<P> An Egyptian device that dates to c. 1500 BC, similar in shape to a bent T - square, measured the passage of time from the shadow cast by its crossbar on a nonlinear rule . The T was oriented eastward in the mornings . At noon, the device was turned around so that it could cast its shadow in the evening direction . </P> <P> A sundial uses a gnomon to cast a shadow on a set of markings calibrated to the hour . The position of the shadow marks the hour in local time . The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system . The importance of the number 12 is due the number of lunar cycles in a year and the number of stars used to count the passage of night . </P> <P> The most precise timekeeping device of the ancient world was the water clock, or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I (1525--1504 BC). They could be used to measure the hours even at night, but required manual upkeep to replenish the flow of water . The Ancient Greeks and the people from Chaldea (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations . Arab inventors and engineers in particular made improvements on the use of water clocks up to the Middle Ages . In the 11th century, Chinese inventors and engineers invented the first mechanical clocks driven by an escapement mechanism . </P> <P> The hourglass uses the flow of sand to measure the flow of time . They were used in navigation . Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe (1522). Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe . Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages . Richard of Wallingford (1292--1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330 . Great advances in accurate time - keeping were made by Galileo Galilei and especially Christiaan Huygens with the invention of pendulum driven clocks along with the invention of the minute hand by Jost Burgi . </P>

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