The clock is one of the most important discoveries in the history of western science. The division of time into regular, predictable units is essential to the operation of society. Even in ancient times, humanity accepted the need of a methodical system of chronology.
The first docks to be used were based on the daily motion of the sun across the heavens, the waxing and waning of the moon, and the yearly cycle of the sun around the zodiac; these provided us with standards of the day, month, and year. Others composed variations on this theme. The Indian astronomers developed a scheme whereby the passage of the moon against its background of stars was used for timekeeping. The moon cycles through its phases in the synodic period of 29.53 days; they defined a unit of time, the tithi, as 1/30th of this (or a little less than a day). Both the tithi and the movement of the sun in the zodiac have been used by Indians for calendrical purposes.
The most obvious natural cyclic event is the alternation of day and night, and this provides the primary unit of time of most calendars. It corresponds to the period of rotation of the earth about its axis or, from our human viewpoint, the time taken by the sun to return to the same place after completing its journey across the sky. The passage of the days is marked, except at certain times of the year in the Polar Regions, by the alternation of night and day, and these provide the ticks of this celestial clock. (Neugebauer, O. 1949: v.40 pp.240-255).
The first problem in using the day is to decide when it begins and ends its phase so that events may be assigned to a day without ambiguity.................
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