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Event

13th General Conference on Weights and Measures

A formal meeting room is depicted with attendees seated around a long table, facing a speaker at the head, under dramatic lighting and a large clock on the wall, symbolizing precision and timekeeping.

The 1967 meeting in Paris at which the world's metrological authorities replaced the old astronomical definition of the second — a fraction of the mean solar day — with an atomic one. Henceforth one second was, by decree, 9,192,631,770 oscillations of the caesium-133 hyperfine transition. It was the first SI unit to be defined entirely in terms of an atomic constant.

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