calendar
calendar Way of reckoning time for regulating religious, commercial and civil life, and for dating events in the past and future. Ancient Egyptians had a system based on the movement of the star Sirius and on the seasons. Calendars are based on natural and astronomical regularities: tides and seasons, movements of the Sun and Earth, and phases of the Moon. The basic units are day, month and year. The main difficulty in compiling a calendar is that the month is not an exact number of days and the year not an exact number of months. For convenience, months and days are assigned a whole number of days, and extra days (intercalations) are added at intervals to compensate. In the modern Gregorian, or New Style, calendar, an extra day (February 29) is added every four years (leap year). The Gregorian calendar was based on the Julian, or Old Style, solar calendar. This was introduced by Julius Caesar in the 1st century bc and was developed from an earlier Moon-based calendar. The Jewish calendar is semilunar, made up of 12 common years and 7 leap years. The leap years have an additional month (Adar II). The Muslim calendar is wholly lunar, the year always consisting of 12 months without intercalations. The months have alternately 30 and 29 days, except for the twelfth month (Dulhajj) which has one intercalary day in 11 years out of a cycle of 30 calendar years.
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calendar