italy, France, Holland, and Zeeland; in 1700 in Denmark, Norway, and the Protestant states of Germany; in 1753 in the British Empire; in 1918 in Russia and in 1923 in Greece; and by the end of the 19th century it had been adopted in Japan and Korea and at the beginning of the 20th century in China.
The Gregorian calendar is quite accurate, accumulating an error of one day in about 3,300 years. Yet it also shows some weaknesses: Because of its unequal units, it cannot be divided into equal halves or quarters, and years and months may begin on any day of the week, making it difficult to predict the date of movable feasts.
See also Astronomical Calendar; Julian Calendar; Revolution of Earth; Equinoxes; Leap Years; Measurements of Time
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