Does Anybody Really Know What Time It Is?
So, what, exactly, is the watch on your wrist, Big Ben in London, or the national atomic clock in Boulder, Colorado, actually measuring? The first definition of a second was 1/86,400 of the average solar day; in other words, a division of the average period of rotation of Earth on its axis relative to the Sun. This definition lasted until the mid-20th century, when the needs of international air and sea navigation and international communications required much more precise measurements of time. In 1956, the International Committee on Weights and Measures redefined the second to be 1/31,556,925.9747 of the length of the year 1900. This definition, known as the second of Ephemeris Time, was ratified by the General Conference on Weights and Measures in 1960. The definition was not to last very long, however, because of new developments in atomic physics.
In 1949, Harvard professor Norman Ramsey had developed a method of studying the structure of atoms by sending them through two oscillating electromagnetic fields. The procedure allowed a microwave oscillator to be synchronized with the unvarying atomic oscillations and could be used to measure the passage of time with great precision, thus providing the basis for the modern cesium atomic clock. In 1964, the International Committee on Weights and Measures acknowledged this new type of clock by provisionally defining the second based on the microwave frequency that drives the transition between two energy levels of a cesium-133 atom. In 1967, this definition became the sole definition of the second.
NASA Marshall Space Flight Center