زمان ِ روزیجی
Fr.: Temps des éphémérides
The uniform time-scale used as the independent variable
to calculate the orbits in the solar system prior to 1984. Ephemeris Time was adopted in
1960 to deal with irregularities in the → Earth's rotation
that had been found to affect the
course of mean solar time. The definition of Ephemeris Time is based on Newcomb's analytical
theory of the Earth's motion around the Sun (Newcomb 1898), according to which the geometric
mean longitude of the Sun with respect to the Earth-Moon barycenter is expressed by:
L = 279° 41' 48".04 + 129 602 768".13 T + 1''.089 T2,
where L refers to the → mean equinox
of date while T measures time from noon
1900 January 0 GMT in Julian centuries of 36525 days.
Ephemeris Time is therefore defined as
the instant near the beginning of the calendar year A.D. 1900 when the mean longitude
of the Sun was 279° 41' 48''.04, at which instant the measure of ET was
1900 January 0, 12h precisely. In this system the fundamental unit was the
→ ephemeris second, which was defined so that the
→ tropical year at the epoch
1900.0 should be exactly 31 556 925,9747 seconds of
ephemerides. Ephemeris Time was inconvenient in many ways and
was supeseded with the → Terrestrial Dynamical Time (TDT), whose
fundamental unit is the SI second.
→ ephemeris; → time.