Ephemeris Time ET

Ephemeris Time (ET)


a uniform time scale corresponding to I. Newton’s fundamental laws of dynamics and determined by the gravitational theory of the motion of the earth in its orbit around the sun developed by S. Newcomb in the 19th century.

The ephemeris second, equal to 1/31556925.9747 of the tropical year, has been adopted as the unit of measurement in ephemeris time. The beginning of the ephemeris time scale coincides with noon of Dec. 31, 1889, when the mean tropical longitude of the sun, according to Newcomb’s theory, was 279°41 ’48.04” (this is the fundamental epoch of Newcomb’s planetary theories and is designated in astronomy as 1900 January 0d 12h ET).

Ephemeris time as an independent variable in differential equations of the motion of the bodies of the solar system, solved by the methods of celestial mechanics, acts as an argument of the gravitational theories of motion of these bodies and the ephemerides calculated on their basis (this is the origin of the term “ephemeris time”).

Ephemeris time was introduced in 1950 by a decision of the international conference on fundamental astronomical constants held in Paris. The magnitude of the discrepancy ΔT = ET – UT between ephemeris time ET and universal time UT, defined by the rotation of the earth, whose irregularity was finally proved in 1935, can be computed by comparing the moment of universal time at which the observed coordinates of a celestial object were obtained with the moment of ephemeris time whose ephemeris coordinates coincide with the observed coordinates. By analyzing discrepancies between ephemeris and observed values of the longitudes of the moon, sun, Mercury, and Venus, the British astronomer H. Spencer-Jones found in 1939 that these discrepancies change proportionally with the velocity of the apparent motions of the celestial objects. Thus it was found that the correction of ephemeris time ΔT can be determined with greatest precision by using observations of the moon, which moves across the sky more rapidly than other celestial objects. From observations of the moon, Spencer-Jones derived the formula, later adopted (1952) by the International Astronomical Union, for computing ΔT in seconds:

ΔT = + 24.349 + 72.318T + 29.950T2 + 1.82144B

where T is the interval of time that has passed from the fundamental epoch to the given moment expressed in Julian centuries of 36,525 days and B is the discrepancy between the computed and observed values of the moon’s longitude (fluctuation of the moon’s longitude). B is determined from observations of the phenomenon of the occultation of stars by the moon and from measurements of the moon’s position with respect to the stars. Determination of the corrections ΔT is an important problem of contemporary astronomy. Tables of values of the corrections for different epochs are given in astronomical yearbooks.

The appearance of highly stable frequency standards and scales of atomic time associated with them makes it possible to obtain virtually exact approximations of ephemeris time using the scale of international atomic time (TAI) established by the International Time Bureau in Paris: ET = TAI + 32.18 sec.


Spravochnoe rukovodstvo po nebesnoi mekhanike i asirodinamike, 2nd ed. Edited by G. N. Duboshin. Moscow, 1976.