Atomic Standards of Frequency and the Second of Ephemeris Time

Abstract

ON the authorization of the twelfth General Conference, the International Committee of Weights and Measures assigned, in October 1964 (ref. 1), the frequency 9 192 631 770 hertz to the well-known transition of caesium-133 at zero field, for temporary use pending consideration of a new definition of the second; the second, as at present defined, is the second of ephemeris time. The number 9 192 631 770 with a quoted probable error of ± 20 is that originally determined by Markowitz et al.² by the comparison, during the period 1954.0 to 1958.5, of an integrated measure of atomic time based on the frequency of the caesium transition with the measure of ephemeris time (E.T.) as determined by observations of the Moon with moon-cameras. Subsequent comparisons over longer periods have tended to confirm that this frequency lies within the uncertainty quoted.