Dating ancient events
This number, 2 × 12, or 24, was derived in Babylonia from the Sumerian sexagesimal method of reckoning, based on gradations of 60 (5 × 12 = 60) rather than on multiples of 10.
In Babylonia, for most purposes, both daylight and night were divided into three equal watches, and each watch was subdivided into half- and quarter-watches.
In either case the result was a year of 365 days, a period incompatible with the 29 ) mentions a wide variety: the cry of migrating cranes, which indicated a time for plowing and sowing; the time when snails climb up plants, after which digging in vineyards should cease; and so on.
An unwitting approximation to the tropical year may also be obtained by intercalation, using a simple lunar calendar and observations of animal behaviour.
Later the Babylonians, Jews, and Greeks counted a day from sunset to sunset, whereas the day was said to begin at dawn for the Hindus and Egyptians and at midnight for the Romans.
The Teutons counted nights, and from them the grouping of 14 days called a day was subdivided.
Most primitive tribes used a dawn-to-dawn reckoning, calling a succession of days so many dawns, or suns.
In West Africa some tribes used a four-day interval; in central Asia five days was customary; the Assyrians adopted five days and the Egyptians 10 days, whereas the Babylonians attached significance to the days of the lunation that were multiples of seven.
In ancient Rome, markets were held at eight-day intervals; because of the Roman method of inclusive numeration, the market day was denoted week may owe its origin partly to the four (approximately) seven-day phases of the Moon and partly to the Babylonian belief in the sacredness of the number seven, which was probably related to the seven planets.
The word is derived from the Latin chronology, since this is concerned with reckoning time by regular divisions, or periods, and using these to date events.
It is essential, too, for any civilization that needs to measure periods for agricultural, business, domestic, or other reasons.