Terrestrial time Terrestrial Time is a theoretical uniform time scale, defined to provide continuity with the former Ephemeris Time (ET). ET was an independent time-variable, proposed (and its adoption agreed) in the period 1948–1952Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac, Nautical Almanac Offices of UK and US (1961), at pp. 9 and 71. with the intent of forming a gravitationally uniform time scale as far as was feasible at that time, and depending for its definition on Simon Newcomb's Tables of the Sun (1895), interpreted in a new way to accommodate certain observed discrepancies.See G M Clemence's proposal of 1948, contained in his paper: "On the System of Astronomical Constants", Astronomical Journal (1948) vol.53 (6), issue #1170, pp 169–179; also G M Clemence (1971), "The Concept of Ephemeris Time", in Journal for the History of Astronomy v2 (1971), pp. 73–79 (giving details of the genesis and adoption of the ephemeris time proposal); also article Ephemeris time and references therein. Newcomb's tables formed the basis of all astronomical ephemerides of the Sun from 1900 through 1983: they were originally expressed (and published) in terms of Greenwich Mean Time and the mean solar day,Newcomb's Tables of the Sun (Washington, 1895), Introduction, I. Basis of the Tables, pp. 9 and 20, citing time units of Greenwich Mean Noon, Greenwich Mean Time, and mean solar dayW de Sitter, on p. 38 of Bulletin of the Astronomical Institutes of the Netherlands, v4 (1927), pp.21–38, "On the secular accelerations and the fluctuations of the moon, the sun, Mercury and Venus", which refers to "the 'astronomical time', given by the earth's rotation, and used in all practical astronomical computations", and states that it "differs from the 'uniform' or 'Newtonian' time". but later, in respect of the period 1960–1983, they were treated as expressed in terms of ET,See p. 612 in Explanatory Supplement to the Astronomical Almanac, ed. P K Seidelmann, 1992, confirming introduction of ET in the 1960 edition of the ephemerides. in accordance with the adopted ET proposal of 1948–52. ET, in turn, can now be seen (in light of modern results)See especially F R Stephenson (1997), and Stephenson & Morrison (1995), book and papers cited below. as close to the average mean solar time between 1750 and 1890 (centered on 1820), because that was the period during which the observations on which Newcomb's tables were based were performed. While TT is strictly uniform (being based on the SI second, every second is the same as every other second), it is in practice realised by International Atomic Time (TAI) with an accuracy of about 1 part in 1014.
Terrestrial time Terrestrial Time is a theoretical uniform time scale defined to provide continuity with the former Ephemeris Time (ET) ET was an independent time-variable proposed (and its adoption agreed) in the period 1948–1952Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac Nautical Almanac Offices of UK and US (1961) at pp 9 and 71 with the intent of forming a gravitationally uniform time scale as far as was feasible at that time and depending for its definition on Simon Newcomb's Tables of the Sun (1895) interpreted in a new way to accommodate certain observed discrepanciesSee G M Clemence's proposal of 1948 contained in his paper "On the System of Astronomical Constants" Astronomical Journal (1948) vol53 (6) issue #1170 pp 169–179; also G M Clemence (1971) "The Concept of Ephemeris Time" in Journal for the History of Astronomy v2 (1971) pp 73–79 (giving details of the genesis and adoption of the ephemeris time proposal); also article Ephemeris time and references therein Newcomb's tables formed the basis of all astronomical ephemerides of the Sun from 1900 through 1983 they were originally expressed (and published) in terms of Greenwich Mean Time and the mean solar dayNewcomb's Tables of the Sun (Washington 1895) Introduction I Basis of the Tables pp 9 and 20 citing time units of Greenwich Mean Noon Greenwich Mean Time and mean solar dayW de Sitter on p 38 of Bulletin of the Astronomical Institutes of the Netherlands v4 (1927) pp21–38 "On the secular accelerations and the fluctuations of the moon the sun Mercury and Venus" which refers to "the 'astronomical time' given by the earth's rotation and used in all practical astronomical computations" and states that it "differs from the 'uniform' or 'Newtonian' time" but later in respect of the period 1960–1983 they were treated as expressed in terms of ETSee p 612 in Explanatory Supplement to the Astronomical Almanac ed P K Seidelmann 1992 confirming introduction of ET in the 1960 edition of the ephemerides in accordance with the adopted ET proposal of 1948–52 ET in turn can now be seen (in light of modern results)See especially F R Stephenson (1997) and Stephenson & Morrison (1995) book and papers cited below as close to the average mean solar time between 1750 and 1890 (centered on 1820) because that was the period during which the observations on which Newcomb's tables were based were performed While TT is strictly uniform (being based on the SI second every second is the same as every other second) it is in practice realised by International Atomic Time (TAI) with an accuracy of about 1 part in 1014