Early timekeeping devices
Many ancient civilizations observed astronomical bodies, often
the Sun and Moon, to determine times, dates, and seasons.
Methods of sexagesimal timekeeping, now common in
Western society, first originated nearly 4,000 years ago in
Mesopotamia and Egypt; a similar system was developed later
in Mesoamerica. he first calendars may have been created
during the last glacial period, by hunter-gatherers who
employed tools such as sticks and bones to track the phases of
the moon or the seasons.Stone circles, such as England's Stonehenge, were built in various parts of the world, especially in Prehistoric Europe, and are thought to have been used to time and predict seasonal and annual events such as equinoxes or solstices. As those megalithic civilizations left no recorded history, little is known of their calendars or timekeeping methods.
3500 BC – 500 BC
Sundials have their origin in shadow clocks, which were the first devices used for measuring the parts of a day. 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.
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. 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. 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, 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. 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