text: In this way, decisions made by the original designers of ephemeris time influenced the length of today's standard SI second, and in turn, this has a continuing influence on the number of leap seconds which have been needed for insertion into current broadcast time scales, to keep them approximately in step with mean solar time.
  Definition (1952)
  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). Its detailed definition was based on Simon Newcomb's Tables of the Sun (1895), implemented in a new way to accommodate certain observed discrepancies:
  In the introduction to Tables of the Sun, the basis of the tables (p. 9) includes a formula for the Sun's mean longitude at a time, indicated by interval T (in units of Julian centuries of 36525 mean solar days), reckoned from Greenwich Mean Noon on 0 January 1900:
  Ls = 279° 41' 48".04 + 129,602,768".13T +1".089T2 . . . . . (1)
  Spencer Jones' work of 1939 showed that differences between the observed positions of the Sun and the predicted positions given by Newcomb's formula demonstrated the need for the following correction to the formula:
  ΔLs = + 1".00  + 2".97T  + 1".23T2  + 0.0748B
  where "the times of observation are in Universal time, not corrected to Newtonian time," and 0.0748B represents an irregular fluctuation calculated from lunar observations.