Time

We measure time, in general, with a clock. However, not all clocks are based on the same time scale. Measurement systems and types of time differ widely. We have just seen that clocks in different places are calibrated to different schemes, and so are likely to tell different times.

The first clock was natural—the motion of the Sun through the sky. This led to the invention of the sundial to measure time, as well as the evening version using the Moon's position. Later came the hourglass, candles, and clocks which burned wax, or oil, at a specified rate, and water clocks, which allowed water to flow at a specified rate. Early clocks of greater accuracy used a pendulum arrangement invented in the 1656 by the Dutch astronomer Christiaan Huygens (1629-1695). Balance wheels then replaced pendulums in some clocks, which allowed clocks to become portable. In recent times, coupled A sundial in Death Valley, California. Sundials were the first devices used to measure time. JLM Visuals. Reproduced by permission. pendulum clocks were used to keep extremely accurate times.

Most of today's clocks and watches use a quartz crystal to keep the time. The specially manufactured quartz crystal, with a specific frequency voltage passing through it, will vibrate at a constant (characteristic) frequency. This frequency, when amplified, drives an electric motor which makes the hands of the clock turn, or makes the digital display change accordingly.

Atomic clocks are the most accurate clocks, and are used by the United States Naval Observatory. They vibrate at a constant sustainable frequency. The process is analogous to setting a pendulum in motion, or a quartz crystal vibrating, except the atoms "vibrate" between two different energies. This "vibration" between energy levels is associated with a specific frequency, just like the oscillating quartz crystal. This atomic frequency is what drives the clock. The current standard for atomic clocks is one that uses an isotope of Cesium, ¹³³Cs. In 1967, the second was redefined to allow for this accuracy. The second is now defined as 9,192,631,770 times one period, the time for one complete oscillation, of ¹³³Cs.

Research continues into ion clocks such as the hydrogen maserclock. This type of clock is more accurate than a ¹³³Cs clock; however, after several days its accuracy drops to, or below, that of a ¹³³Cs clock.