Quote:
Originally Posted by mbyron
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!
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2 is easy. cesium 132 wasn't quite long enough, and cesium 134 was too long.