calendar, System for dividing time over extended periods, such as days, months, or years, and arranging these divisions in a definite order. A calendar is essential for the study of chronology, which reckons time by regular divisions, or periods, and uses these to date events. It is also vital for any civilization that needs to measure periods for agricultural, business, domestic, or other reasons. The lunation, or period in which the moon completes a cycle of its phases (291/2 days), is the basis for the month; most ancient calendars were collections of months. Days and seasons, which are a solar phenomena, do not have periods that evenly divide, so ancient calendars employed various means, such as the periodic insertion of an intercalary month, to reconcile the months with the seasons. The Gregorian calendar used almost universally today is a modification of the Julian calendar adopted by Julius Caesar, which used a 3651/4–day year with 12 months that came to have the number of days we know today. See also calendar, Jewish; calendar, Muslim; sidereal period.