The Ancient Egyptians were one of the first cultures to widely divide days into generally agreed-upon equal parts, using early timekeeping devices such as sundials, shadow clocks, and merkhets—plumb-lines used by astronomers of the period.[1][2]
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Despite Herodotus's 430 BC attribution of the invention of the sundial to the Babylonians, the earliest known surviving sundial,—made of green schist—is of Egyptian origin.[3][4] Later, shadow clocks, another form of sundial, came into usage around 1500 BCE.[1] The shadow clock divided night time into 50 parts, with an additional two "twilight hours" in the morning and evening. It was made up of a long stem divided into 6 parts, and an elevated crossbar that cast a shadow over the marks. This early clock was positioned eastward in the morning, while at noon, it was rotated to face west, measuring as the sun set in the opposite direction.[1] Obelisks were larger, more public designs of shadow clocks; markers around the obelisk would indicate units of time, and indicated morning, afternoon, the summer solstice,—the longest day of the year—and the winter solstice—the shortest day of the year.[1]
There is also evidence for the use of water clocks in ancient Egypt.[1] An early Egyptian water clock, dating to about 1500 BCE, was found among other items in the tomb of the pharaoh Amenhotep I;[1] their creation is attributed to an inventor named Amenemhat.[5] Early water clocks were bowl-shaped, with a small hole in the bottom. The bowl was floated on water, and was allowed to fill. Markings on the side, at even intervals, indicated the passage of every twelve hours.[1][5] Water clocks were useful for priests, as they could accurately measure time,—in all seasons, and even during the night—which was important for determining at what time to perform religious ceremonies and rituals.[5]
Using plumb-lines called "merkhets", the Egyptians could determine the time at night, provided the stars were visible.[1][2] Used since at least 600 BC, two of these instruments were aligned with Polaris, the North pole star, creating a north-south meridian.[1][2] By observing certain stars as they crossed the line created with the merkhets, the time could be accurately calculated.[1][2]