Timekeepng Devices

Sundials have their origin in shadow clocks, which were the first devices used for measuring the parts of a day.^[12] The oldest known shadow clock is from Egypt, and was made from green schist. Ancient Egyptian obelisks, constructed about 3500 BC, are also among the earliest shadow clocks.^[6][13][14]

The Luxor Obelisk in Place de la Concorde, Paris, France

                                                                                                                                                        Egyptian shadow clocks divided daytime into 10 parts, with an additional four "twilight hours"—two in the morning, and two in the evening. 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.^[6][15] 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.^[16]
Although accurate, shadow clocks relied on the sun, and so were useless at night and in cloudy weather.^[15][17] The Egyptians therefore developed a number of alternative timekeeping instruments, including water clocks, hourglasses, 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.^[18] 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.^[15][19][20] The ancient Egyptians are also believed to be the inventors of the hourglass, which consisted of two vertically aligned glass chambers connected by a small opening. When the hourglass was turned over, grains of sand fell at a constant rate from one chamber to the other.^[17] 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.