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A Historical Review of U. S. Contributions to the Atomic Definition of the SI Second



Michael A. Lombardi


Before atomic clocks, the second was defined by dividing astronomical events, such as the solar day or the tropical year, into smaller parts. This permanently changed in 1967, when the SI second was redefined as the duration of 9 192 631 770 energy transitions of the cesium atom. The new definition meant that seconds were now measured by counting oscillations of atoms, and minutes and hours were now multiples of the second rather than divisions of the day. The benefits of atomic timekeeping to modern society have been immense and many technologies that we now take for granted, such as global navigation satellite systems (GNSS), mobile telephones, and the "smart grids" that provide our electric power, depend upon atomic clock accuracy. The current year (2017) marks the 50th anniversary of the first definition of the International System of Units (SI) second that was based on atomic transitions. In recognition of this anniversary, this paper provides a historical review of work conducted in the United States of America that contributed to the atomic definition of the SI second a half century ago.
Journal of Research (NIST JRES) -


atomic clock, cesium, second, time, UTC


Lombardi, M. (2017), A Historical Review of U. S. Contributions to the Atomic Definition of the SI Second, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed April 21, 2024)
Created June 1, 2017, Updated November 10, 2018