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second unit of time

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.

A consistent method of sending signals must be developed before the second is redefined, such as fiber-optics. The definition of a unit refers to an idealized situation that can be reached in the practical realization with some uncertainty only. In 1656, Dutch scientist Christiaan Huygens invented the first pendulum clock. It had a pendulum length of just under a meter which gave it a swing of one second, and an escapement that ticked every second. It was the first clock that could accurately keep time in seconds. By the 1730s, 80 years later, John Harrison's maritime chronometers could keep time accurate to within one second in 100 days.

Pertaining to the gear transmission ratio at which drive shaft speed is greater than that second of low gear but not so great as that of other gears for a given engine crankshaft speed.

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The basic unit of time in the International System of Units , equal to the duration of 9,192,631,770 cycles of radiation in a transition, or energy level change, of the cesium atom. Sundials and water clocks were among the earliest timekeeping devices, and units of time were measured in degrees of arc. Conceptual units of time smaller than realisable on sundials were also used. A mechanical clock, one which does not depend on measuring the relative rotational position of the Earth, keeps uniform time called mean time, within whatever accuracy is intrinsic to it. That means that every second, minute and every other division of time counted by the clock will be the same duration as any other identical division of time.