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Best 30 Second Hand Stores in Kirkwood, MO with Reviews Starting in the 1950s, atomic clocks became better timekeepers than Earth's rotation, and they continue to set the standard today. In science, a second is the time it takes for a caesium atom to vibrate 9,192,631,770 times. Scientists measure the second this way because the length of a day changes all the time. For example, when the dinosaurs lived, a day was about an hour shorter. Vibrations of atoms on the other hand always take the same time. The Earth's motion was described in Newcomb's Tables of the Sun , which provided a formula for estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892. Metric prefixes are frequently combined with the word second to denote subdivisions of the second, e.g., the millisecond and nanosecond . Though SI prefixes may also be used to form multiples of the second (such as “kilosecond”, or one thousand seconds), such u