What is a clock — what takes it to tell time. Clocks don’t measure time; they measure the motion of something, and the measurement is called ‘time’. Pretty much any process of change that we adopt as a standard can be a clock. But some clocks are more useful than others.
The name ‘clock’ is reserved for objects whose timekeeping ability is enhanced by periodicity: some mechanism that spaces out the intervals between the moments when processes recur.
Some examples are: the swings of a pendulum, repeated sunrises, cycles of a shadow on a sundial, bouncing mechanical springs, vibrations of a quartz crystal, radioactive decay that occurs at a predictable rate, and repeated reflections of a photon between relatively stationary mirrors.
A good clock doesn’t just change. It ticks and time is measured by counting; discrete steps to divide up the infinity of time. We can count how many beats for each oscillation; how many oscillations for every tick of a stopwatch; how many ticks of a stopwatch between intervals of the clock on the bell-tower, and so on. The more regular the ticks, the more accurate the clock.
- The standard clock of Coordinated Universal Time (U.T.C. time) adopted worldwide in 1964 is an atomic clock that measures the number of photons emitted from a cesium-133 atom's orbital electron. This caesium clock is very regular and accurate within one second in 100 million years. It can easily be copied and constructed elsewhere.
- The standard clock is the basis for the S.I. second (Système International second) which in 1967 was defined to be the numerical measure of the time it takes for a motionless (relative to the Greenwich observatory), designated, standard caesium atomic clock to emit exactly 9,192,631,770 cycles of radiation from caesium-133 atoms during their transition between the two hyperfine levels of the ground state of the atom.