Accepted systems for the division of time into days, months, and years, calendars reflect a human effort to measure and order extended periods of the future. Scholars divide calendars into three general categories—lunar, solar, and lunisolar—depending upon whether their temporal divisions are principally based upon the movements of the moon or the sun or on a compromise between both. Regarding Western forms of chronology, in which solar reckoning of time predominates, historians have studied calendar reform, which encapsulates many scientific advancements and cosmological debates of the Renaissance and Enlightenment. In their studies of other parts of the world, where lunar calendars tend to prevail, commentators have tended to seek a comparative understanding of the various means of envisioning and organizing time across the globe and the cultural implications of such systems.Used to regulate civil and religious observances, as well as agricultural and business affairs, calendars offer valuable insights into the historical development of cultural and scientific standards within a society.
Historically, the creation of a calendar that provides both a satisfactory representation of time and practical value for the ordering of life has proved a vexing problem. Technical difficulties associated with measuring time on Earth and ignorance of the true structure of the solar system proved the greatest impediments to accurate measurement. Such problems began with the length of the day, the basic division of calendrical time. Early astronomers computed the amount of time required for the Earth to complete one rotation about its axis in relation to either the Sun—the solar day—or another, more distant star—the sidereal day. Because of practical variations in the computed values, however, astronomers found it necessary to use an average value, the mean solar day, for use in devising a calendar. The next important calendrical division, the month, was commonly associated with the orbit of the moon around the Earth. Ancient observers recorded months based upon the cycling of the moon through its phases, from full to new and full again—a process that occurs approximately every 29 and one half days and is called the synodic month. Early calendar-makers then divided time into years by examining the period required for the Earth to complete one full orbit around the sun. Several means of determining this value have been applied since antiquity, but among the most useful has been the measurement of the tropical year. Determining the length of the tropical year involves calculating the period of time elapsed between the sun's passage through successive vernal equinoxes—moments marking the beginning of Spring when the sun is directly above the equator and the hours of day and night are equal. Prior to the twentieth century, such calculations produced inaccuracies on the scale of several minutes. Due to such errors, and because the values used in dividing time according to lunar or solar reckoning produced such widely divergent results, calendar-makers found it necessary to introduce intercalations to the calendar year, adding days where appropriate in order to maintain consistency between the accepted values for the length of a month and the actual time required for the Earth to complete its transit around the sun.
The origins of the calendar in the West can be traced to the astronomical reckoning of the ancient Egyptians, who devised a system based upon a year of 360 days with an additional five-day intercalation. Later, the Greek astronomer Eudoxes amended this calendar, calculating the year at 365 days and six hours, which more closely approximates the true value of the tropical year. This system was then adopted by the Romans in 45 bc to replace their flawed calendar. Forwarded by Julius Caesar under the advisement of the astronomer Sosigenes, the Julian Reform called for an abandonment of lunation and for reliance on a completely solar calendar. The system instituted the leap year, adding one day to the calendar every four years, but even with the leap year fix it was unsatisfactory—the Julian year was still 11 minutes and 14 seconds longer than the tropical year. The resulting discrepancy produced significant conflicts between calendar dates and the observable seasons over the course of the ensuing centuries. Nevertheless, the new system proved vastly superior to its predecessor. It was accepted throughout the Roman world, and in 325 ad made the official doctrine of Christendom by the first Council of Nicea.
By the late sixteenth century, advances in astronomy had radically pushed the possibility of calendar reform ahead. The real impetus for updating the calendar, however, came from a need to establish the correct time for the celebration of Easter. In 1582 Pope Gregory XIII issued a papal bull instituting a reform of the Julian calendar in which several days were dropped to reset a system that had strayed dramatically. Building upon the work of the astronomer Christoph Clavius, whose calculations had produced a closer approximation of the tropical year, the Gregorian Reform removed centurial leap years, except for those divisible by 400. While the reform was rapidly accepted throughout Catholic Europe, Protestant countries were resistant. Over time, new scientific discoveries concerning the mechanics of the universe were brought to light, causing astronomers, regardless of religious persuasion, to acknowledge the errors of the old system. Notably, as the work of Sir Isaac Newton in the seventeenth century effectively annihilated the last remnants of the ancient and flawed Ptolemaic cosmological model and presented a new vision of the universe, resistance began to erode. Eventually the German states fell into line by 1699, in large part due to the lobbying efforts of Newton's contemporary, Gottfried Wilhelm Leibniz. Still, the change was not accepted until 1752 in England, where, tradition has it, the loss of 11 days in the calendar sparked protest—the so-called British time riots—throughout the county.
The Gregorian calendar, while much more accurate than its Julian predecessor, was still not completely in accordance with empirical reality. And while considerable advancements have since been made in the measurement of time, astronomical variables—such as changes in the motion of the Earth and the sun, and the inevitable, if slight, drift of the equinoxes—make the proposition of a perfectly accurate calendar an unattainable one. Meanwhile, scholars continue to study developments in the calendar over the centuries, and the cultural importance of these changes. Additionally, a variety of non-western calendars employed around the world—such as the lunisolar Jewish calendar, and the lunar-based Chinese and Islamic calendars—have attracted the interest of cultural critics. Other scholars continue to examine historical calendars, such as the elaborate lunar system employed by the pre-columbian Mayan civilization of Central America, for insights into astronomical influence on the construction of society.