Quote:
Originally Posted by BadNewsRef
If that is the measurement of a second how could it have been scientifically defined prior to the 20th Century?
And why is that the measurement?
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1. Prior to the development of chronometers capable of measuring microseconds, the sidereal second was sufficiently precise.
2. It matches a sidereal second pretty closely. If you're asking why cesium 133, I don't know the answer to that one. Sorry!