Back when humans began to track the passage of time thousands of years ago, they did it by watching the apparent movement of the sun across the sky — which actually was due to the Earth’s rotation — and basing units of time on that journey. Traditionally, for example, a second was defined as 1/86,400 of the average length of a solar day.
But with the advent of atomic clocks, which were far more reliable than the motion of the Earth itself, it became necessary to change that standard. In 1967, the second was redefined as the time that it took for an atom of the isotope cesium 133 to oscillate 9,192,631,770 cycles [source: Sciencemuseum.org.uk].
Electrons orbit the center of an atom, which is called the nucleus. Imagine an extremely tiny version of our solar system, with planets revolving around the sun, and you’ll get the general idea. Physicists have discovered that electrons are amazingly regular in their movements — they tend to remain within a narrow range of orbits, with the distance from the nucleus determined by how much radiation they’re emitting at a given moment. The distance between the lowest orbit and the highest orbit that an electron moves in is the frequency.
In the case of cesium, which is used in atomic clocks, scientists focus on just one of the element’s 55 electrons — the outermost one, which occupies an orbit that’s conspicuously higher than the rest. The difference in energy between the outermost electron’s closest orbit to the nucleus and its farthest orbit corresponds to a radio frequency of 9,192,631,770 cycles. That’s the part that scientists actually use to calculate time and break it into incredibly brief units of less than a billionth of a second [source:Sciencemuseum.org.uk].