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 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 ).

Ephemeris time based on the standard adopted in 1952 was introduced into the Astronomical Ephemeris ( UK ) and the American Ephemeris and Nautical Almanac, replacing UT in the main ephemerides in the issues for 1960 and after.

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.

* TT is in effect a continuation of ( but is more precisely uniform than ) the former Ephemeris Time ( ET ).

For continuity with their predecessor Ephemeris Time ( ET ), TT and TCG were set to match ET at around Julian Date 2443 144. 5 ( 1977-01-01T00Z ).

TDB is a successor of Ephemeris Time ( ET ), in that ET can be seen ( within the limits of the lesser accuracy and precision achievable in its time ) to be an approximation to TDB as well as to Terrestrial Time ( TT ) ( see Ephemeris time-Implementations ).

For continuity with its predecessor Ephemeris Time, TCB was set to match ET at around Julian Date 2443144. 5 ( 1977-01-01T00Z ).

For continuity with its predecessor Ephemeris Time, TCG was set to match ET at around Julian Date 2443144. 5 ( 1977-01-01T00Z ).

*< cite id = refClem1971 > G M Clemence ( 1971 ), " The Concept of Ephemeris Time ", Journal for the History of Astronomy, vol. 2 ( 1971 ), pp. 73 – 79 .</ cite >

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

To this the Portuguese used the astronomical tables ( Ephemeris ), precious tools for oceanic navigation, which have experienced a remarkable diffusion in the fifteenth century.

To address this the Portuguese used the astronomical tables ( Ephemeris ), a precious tool for oceanic navigation, which spread widely in the fifteenth century.

Balbus kept a diary of the chief events in his own and Caesar's life ( Ephemeris ), which has been lost ( Suetonius, Caesar, 81 ).

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.

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 ).

* Various authors, The Rosicrucian Ephemeris: 2000 – 2100 12H Tdt ( Noon ), ISBN 978-0-911274-24-0, June 1992

While in the city, he edited a Greek-language newspaper, Ephemeris ( i. e. Daily ), and published a proposed political map of Great Greece which included Constantinople and many other places, including a large number of places where Greeks were in the minority ( such as Constantinople ).

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.

This led to the Improved Lunar Ephemeris ( ILE ), which, with some minor successive improvements, was used in the astronomical almanacs from 1960 through 1983 ( ILE j = 0 from 1960 to 1967, ILE j = 1 from 1968 to 1971, ILE j = 2 from 1972 to 1983 ), and was used to bring men to the moon.

*< 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.

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

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.

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.

*< 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.

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 data was from JPL Horizons Ephemeris System.

To rectify the sphere for use, first slacken the screw r in the upright stem R, and taking hold of the arm Q, move it up or down until the given degree of latitude for any place be at the side of the stem R ; and then the axis of the sphere will be properly elevated, so as to stand parallel to the axis of the world, if the machine be set north and south by a small compass: this done, count the latitude from the north pole, upon the celestial meridian L, down towards the north notch of the horizon, and set the horizon to that latitude ; then, turn the nut b until the sun Y comes to the given day of the year in the ecliptic, and the sun will be at its proper place for that day: find the place of the moon's ascending node, and also the place of the moon, by an Ephemeris, and set them right accordingly: lastly, turn the winch W, until either the sun comes to the meridian L, or until the meridian comes to the sun ( according as you want the sphere or earth to move ) and set the hour-index to the XII, marked noon, and the whole machine will be rectified.

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 ).

Ephemeris data generated by JPL Horizons indicates that Opportunity would be able to observe the transit from the start until local sunset at about 19: 23 UTC, while Spirit would be able to observe it from local sunrise at about 19: 38 UTC until the end of the transit.