calendar

calendar A scheme for organizing periods of time—days, weeks, months, years—and numbering or naming them. Many different calendars are in use, but only the Gregorian calendar has world-wide recognition. Calendars usually attempt to recognize astronomical cycles, notably the phases of the Moon and the length of the year, but they do not necessarily closely follow them. For example, the months in the Gregorian calendar have no connection with the actual phases of the Moon and are no more than arbitrary subdivisions of the year. The Gregorian calendar is a solar calendar designed to relate the dates as precisely as possible to the annual seasonal cycle. The year normally has 365 days, but periodically an additional day is inserted (or intercalated) to maintain the correspondence. Thus the vernal equinox will always occur on or within one day of March21.

 The Jewish calendar, on the other hand, can be described as lunisolar. Its months are tied closely to the lunar cycle and are either 29 or 30 days in length. There are normally 12 such months in a year, but so that close correspondence with the seasons is not quickly lost, an additional month is intercalated in a leap year. The Jewish calendar is based on the 19-year Metonic cycle, which contains 235 months. Seven of the years of this cycle are therefore leap years.

 The Islamic calendar is wholly lunar and the year always consists of 12 lunar months. Odd-numbered months have 30 days and even-numbered ones 29 days, so that the normal year has 354 days. Hence the Islamic year regresses through the seasons in a period of about 33 years, but this period has no significance in the Islamic calendar. There is, however, a calendrical sequence of 30 years, eleven of which are made leap years by adding one extra day to the first month. A very close correspondence is thus maintained with the astronomical lunar cycle, accurate to one day in 30 000 years. See also julian calendar.