text: The term ephemeris time (often abbreviated ET) can in principle refer to time in association with any ephemeris (itinerary of the trajectory of an astronomical object). In practice it has been used more specifically to refer to:
  a former standard astronomical time scale adopted in 1952 by the IAU, and superseded during the 1970s.  This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating mean solar time.  The intent was to define a uniform time (as far as was then feasible) based on Newtonian theory (see below: Definition of ephemeris time (1952)).  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.