Editing Ephemeris time (section)

==JPL ephemeris time argument T<sub>eph</sub>==
High-precision [[ephemeris|ephemerides]] of sun, moon and planets were developed and calculated at the [[Jet Propulsion Laboratory]] (JPL) over a long period, and the latest available were adopted for the ephemerides in the [[Astronomical Almanac]] starting in 1984. Although not an IAU standard, the ephemeris time argument T<sub>eph</sub> has been in use at that institution since the 1960s. The time scale represented by T<sub>eph</sub> has been characterized as a [[principle of relativity|relativistic]] coordinate time that differs from [[Terrestrial Time]] only by small periodic terms with an amplitude not exceeding 2 milliseconds of time: it is linearly related to, but distinct (by an offset and constant rate which is of the order of 0.5&nbsp;s/a) from the [[Barycentric Coordinate Time|TCB]] time scale adopted in 1991 as a standard by the [[IAU]]. Thus for clocks on or near the [[geoid]], T<sub>eph</sub> (within 2 milliseconds), but not so closely TCB, can be used as approximations to Terrestrial Time, and via the standard ephemerides T<sub>eph</sub> is in widespread use.<ref name="EMS1998"/>

Partly in acknowledgement of the widespread use of T<sub>eph</sub> via the JPL ephemerides, IAU resolution 3 of 2006<ref>{{cite web| url = http://www.iau.org/static/resolutions/IAU2006_Resol3.pdf| title = IAU 2006 resolution 3}}</ref> (re-)defined [[Barycentric Dynamical Time]] (TDB) as a current standard. As re-defined in 2006, TDB is a linear transformation of [[Barycentric Coordinate Time|TCB]]. The same IAU resolution also stated (in note 4) that the "independent time argument of the JPL ephemeris [[DE405]], which is called T<sub>eph</sub>" (here the IAU source cites<ref name="EMS1998"/>), "is for practical purposes the same as [[Barycentric Dynamical Time|TDB]] defined in this Resolution". Thus the new TDB, like T<sub>eph</sub>, is essentially a more refined continuation of the older ephemeris time ET and (apart from the {{nowrap|< 2 ms}} periodic fluctuations) has the same mean rate as that established for ET in the 1950s.