ABSTRACT
A physical second? Accompanying the announcement in Nature of the success of the NPL caesium standard was a note by Sir Edward Bullard, the Director of NPL. He pointed out that observations of the Moon over a period of four years would be necessary to determine Ephemeris Time to the accuracy achieved by the NPL atomic clock in a matter of minutes. “The natural way of escape from this difficulty,” he wrote, “is to define a ‘physical second’ in terms of the natural period of the caesium atom, choosing the numerical value so that it agrees as well as may with the current estimates of the second of Ephemeris Time.” Bullard chose his words carefully, for he intended “physical second” to be understood as being in distinction to “astronomical second”. He was articulating what many physicists had been thinking, and no doubt some astronomers too, that the time was near when timekeeping would be taken out of the hands of astronomers and handed over to physicists.
