<P> Egyptian shadow clocks divided daytime into 12 parts with each part further divided into more precise parts . One type of shadow clock consisted of a long stem with five variable marks and an elevated crossbar which cast a shadow over those marks . It was positioned eastward in the morning, and was turned west at noon . Obelisks functioned in much the same manner: the shadow cast on the markers around it allowed the Egyptians to calculate the time . The obelisk also indicated whether it was morning or afternoon, as well as the summer and winter solstices . A third shadow clock, developed c. 1500 BC, was similar in shape to a bent T - square . It measured the passage of time by the shadow cast by its crossbar on a non-linear rule . The T was oriented eastward in the mornings, and turned around at noon, so that it could cast its shadow in the opposite direction . </P> <P> Although accurate, shadow clocks relied on the sun, and so were useless at night and in cloudy weather . The Egyptians therefore developed a number of alternative timekeeping instruments, including water clocks, and a system for tracking star movements . The oldest description of a water clock is from the tomb inscription of the 16th - century BC Egyptian court official Amenemhet, identifying him as its inventor . There were several types of water clocks, some more elaborate than others . One type consisted of a bowl with small holes in its bottom, which was floated on water and allowed to fill at a near - constant rate; markings on the side of the bowl indicated elapsed time, as the surface of the water reached them . The oldest - known waterclock was found in the tomb of pharaoh Amenhotep I (1525--1504 BC), suggesting that they were first used in ancient Egypt . Another Egyptian method of determining the time during the night was using plumb - lines called merkhets . In use since at least 600 BC, two of these instruments were aligned with Polaris, the north pole star, to create a north--south meridian . The time was accurately measured by observing certain stars as they crossed the line created with the merkhets . </P> <P> Water clocks, or clepsydrae, were commonly used in Ancient Greece following their introduction by Plato, who also invented a water - based alarm clock . One account of Plato's alarm clock describes it as depending on the nightly overflow of a vessel containing lead balls, which floated in a columnar vat . The vat held a steadily increasing amount of water, supplied by a cistern . By morning, the vessel would have floated high enough to tip over, causing the lead balls to cascade onto a copper platter . The resultant clangor would then awaken Plato's students at the Academy . Another possibility is that it comprised two jars, connected by a siphon . Water emptied until it reached the siphon, which transported the water to the other jar . There, the rising water would force air through a whistle, sounding an alarm . The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations . </P> <P> Greek astronomer, Andronicus of Cyrrhus, supervised the construction of the Tower of the Winds in Athens in the 1st century BC . </P>

Who knew what time it was when the first clock was invented