Editing Terrestrial Time (section)

==Relativistic relationships==

Observers in different locations, that are in relative motion or at different altitudes, can disagree about the rates of each other's clocks, owing to effects described by the [[theory of relativity]].  As a result, TT (even as a theoretical ideal) does not match the proper time of all observers.

In relativistic terms, TT is described as the [[proper time]] of a clock located on the [[geoid]] (essentially [[mean sea level]]).<ref>For example, IAU Commission 4 (Ephemerides), Recommendations to IAU General Assembly 1976, Notes on Recommendation 5, note 1, as well as other sources, indicate the time scale for apparent geocentric ephemerides as a proper time.</ref>
However,<ref name=guin86>B Guinot (1986), [http://adsabs.harvard.edu/abs/1986CeMec..38..155G "Is the International Atomic Time a Coordinate Time or a Proper Time?"], Celestial Mechanics, 38 (1986), pp.155-161.</ref>
TT is now actually defined as a [[coordinate time scale]].<ref>IAU General Assembly 1991, Resolution A4, Recommendations III and IV, define TCB and TCG as coordinate time scales, and TT as a linear scaling of TCG, hence also a coordinate time.</ref>
The redefinition did not quantitatively change TT, but rather made the existing definition more precise.  In effect it defined the geoid (mean sea level) in terms of a particular level of [[gravitational time dilation]] relative to a notional observer located at infinitely high altitude.

The present definition of TT is a linear scaling of [[Geocentric Coordinate Time]] (TCG), which is the proper time of a notional observer who is infinitely far away (so not affected by gravitational time dilation) and at rest relative to Earth. TCG is used to date mainly for theoretical purposes in astronomy. From the point of view of an observer on Earth's surface the second of TCG passes in slightly less than the observer's SI second. The comparison of the observer's clock against TT depends on the observer's altitude: they will match on the geoid, and clocks at higher altitude tick slightly faster.