Terrestrial Time is a theoretical uniform time scale, defined to provide continuity with the former Ephemeris Time ( ET ).

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.

Ephemeris time ( ET ), adopted as standard in 1952, was originally designed as an approach to a uniform time scale, to be freed from the effects of irregularity in the rotation of the earth, " for the convenience of astronomers and other scientists ", for example for use in ephemerides of the Sun ( as observed from the Earth ), the Moon, and the planets.

Ephemeris time was defined in principle by the orbital motion of the Earth around the Sun, ( but its practical implementation was usually achieved in another way, see below ).

Previous to the 1960 change, the ' Improved Lunar Ephemeris ' had already been made available in terms of ephemeris time for the years 1952-1959 ( computed by W J Eckert from Brown's theory with modifications recommended by Clemence ( 1948 )).

*< cite id = refMark1955 > W Markowitz, R G Hall, S Edelson ( 1955 ), " Ephemeris time from photographic positions of the moon ", Astronomical Journal, vol.

*< cite id = refWink1977 > G M R Winkler and T C van Flandern ( 1977 ), " Ephemeris Time, relativity, and the problem of uniform time in astronomy ", Astronomical Journal, vol. 82 ( Jan. 1977 ), pp. 84 – 92 .</ cite >

( It was not yet known in Halley's time that what is actually occurring includes a slowing-down of the Earth's rate of rotation: see also Ephemeris time – History.

) The small difference accumulates every day, which leads to an increasing difference between our clock time ( Universal Time ) on the one hand, and Atomic Time and Ephemeris Time on the other hand: see ΔT.

It was designed for continuity with ET, and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET ( see, under Ephemeris time, Redefinition of the second and Implementations.

( If its begin and end are defined using mean solar time ( the legal time scale ) then its duration was 31622401. 141 seconds of Terrestrial Time ( or Ephemeris Time ), which is slightly shorter than 1908 ).

Historical Julian dates were recorded relative to GMT or Ephemeris Time, but the International Astronomical Union now recommends that Julian Dates be specified in Terrestrial Time, and that when necessary to specify Julian Dates using a different time scale, that the time scale used be indicated when required, such as JD ( UT1 ).

Ephemeris time and its successor time scales described below have all been intended for astronomical use, e. g. in planetary motion calculations, with aims including uniformity, in particular, freedom from irregularities of Earth rotation.

* Ephemeris Time ( ET ) was from 1952 to 1976 an official time scale standard of the International Astronomical Union ; it was a dynamical time scale based on the orbital motion of the Earth around the Sun, from which the ephemeris second was derived as a defined fraction of the tropical year.

It was in use for the official almanacs and planetary ephemerides from 1960 to 1983, and was replaced in official almanacs for 1984 and after, by numerically integrated Jet Propulsion Laboratory Development Ephemeris DE200 ( based on the JPL relativistic coordinate time scale T < sub > eph </ sub >).

The Lahiris Ephemeris published annually is the most widely used English almanac in Vedic astrology apart from the many Panchāngas published in local languages, which are mostly based on the National Panchānga.

Later work has shown this to be true, and astronomers now make a distinction between Universal Time, which is based on the Earth's rotation, and Terrestrial Time ( formerly Ephemeris time ), which is a uniform measure of the passage of time ( see also ΔT ).

A later version of Washington mean time based on the merdian of the clock room at the exact center of the New Naval Observatory ( 77 ° 4 ' 2. 24 " W or ) was still being used in 1950 on a few pages of the American Ephemeris and Nautical Almanac, even though most of its pages used Greenwich Civil Time, the American name for the midnight epoch Greenwich Mean Time.

* 1504 — While shipwrecked on the island of Jamaica, Christopher Columbus successfully predicted a lunar eclipse for the natives, using the Ephemeris of the German astronomer Regiomontanus.

