CALENDAR, a system of dividing time into years, months, weeks and days for use in civil life, or a register of these or similar divisions. Among the old Romans, for want of such a register, it was the custom of the pontifex znaximus, on the first day of the month, which began with the new moon, to proclaim (calare) the month, with the festivals occurring in it. Hence, calends (the first of the month) and calendar. The periodical oc currence of certain natural phenomena gave rise to the first division of time. The apparent daily revolution of the sun about the earth occasioned the division into days. The time at which a day begins and ends has been differently fixed, the reckoning being from sunrise to sunrise, from sunset to sunset, from noon to noon, or from midnight to midnight. The changes of the moon, which were observed to recur every 29 or 30 days, suggested the division into months, but the month now used, though nearly equal ti to a lunation, is really an arbitrary unit; and, as a still longer measure of time was found necessary for many purposes, it was supplied by the apparent yearly revolution of the sun round the earth, producing the changing sea sons. The time of this revolution is now known to be 365 days, 5 hours, 48 minutes and 46 seconds, but as it has at various times been reckoned differently, this has given rise to cor responding changes in the calendar. This unit of time is called a solar year. The division into weeks, which has been almost universally adopted, is not founded on any natural phe nomenon, and, as it originated in the East, has been attributed to the divine command to Moses in regard to the observation of the seventh day as a day of rest. By other author ities it has been ascribed to the number of the principal planets, a theory supported by the names given to the days. It was not used by the Greeks, nor by the Romans, till the time of Theodosius. The great influence of the sun's course upon the seasons naturally attracted the attention of men at all periods to this phe nomenon; accordingly all nations in any de gree civilized have adopted the year as the longest unit of time. The year of the ancient Egyptians was based on the changes of the seasons alone, without reference to the lunar month, and contained 365 days, which were divided into 12 months of 30 days each, with five supplementary days at the end of each year. 'TJewish year consisted of lunar
months, of which they reckoned 12 in the year, intercalating a 13th when necessary to main tain the correspondence of the particular months with the regular recurrence of the seasons. The Greeks in the earliest period also reckoned by lunar and intercalary months. They divided the month into three decades, a system also adopted long afterward at the time of the French Revolution. It possesses the ad vantage of making the smaller division an exact measure of the larger, and under it the num ber of a day in the 10-day period readily sug gests its number in the month. The Greeks in the time of Solon had a year of 12 months alternately of 29 and 30 days, the total num ber of days being 354, and the year being very nearly equal to a lunar one. Soon afterward a month of 30 days began to be intercalated every other year in order to reconcile their year with that founded on the sun's movement, but as the error was still very large the intercalary month was afterward omitted once in four times. The Jewish and also the Greek year thus both va ried in duration according as the intercalary month was introduced or omitted. This, with the uncertainty as to the exact duration of the year, was a constant source of confusion.
Various plans for the reformation of the calendar were proposed from time to time; but all proved insufficient till Meton and Eucte mon finally succeeded in bringing it to a much greater degree of accuracy by fixing on the period of 19 years, in which time the new moons return upon the same days of the year as before (as 19 solar years are very nearly equal to 235 lunations). (See Crct.z). This mode of computation, first adopted by the Greeks about 432 a.c., was so much approved of that it was engraven with golden letters on a tablet at Athens. Hence the number showing what year of the moon's cycle any given year is is called the golden number. This period of 19 years was found, however, to be about six hours too long. This defect Calippus, about 102 years later, endeavored to remedy, but still failed to make the beginning of the seasons re turn on the same fixed day of the year.