The Basics


What was the first clock?

For thousands of years the second, and all other units used to measure time, were based on the rotation of Earth. The first method of measuring time shorter than a day dates back to 3500 B.C.E., when a device known as the gnomon was used. The gnomon was a stick placed vertically into the ground which, when struck by the sun’s light, produced a distinct shadow. By measuring the relative positions of the shadow throughout the day, the length of a day was able to be measured. The gnomon was later replaced by the first hemispherical sundial in the third century B.C.E. by the astronomer Berossus (born about 340 B.C.E.).


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