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.).