calendar [Lat., from Kalends], system of reckoning time for the practical purpose of recording past events and calculating dates for future plans. The calendar is based on noting ordinary and easily observable natural events, the cycle of the sun through the seasons with equinox and solstice, and the recurrent phases of the moon.
Measures of Time
The earth completes its orbit about the sun in 365 days 5 hr 48 min 46 sec—the length of the solar year. The moon passes through its phases in about 291/2 days; therefore, 12 lunar months (called a lunar year) amount to more than 354 days 8 hr 48 min. The discrepancy between the years is inescapable, and one of the major problems since early days has been to reconcile and harmonize solar and lunar reckonings. Some peoples have simply recorded time by the lunar cycle, but, as skill in calculation developed, the prevailing calculations generally came to depend upon a combination.
The fact that months and years cannot be divided exactly by days and that the years cannot be easily divided into months has led to the device of intercalation (i.e., the insertion of extra days or months into a calendar to make it more accurate). The simplest form of this is shown in ancient calendars which have series of months alternating between 30 and 29 days, thus arriving at mean months of 291/2 days each. Similarly four years of about 3651/4 days each can be approximated by taking three years of 365 days and a fourth year of 366. This fourth year with its intercalary day is the leap year. If calculations are by the lunar cycle, the surplus of the solar over the lunar year (365 over 354) can be somewhat rectified by adding an intercalary month of 33 days every three years.
Reckoning of day and year was considered necessary by many ancient peoples to determine sacred days, to arrange plans for the future, and to keep some intelligible record of the past. There were, therefore, various efforts to reconcile the count in solar, lunar, and semilunar calendars, from the Egyptians and the Greeks to the Chinese and the Maya. The prevailing modern method of constructing a calendar in the Christian West came originally from the Egyptians, who worked out a formula for the solar year (12 months of 30 days each, five extra days a year, and an extra day every four years) that was to be adopted later by the Romans.
Development of the Modern Calendar
The Early Roman Calendar
In its most primitive form the Roman calendar apparently had 10 months, which were (to use corresponding English terms whenever possible): March (31 days), April (29 days), May (31 days), June (29 days), Quintilis (31 days), Sextilis (29 days), September (29 days), October (31 days), November (29 days), and December (29 days). To fill out the 365 days a number of blank days or occasional intercalary months were used. Later, January (29 days) and February (28 days) were added at the end of the year.
In the time of the early republic the so-called year of Numa was added. The Romans thus arrived at a cycle of four years: the first year and the third year had four months of 31 days, seven of 29, and one, February, of 28; the second year had a February of 23 days and an intercalary month of 27 days; the fourth year had a February of 24 days and an intercalary month. The chief trouble with this system was that in a four-year cycle there were four days too many. What was worse, the pontifex maximus was given the power soon after 200 BC to regulate the calendar, and the practice grew of using the intercalations for the promotion of political ends to lengthen or to shorten an official's term.
The Julian Calendar
When Julius Caesar became pontifex maximus, the Roman calendar had been so much abused that January was falling in autumn. At this point the methods of the Egyptian calendar were borrowed for the Roman. Julius Caesar, on the advice of the astronomer Sosigenes, added 90 days to the year 46 BC (67 days between November and December, 23 at the end of February). This caused the spring of 45 BC to begin in March. To retain this position of the seasons, he changed the length of most of the months: March, May, Quintilis (later named July after Julius Caesar), and October he left as they were; he added 2 days each to January and Sextilis (later named August to honor the Emperor Augustus); February was 28 days long except that in every fourth year a day was inserted between the 23d and the 24th of the month.
In Roman computation three days in the month were used for counting the date. These three were the Kalends (1st day of the month), the Nones (the 7th day in March, May, July, and October, the 5th in the other months), and the Ides (the 15th day in March, May, July, and October, the 13th in the other months). The days were counted before, not after, the Kalends, Nones, and Ides. Thus, Jan. 10 was the fourth day before the Ides of January or the fourth day of the Ides of January, because the Romans counted inclusively. Jan. 25 was the eighth of the Kalends of February, Feb. 3 was the third of the Nones of February. Feb. 23 was the seventh of the Kalends of March and remained so when an intercalary day was inserted every fourth year between it and Feb. 24; hence in a leap year there were two days counted as the sixth of the Kalends of March. The leap year was therefore called bissextile [Lat.,=sixth twice]. There is a legend that alterations in the length of the months were made later by Augustus to flatter his own vanity, but there seems to be no foundation for this story.
The Gregorian Calendar
The Julian year is 365 days 6 hr, hence a little too long. Therefore, by the 16th cent. the accumulation of surplus time had displaced the vernal equinox to Mar. 11 from Mar. 21, the date set in the 4th cent. In 1582 Pope Gregory XIII rectified this error. He suppressed 10 days in the year 1582 and ordained that thereafter the years ending in hundreds should not be leap years unless they were divisible by 400. The year 1600 was a leap year under both systems, but 1700, 1800, and 1900 were leap years only in the unreformed calendar. The reform was accepted, immediately in most Roman Catholic countries, more gradually in Protestant countries, and in the Eastern Church the Julian calendar was retained into the 20th cent. The present generally accepted calendar is therefore called Gregorian, though it is only a slight modification of the Julian.
The reform was not accepted in England and the British colonies in America until 1752. By that date the English calendar was 11 days different from that of continental Europe. For the intervening period before the reform was introduced into the English calendar, the Gregorian style is called the New Style (N.S.), and the Julian the Old Style (O.S.). New Style years begin Jan. 1, but Old Style years began usually Mar. 25. Thus Washington's birthday, which is Feb. 22, 1732 (N.S.), was Feb. 11, 1731 (O.S.). To avoid confusion sometimes both styles are given; thus 11 Feb. 1731/22 Feb. 1732.