* A Free 6000 Year Ephemeris Provided by Astro. com -- Based on Swiss Ephemeris, by Astrodienst Zürich, Switzerland ( available in 8 languages )

Ephemeris time was consequently developed as a standard that was free from the irregularities of Earth rotation, by defining the time " as the independent variable of the equations of celestial mechanics ", and it was at first measured astronomically, relying on the existing gravitational theories of the motions of the Earth about the Sun and of the Moon about the Earth.

At that point, he took up a full-time position on the staff of American Ephemeris and Nautical Almanac in Cambridge, Massachusetts.

According to Neil Michelsen's " The American Ephemeris ," on 24 August 1987 there was an exceptional alignment of planets in the solar system.

Newcomb's Tables of the Sun is the short title and running head of a work by the American astronomer and mathematician Simon Newcomb entitled " Tables of the Motion of the Earth on its Axis and Around the Sun " on pages 1 – 169 of " Tables of the Four Inner Planets " ( 1895 ), volume VI of the serial publication Astronomical Papers prepared for the use of the American Ephemeris and Nautical Almanac.

# Booker rebuked ; or, Animadversions on Booker's Teiescopium Uranicum or Ephemeris, 1665, which is very erroneous, & c. London, 1665, quarto, in one sheet, which made much sport among people, having had the assistance therein of Jo.

In astrology, the adjusted calculation date, or artificial birthday is the day on which the planetary position shown in an Ephemeris coincides with the progressed positions of the planets and other bodies in the heavens.

In 1976 the IAU resolved that the theoretical basis for its current ( 1952 ) standard of Ephemeris Time was non-relativistic, and that therefore, beginning in 1984, Ephemeris Time would be replaced by two relativistic timescales intended to constitute dynamical timescales: Terrestrial Dynamical Time ( TDT ) and Barycentric Dynamical Time ( TDB ).

This Ephemeris Time standard was non-relativistic and did not fulfil growing needs for relativistic coordinate time scales.

The first uniform time system Ephemeris Time was adopted in 1952.

that the JPL Ephemeris Tape System was " used for virtually all computations of spacecraft trajectories in the US space program ", and that it had, as its current lunar ephemeris, an evaluation of the Improved Lunar Ephemeris incorporating a number of corrections: sources are named as ' The Improved Lunar Ephemeris ' ( documentation which was the report of the Eckert computations carried out by the SSEC, complete with lunar position results from 1952 – 1971 ), with corrections as described by Eckert et al.

*< cite id = refILE1954 > W J Eckert et al., Improved Lunar Ephemeris 1952 – 1959: A Joint Supplement to the American Ephemeris and the ( British ) Nautical Almanac, ( US Government Printing Office, 1954 ).

The Boston Ephemeris was an almanac written by Mather in 1686.

The data was from JPL Horizons Ephemeris System.

As UT is slightly irregular in its rate, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time ( TT ).

*< cite id = refESAE >' 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 ).</ cite >

6, part 1, of Astronomical Papers prepared for the use of the American Ephemeris and Nautical Almanac ( 1895 ), at pages 1 – 169 ).</ cite >

TDB according to the 2006 redefinition can now be treated as equivalent, for practical astronomical purposes, to the long-established JPL ephemeris time argument T < sub > eph </ sub > as implemented in JPL Development Ephemeris DE405 ( in use as the official basis for planetary and lunar ephemerides in the Astronomical Almanac, editions for 2003 and succeedng years ).

Until 1833, the equation of time was tabulated in the sense ' mean minus apparent solar time ' in the British Nautical Almanac and Astronomical Ephemeris published for the years 1767 onwards.

*< cite id = refNAAE1871 >' AE 1871 ': " Nautical Almanac & Astronomical Ephemeris " for 1871, ( London, 1867 ).

*< cite id = refES1961 >' 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 '), London ( HMSO ), 1961.

* 1408 — Chinese Ephemeris Table ( copy in Pepysian Library, Cambridge, UK ( refer book ' 1434 '); Chinese tables believed known to Regiomontanus ).