Until the middle of the 20th century precise timekeeping was based on the apparent motions of the celestial bodies (in particular, the sun). Key moments of the day were regulated by the rising, the noon transit or the setting of the sun. Due to the large seasonal variation in the times of sunrise or sunset in temperate latitudes astronomers adopted the practice of measuring time from the noon transit of the sun as this moment can be determined with fair precision even with modest means.
However, when measured against an uniform time scale (as generated by an ideal clock) the times of successive noon transits of the sun show a yearly and a half-yearly variation which is known as the “equation of time” due to the non-uniform apparent motion of the sun in the ecliptic (as viewed from a stationary earth) and the angle of the sun’s apparent orbit with the celestial equator.
Although this variation was already known (from theoretical principles) by Hellenistic astronomers as Claudius Ptolemy, who worked in Alexandria around 150 CE, it had little importance in timekeeping until the invention of the pendulum clock in 1656 by the Dutch scientist Christiaan Huygens.
Modern astrodynamical theories of the motions of the sun, the moon and the planets are based on a smoothly increasing and uniform time scale known as Terrestrial Time (TT). This time scale is related to International Atomic Time (Temps Atomique International or TAI) that in turn is defined by a world-wide array of interlinked atomic clocks. These clocks are based on the International System of Units (Systeme International or SI) definition of the second, defined in 1967 by the 13th General Conference on Weights and Measures (Conférence Générale des Poids et Mesures) as follows:
“The second is the duration of 9.192.631.770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.”
Terrestrial Time differs from the commonly used Coordinated Universal Time (UTC), a time scale that is based on the slightly erratic rotation of the Earth around its axis and formerly known as Greenwich Mean Time (GMT), by a slowly growing interval of time that now amounts to a little more than a minute. The difference TT–UT1 (or ΔT for short) has an effect on any computation involving a topocentric observer, but is of particular importance in eclipse and occultation calculations (cf. Meeus, 1998, for details).
[For the moment, I have glossed over the subtle differences between UTC, UT0, UT1, UT2 and related time scales. These will be treated in more detail at a later stage]
Note that Terrestrial Time was formerly known as Ephemeris Time (ET, used from 1952 to 1983) and Dynamical Time (TD, used from 1984 to 2000). More information on the various physical, astronomical and civil timescales can be found on the International Earth Rotation and Reference Systems Service (IERS) website.
* The leap second is inserted at the end of the day cited
and is denoted as 23:59:60 UTC