The Christian Ecclesiastical Calendar
The church calendar with its movable feasts shows an interesting example of a harmony of several different systems. The key is the reconciliation of the seven-day week with the Roman calendar (see week). The resurrection of Jesus has always been traditionally reckoned as having taken place on a Sunday (first day of the week); hence the annual feast celebrating the event, Easter, should fall on a Sunday. The Bible places the Passion with relation to the Passover. Since the Jewish Passover is on the evening of the 14th (eve of the 15th) Nisan (see below), it may fall on any day of the week; hence Easter must fall on a Sunday near the 14th Nisan. In ancient times some Eastern Christians celebrated Easter on the 14th Nisan itself; these were called Quartodecimans [Lat.,=fourteenth]. In 325 the First Council of Nicaea determined that Easter should fall on the Sunday following the next full moon after the vernal equinox, the full moon being theoretically the 14th day, and Nisan beginning with a new moon in March. The vernal equinox was considered by the church to fall on Mar. 21. The paschal, or Easter, moon is the full moon, the 14th day of which falls after (but not on) Mar. 21.
Today Easter is calculated according to a system that does not take all factors of the lunar period into consideration, and it nearly always varies somewhat from what it should be according to true astronomical calculation. Several different systems have been used for determining Easter. In the 6th and 7th cent. in England, there was a great dispute between Christians who derived their rite from the Celts and Christians who had been converted as a result of the mission of St. Augustine. The dispute was settled at the Synod of Whitby in favor of the Roman system, which prevailed from that time over the entire West. For a conventional means of computing Easter, see the Anglican Book of Common Prayer.
The Jewish Calendar
The Jewish calendar is today a lunisolar or semilunar calendar, i.e., an adjustment of a lunar calendar to the solar year. The months are Tishri (30), Heshvan—sometimes also called Marheshvan—(29 or 30), Kislev (29 or 30), Tebet (29), Sebat or Shebat (30), Adar (29), Nisan (30), Iyar (29), Sivan (30), Tammuz (29), Ab (30), and Elul (29). The intercalary month of 30 days, Adar II, is added after Adar, Nisan being in ancient times the first month. The intercalation is arranged to take place seven times in 19 years; this is called the Metonic cycle after the Greek astronomer Meton who proposed it about 432 BC to express the relation between a lunar and solar year. The common year is referred to as a defective, regular, or perfect year, depending upon whether its length is 353, 354, or 355 days; the leap year may have 383 (defective), 384 (regular), or 385 (perfect) days. The Jewish civil year begins about the autumnal equinox, with the festival of Rosh ha-Shanah (the first of Tishri), which in 1999 fell on Sept. 11, marking the start of the Jewish year 5760.
The Islamic Calendar
The Islamic calendar is the only widely used purely lunar calendar, its year varying from 354 to 355 days. Hence the seasons and months have no connection, and there are about 33 years to every 32 Gregorian years. The months are Muharram (30), Safar (29), 1st Rabia (30), 2d Rabia (29), 1st Jumada (30), 2d Jumada (29), Rajab (30), Shaban (29), Ramadan (the fast, 30), Shawwal (29), Dhu-l-Kada (30), and Dhu-l-Hijja (month of the pilgrimage, 29 or 30). The first day of the Islamic calendar, Muharram 1, descr='[AH]' 1, was July 16, 622, in the Western calendar (descr='[AH]' [Anno Hegirae=in the year of the Hegira] is used to indicate the Islamic year). Muharram 1, descr='[AH]' 1420 was Apr. 17, 1999.
The old Chinese calendar was devised to have six 60-day cycles, each cycle having 10-day periods and three such periods going to make up a month. By the 5th cent. BC the solar year was calculated at 365.2444 solar days and the solar month at 29.53059 days. The difference between solar time and the cycles was adjusted by intercalary months and shorter intercalary periods. The years were arranged in major cycles of 60 years with minor cycles of 5 years each. An interesting calendar is that of the Maya, who used a year of 365 days divided into 18 20-day periods, with a 5-day period at the end. A cycle of 260 days was used to name days. These two recurrent cycles resulted in a great cycle of 52 years. This calendar was carefully calibrated, but the year was never readjusted to the error in its length; instead, the feasts and dates were adjusted to the calendar. The Aztec calendar was very similar. Many attempts have been made to devise new calendars, adjusting the months more regularly to the solar year, discarding the week, making the months equal in length, and the like, but they have never been widely adopted. The most celebrated is the French Revolutionary calendar.
Reckoning the Dates Assigned to Years
The Athenian system of identifying years by archons, the Roman system of identifying them by consuls, and the system of reckoning by the year of the reign of a given king or other ruler offer enormous difficulties, and the establishment of chronology is one of the major problems in ancient and medieval history. (The classic work on chronology is that of the Benedictines, first published in 1750, L'Art de vérifier les dates des faits historiques [the art of verifying the dates of historical acts].) For the method of computing years from a fixed point (e.g., the birth of Jesus and the Hegira), see era. The adoption of such era systems has made computation of time much easier.
See P. W. Wilson, The Romance of the Calendar (1937); H. Watkins, Time Counts: The Story of the Calendar (1954); K. G. Irwin, The Three Hundred Sixty-Five Days (1963); J. E. S. Thompson, Mayan Hieroglyphic Writing (3d ed. 1971); F. Parise, ed., The Book of Calendars (1982).