Calendar

The prominence of the Moon in premodern societies with limited lighting and the regularity of its phases resulted in the synodical lunar month being basic to many traditional calendars. In the Muslim calendar, twelve lunar months of twenty-nine or thirty days are reckoned in a year of 354 days, or a leap year of 355, in a thirty-year cycle. The approximately eleven-day difference between such a synodical lunar calendar and the solar year, however, results in a slippage of months through the seasons. In order to maintain synchrony between lunar months and the seasons, an intercalated month is necessary, a strategy employed in the Jewish calendar with influence from Babylonia: seven leap years intersperse with twelve common years in a nineteen-year cycle.

Ancient Egyptians used the annual heliacal rise (predawn reappearance) of Sirius to help coordinate their lunar months with the seasons and solar year. While maintaining this system for religious observance, they later developed a civil year of 365 days comprising twelve fixed months of thirty days with five additional days. The ancient Maya had a similar five-day end-of-year, but divided the other 360 days into eighteen named periods of twenty days. This yearly calendar intermeshed every fifty-two years with a divinatory cycle of 260 days. Maintaining separate calendars for civic and religious (and/or regional and ethnic) functions is a common practice, useful in the twenty-first century for the retention of local traditions amidst the spread of the Gregorian calendar.