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Book Review
Nature 410, 1027 (26 April 2001) | doi:10.1038/35074156
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An instant in time
Daniel Kleppner1
By the mid-1950s astronomical timekeeping had achieved unprecedented accuracy, based on a deepened understanding of planetary and terrestrial dynamics and the development of the quartz crystal clock. In 1952 the International Astronomical Union introduced a new timescale — ephemeris time, based on the motions of the planets — and in 1956 it redefined the second in terms of the length of a solar year.
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