Hello, > > I was at high school in the 70's. At that time the definition of a > second wasn't linked to the earth's rotation at all. > I still have a list of SI units somewhere, but I can't find find it > right now. However, I remember it having something to do with caesium > and a number of periods.
Yes, that is true. But the numerical value -- the number of periods that constitute a "cesium" second was defined in the 1970s so that it would match the previous astronomical definition of the length of the second. The problem was that the length of the astronomical second that was used in this definition was based on data that was almost a century old, and the length of the day (and therefore the length of the astronomical second) had increased during that century. As a result, the actual astronomical second in the 70s was already longer than the assumed value that was used to define the number of periods of the cesium frequency that would equal an "atomic" second. Therefore, leap seconds were needed immediately, and they are also of the same sign for this reason. Judah Levine Time and Frequency Division NIST Boulder _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
