The atomic second is the precise definition of the modern unit of time known as the second, measured as the amount of time it takes an atom of cesium to vibrate, or shift between two ground states, exactly 9,192,631,770 times.

– The History of the Second –

The decision to divide the day into twenty-four hours, 1440 minutes, or 86,400 seconds was arbitrary: we could just as easily have settled on other numbers. Our current way of dividing the day into hours and then minutes originated in ancient Egypt and Babylon.

Seconds, however, were pretty much irrelevant until the invention of mechanical clocks in the 16th century, at which time they finally could be measured reliably. At the time, it seemed relatively easy to define seconds: there were 86,400 of them in every twenty-four hour day.

– The Tidal Problem –

Twentieth-century science’s drive to develop objective, standardized systems of measurement discovered two problems with the second. (Beyond the most obvious problem, which is that the second, unlike all other metric measures, was never redefined for mathematical simplicity, as were metres and grams). First, the Earth day is not actually 24 hours, and therefore not precisely 86,400 seconds long. This problem was resolved with another method devised during the Renaissance period, the leap year, which tacks on a day every four years to make up for the time inaccuracy.

The second and more serious problem was the discovery that the Earth is not always rotating at precisely the same speed. The difference is not noticeable over short periods of time, but, very gradually, the tidal relationship with the Moon is altering both bodies’ rotation speeds. In Earth’s case, the planet rotates as much as a few thousandths of a second slower each year.

This extremely slow tidal deceleration does not cause a large enough change in the length of the day for us to observe it in a short period of time without the aid of extremely sensitive clocks. (On the other hand, it does mean that the Earth’s day was a few hours shorter, billions of years ago.) This, however, was precisely the problem. Modern scientists hoping for a standardized, objective measurement of time found that the rotation of the Earth was not sufficiently objective or unchanging, because of the gradual deceleration. A few thousandths of a second might not matter when checking a bus schedule, but it does become significant in the realm of advanced nuclear physics.

– Modern Scientific Redefinition –

The solution was to find a more constant physical property which was precisely equal to the Earth-day second, when this issue became a problem during the 1950s. The answer was provided by chemistry: the rate of vibration of cesium atoms, which, at the time, vibrated exactly 9,192,631,770 times in the space of an Earth-second. As a result, the metric system switched definitions. Now, technically, the second is based on the cesium vibration frequency rather than the rotation of the Earth.

Five decades later, the Earth has slowed enough that the difference can be detected with advanced scientific instruments, and so that, if we still defined the second as 1/86,400th of an Earth day, it would be equal to more than 9,192,631,770 vibrations of a cesium atom. This illustrates the inherent, ironic long-term problem with the SI metric definition of the atomic second: if human beings are still using metric atomic seconds in thousands or tens of thousands of years, by then 24 hours will be noticeably shorter than an actual day.