Today’s scientific research requires super accurate clocks. The best mechanical clocks are inadequate because they might gain or lose at least four seconds a year.

Modern quartz watches are more reliable. They depend on the unvarying frequency of vibration of a quartz crystal when an electric current is applied. But there is still a mechanical element to quartz crystal clocks. The frequency of vibration depends on the crystal’s size and shape, and no two crystals are precisely alike. Quartz clocks are accurate to about one second in ten years.

The most accurate clocks these days are atomic clocks, which have an accuracy exceeding one second in a thousand years. Atomic clocks keep time by counting the vibrations of atoms. This property is unaffected by outside conditions because every chemical element has a distinct and inherently stable characteristic frequency of vibration.

The cesium atomic clock, for example, has over nine billion vibrations a second. Even more precise are atomic clocks which use hydrogen. These are accurate to about one second in nearly two million years. But still scientists are not satisfied! They are working on a mercury atomic clock, which they hope will be accurate to one second in two billion years.

So next time you check your watch, remember it only tells you the approximate time!