Editing Ephemeris time (section)

===Secondary realizations by lunar observations===
Although ephemeris time was defined in principle by the orbital motion of the Earth around the Sun,<ref>[[#refClem1948|Clemence (1948)]], at pp. 171—3.</ref> it was usually measured in practice by the orbital motion of the Moon around the Earth.<ref>[[#refMark1955|W Markowitz & others (1955)]]; [[#refMark1959|W Markowitz (1959)]]; also [[#refMark1958|W Markowitz, R G Hall, L Essen, J V L Parry (1958)]].</ref>  These measurements can be considered as secondary realizations (in a [[metrology|metrological]] sense) of the primary definition of ET in terms of the solar motion, after a calibration of the mean motion of the Moon with respect to the mean motion of the Sun.<ref name=guin88>
[[#refGuin1988|B Guinot & P K Seidelmann (1988)]], at p. 305.</ref>

Reasons for the use of lunar measurements were practically based: the Moon moves against the background of stars about 13 times as fast as the Sun's corresponding rate of motion, and the accuracy of time determinations from lunar measurements is correspondingly greater.

When ephemeris time was first adopted, time scales were still based on astronomical observation, as they always had been.  The accuracy was limited by the accuracy of optical observation, and corrections of clocks and time signals were published in arrear.