Calendar, Gregorian

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 [Page 130]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 exacdy 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.