See also Ephemeris time - History for further information and sources.

During the currency of ephemeris time as a standard, the details were revised a little.

Most of the following sections relate to the ephemeris time of the 1952 standard.

Successive definitions of the unit of ephemeris time are mentioned above (History).

When ephemeris time was first adopted, time scales were still based on astronomical observation, as they always had been.

The length of second so defined was practically equal to the second of ephemeris time.

D Brouwer suggested the name 'ephemeris time'.

Clemence's formula, now superseded by more modern estimations, was included in the original conference decision on ephemeris time.

This was in conformity with the ephemeris time scale adopted by the IAU in 1952.

In view of the fluctuation term, practical determination of the difference between ephemeris time and UT depended on observation.

Although not an IAU standard, the ephemeris time argument T has been in use at that institution since the 1960s.

After the caesium atomic clock was invented, such clocks were used increasingly from the late 1950s as secondary realizations of ephemeris time (ET).

A first application of this concept of dynamical time was the definition of the ephemeris time scale (ET).

Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET).

The term ephemeris time (often abbreviated ET) can in principle refer to time in connection with any astronomical ephemeris.

Finally, SCC predicts a cosmological 'clock drift' between atomic clock time and ephemeris time.

The 1961 official reference put it this way: "The origin and rate of ephemeris time are defined to make the Sun's mean longitude agree with Newcomb's expression"

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).

The difference between Terrestrial Time (TT) (the successor to ephemeris time) and atomic time was later defined as follows:

In the meantime, Brown's theory was improved with better constants and the introduction of Ephemeris Time and the removal of some empirical corrections associated with this.

The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time.

The SI second thus inherits the effect of decisions by the original designers of the ephemeris time scale, determining the length of the ET second.

In the past, mean solar time (before the discovery of the non-uniform rotation of the Earth) and ephemeris time (before the implementation of relativistic gravitational equations) were used.

(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.

He developed the ephemeris time scale, which had been adopted by the IAU in 1952 on a proposal formulated by Clemence in 1948, as an international time standard.

See also Ephemeris time - History for further information and sources.

During the currency of ephemeris time as a standard, the details were revised a little.

Most of the following sections relate to the ephemeris time of the 1952 standard.

Successive definitions of the unit of ephemeris time are mentioned above (History).

When ephemeris time was first adopted, time scales were still based on astronomical observation, as they always had been.

The length of second so defined was practically equal to the second of ephemeris time.

D Brouwer suggested the name 'ephemeris time'.

Clemence's formula, now superseded by more modern estimations, was included in the original conference decision on ephemeris time.

This was in conformity with the ephemeris time scale adopted by the IAU in 1952.

In view of the fluctuation term, practical determination of the difference between ephemeris time and UT depended on observation.

Although not an IAU standard, the ephemeris time argument T has been in use at that institution since the 1960s.

After the caesium atomic clock was invented, such clocks were used increasingly from the late 1950s as secondary realizations of ephemeris time (ET).

A first application of this concept of dynamical time was the definition of the ephemeris time scale (ET).

Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET).

The term ephemeris time (often abbreviated ET) can in principle refer to time in connection with any astronomical ephemeris.

Finally, SCC predicts a cosmological 'clock drift' between atomic clock time and ephemeris time.

The 1961 official reference put it this way: "The origin and rate of ephemeris time are defined to make the Sun's mean longitude agree with Newcomb's expression"

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).

The difference between Terrestrial Time (TT) (the successor to ephemeris time) and atomic time was later defined as follows:

In the meantime, Brown's theory was improved with better constants and the introduction of Ephemeris Time and the removal of some empirical corrections associated with this.

The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time.

The SI second thus inherits the effect of decisions by the original designers of the ephemeris time scale, determining the length of the ET second.

In the past, mean solar time (before the discovery of the non-uniform rotation of the Earth) and ephemeris time (before the implementation of relativistic gravitational equations) were used.

(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.

He developed the ephemeris time scale, which had been adopted by the IAU in 1952 on a proposal formulated by Clemence in 1948, as an international time standard.