The term ephemeris time (often abbreviated ET) can in principle refer to time in association with any ephemeris (itinerary of the trajectory of an astronomical object). In practice it has been used more specifically to refer to:
a former standard astronomical time scale adopted in 1952 by the IAU,[1] and superseded during the 1970s.[2] This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating mean solar time. The intent was to define a uniform time (as far as was then feasible) based on Newtonian theory (see below: Definition of ephemeris time (1952)). Ephemeris time was a first application of the concept of a dynamical time scale, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion.[3]
a modern relativistic coordinate time scale, implemented by the JPL ephemeris time argument Teph, in a series of numerically integrated Development Ephemerides. Among them is the DE405 ephemeris in widespread current use. The time scale represented by Teph is closely related to, but distinct (by an offset and constant rate) from, the TCB time scale currently adopted as a standard by the IAU (see below: JPL ephemeris time argument Teph).[4]
Most of the following sections relate to the ephemeris time of the 1952 standard.
An impression has sometimes arisen that ephemeris time was in use from 1900: this probably arose because ET, though proposed and adopted in the period 1948–1952, was defined in detail using formulae that made retrospective use of the epoch date of 1900 January 0 and of Newcomb's Tables of the Sun.[5][6]
The ephemeris time of the 1952 standard leaves a continuing legacy, through its historical unit ephemeris second which became closely duplicated in the length of the current standard SI second (see below: Redefinition of the second).
^'ESAE 1961': 'Explanatory Supplement (1961), esp. p. 9.
^'ESAA (1992)': P K Seidelmann (ed)., especially at pp. 41—42 and at p. 79.
^B Guinot and P K Seidelmann (1988), at p. 304—5.
^E M Standish (1998).
^S Newcomb (1895).
^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.
The term ephemeristime (often abbreviated ET) can in principle refer to time in association with any ephemeris (itinerary of the trajectory of an astronomical...
celestial navigation, an ephemeris (/ɪˈfɛmərəs/; pl. ephemerides /ˌɛfəˈmɛrədiːz/; from Latin ephemeris 'diary', and Greek ἐφημερίς (ephemeris) 'diary, journal')...
transition, newly established, with the ephemeris second. The ephemeris second is a unit in the system of time that, when used as the independent variable...
time standards such as sidereal time and ephemeristime, for most practical purposes, by newer time standards based wholly or partly on atomic time using...
(non-relativistic) former standard of ephemeristime (adopted by the IAU in 1952 and superseded 1976). In 2006, after a history of multiple time-scale definitions and...
century. Time standards based on Earth rotation were replaced (or initially supplemented) for astronomical use from 1952 onwards by an ephemeristime standard...
Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac, p. 9, ... defined ephemeristime ... [was] adopted by the International...
In this role, TT continues Terrestrial Dynamical Time (TDT or TD), which succeeded ephemeristime (ET). TT shares the original purpose for which ET was...
third release of the JPL Ephemeris Tapes, and was a special purpose, short-duration ephemeris. The then-current JPL Export Ephemeris was DE19. These early...
ephemeris second (see Ephemeristime - redefinition of the second). In 1976, however, the IAU resolved that the theoretical basis for ephemeristime was...
1879, Shaka Era, or 22 March 1957. India has adopted pie EphemerisTime in the Indian Ephemeris from 1960 onwards in pursuance of the resolution passed...
basis for ephemeristime (ET) was wholly non-relativistic, and therefore, beginning in 1984 ephemeristime would be replaced by two further time scales with...
of mean solar time (UT2) (result: 9192631830 cycles) and also the second of ephemeristime (ET) (result:9192631770 ± 20 cycles), see "Time Scales", by L...
of the astronomical time scale ephemeristime (ET). In 1967 the International System of Units (SI) standardized its unit of time, the second, on the properties...
second offset is to provide continuity with the older ephemeristime. "History of TAI−UTC". Time Service Dept., United States Naval Observatory. 2009....
the Gregorian calendar are used. For continuity with its predecessor EphemerisTime, TCG was set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z)...
Propulsion Laboratory as Development Ephemeris. The latest releases include DE430 which covers planetary and lunar ephemeris from Dec 21, 1549 to Jan 25, 2650...
rate, astronomers introduced EphemerisTime, which has since been replaced by Terrestrial Time (TT). Because Universal Time is determined by the Earth's...
standard atomic clock was carried out by the use of the astronomical time scale ephemeristime (ET). As of 2013, the most stable atomic clocks are ytterbium...
Equation of EphemerisTime: 1532 (prior to the distinction that is now made between EOT, E, and Δt the latter was known as the equation of time) is Δ t =...
known in Halley's time that what is actually occurring includes a slowing-down of Earth's rate of rotation: see also Ephemeristime – History. When measured...
Chicago Press. Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac. London: Her Majesty's Stationery Office...
sandglass, sand timer, or sand clock) is a device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that...
36525 ephemeris days and the Julian millennium of 365250 ephemeris days are used in astronomical calculations. Fundamentally, expressing a time interval...
Moon's shadow cone passes closest to the centre of Earth; this is in (EphemerisTime). The number of the saros series that the eclipse belongs to is given...
the Gregorian calendar are used. For continuity with its predecessor EphemerisTime, TCB was set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z)...
developed the ephemeristime scale, which had been adopted by the IAU in 1952 on a proposal formulated by Clemence in 1948, as an international time standard...
year begins". The mean tropical year in 2000 was 365.24219 ephemeris days, each ephemeris day lasting 86,400 SI seconds. This is 365.24217 mean solar...
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