For the components of the definition including its retrospective aspect, see G M Clemence (1948), esp. p. 172, and 'ESAE 1961': 'Explanatory Supplement (1961), esp. pages 69 and 87. G M Clemence, "On the System of Astronomical Constants", Astronomical Journal, vol. 53(6) (1948), issue #1170, pp. 169–179. 'ESAE 1961': "Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac" ('prepared jointly by the Nautical Almanac Offices of the United Kingdom and the United States of America', HMSO, London, 1961).
At the IAU meeting in Rome 1952: see ESAE (1961) at sect.1C, p. 9; also Clemence (1971). 'ESAE 1961': "Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac" ('prepared jointly by the Nautical Almanac Offices of the United Kingdom and the United States of America', HMSO, London, 1961). G M Clemence (1971), "The Concept of Ephemeris Time", Journal for the History of Astronomy, vol. 2 (1971), pp. 73–79.
W G Melbourne & others, 1968, section II.E.4-5, pages 15—16, including footnote 7, noted that the Jet Propulsion Laboratory spacecraft tracking and data reduction programs of that time (including the Single Precision Orbit Determination Program) used, as ET, the current US atomic clock time A.1 offset by 32.25 seconds. The discussion also noted that the usage was "inaccurate" (the quantity indicated was not identical with any of the other realizations of ET such as ET0, ET1), and that while A.1 gave "certainly a closer approximation to uniform time than ET1" there were no grounds for considering either the atomic clocks or any other measures of ET as (perfectly) uniform. Section II.F, pages 18—19, indicates that an improved time measure of (A.1 + 32.15 seconds), applied in the JPL Double Precision Orbit Determination Program, was also designated ET. W G Melbourne, J D Mulholland, W L Sjogren, F M Sturms (1968), "Constants and Related Information for Astrodynamic Calculations", NASA Technical Report 32-1306, Jet Propulsion Laboratory, July 15, 1968.