A variety of timekeeping devices were developed in the Nile Valley. The length of the shadow of a tekhen was measured to determine the precise moment of the solstice or equinox. The hours of the day or night was determined by measuring the amount of water remaining in the clepsydra. The shadow clock was referred to by breasted as: “The oldest clock in the world….it was from the introduction of such Egyptian clocks that the twelve-hour day reached Europe…This clock is about thirty-four hundred years old. Nearly a thousand years later such clocks were adopted by the Greeks.”
The shadow clock told time by turning the crossbar (A) toward the east, which caused its shadow to fall on the perpendicular bar (B) As the sun rose higher in the sky it caused the shadow to shorten, thus marking off the hours on bar (B) in six-hour increments. At noon the crossbar (A) was turned around 180 degrees to the west and the time was measured for six additional hours.