ESHS/HSS Annual Meeting

The Scandal of Ephemeris Time: A New Astronomical Definition of the Second in the Age of Atomic Clocks, 1955-1967

• In Event: The Politics of Standardization

English Abstract

Although the first operational cesium atomic clock, completed in 1955, surpassed astronomical time measurements in precision, a new astronomical definition of the second, based on Ephemeris Time, was adopted in 1956 by the International Committee for Weights and Measures and ratified in 1960 by the General Conference on Weights and Measures as the SI unit of time. Physicists who championed atomic timekeeping were frustrated to see a new astronomical second adopted just as atomic standards had become available and proven superior, and later histories have often dismissed the ephemeris second as a “ridiculous” choice.

This paper revisits the controversy, arguing that the adoption of the ephemeris second was more nuanced than a simple opposition between astronomical and atomic time standards. I show that the ephemeris second was adopted not despite, but because of, the development of atomic clocks. Unlike the mean solar second, the ephemeris second could not be realized directly and required timekeeping devices more precise than available quartz clocks, such as atomic clocks. By following how this definition was negotiated across metrological laboratories, astronomical observatories, and international organizations, the paper situates the redefinition of the second within broader efforts to coordinate standards, expertise, and authority across national and institutional boundaries in the Cold War era.

Moving beyond a narrow reading of official reports, the paper combines a critical rereading of these documents with personal accounts, technical literature, and archival sources to reconstruct the ensuing debates over what made a good definition of the second: on one side, a stable definition grounded in celestial mechanics but difficult to realize; on the other, a definition anchored in a precise and perfectible technical standard, yet liable to continual revision as the technology evolved.

Author

• Ion Mihailescu, University of Neuchâtel