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# Earth Revolution Time

Fundamentals | |
---|---|

Title | Earth Revolution Time |

Author(s) | J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, Technical University of Catalonia, Spain. |

Level | Basic |

Year of Publication | 2011 |

**Ephemeris Time**

The *Ephemeris Time* (ET) was adopted in 1952 as the (conceptually uniform) time of Newtonian Mechanics. It was defined in terms of the earth's revolution orbit around the sun, and it was introduced to be free from the effects of irregularity in the unpredictable polar motion and the earth rotation rate.

The ET second was established as the International System of Physical Units (SI) second between 1956 and 1967 and it was defined as the tropic year fraction:

- [math] ET\ second=\frac{1}{31\,556\,925.9747} \qquad \mbox{(1)} [/math]

In 1976, in Grenoble, the IAU resolved that Ephemeris Time would be replaced by two relativistic timescales intended to constitute dynamical timescales (derived from planetary motions in the solar system): The "*Temps Dinamique Baricentrique*" (TDB) and "*Temps Dinamique Terrestre*" (TDT).

*TDB*or*Temps Dinamique Baricentrique*: It is an inertial time in the Newtonian sense and provides the time variable in the equations of motion for the ephemerides related to the Solar System centre of gravity (i.e., for planetary orbits computation around the sun).

*TDT*or*Temps Dinamique Terrestre*: It is a quasi-inertial time in the Newtonian sense and is the independent variable of the satellites equations motion around the earth^{[footnotes 1]}.

In the terminology of General Gravity, TDB is a Coordinate Time and TDT is a Proper Time.

The IAU resolution A4 (1991) established the framework presently used to define the *Baricentric* and *Geocentric Reference Systems* (BRS and GRS, respectively), and associated to these systems there were defined the *Baricentric Coordinate Time*, from the French Temps Coordonné Barycentrique (TCB) and the *Geocentric coordinate Time*, from the French Temps Coordonné Géocentrique (TCG). It was also defined an additional time for the GRS system named *Terrestrial Time* (TT). The TT corresponds to the TDT in the formers definitions (TDT and TDB used before 1991) and is introduced as the theoretical counterpart of the realized time scale TAI+ 32.184s (see the IERS Conventions 2003 [Denis et al., 2004] )^{[1]}.

## Notes

- ^ It is quasi-inertial because earth is subjected to the acceleration caused by its annual translation around the sun.

## References

- ^ [Denis et al., 2004] Denis, D., McCarthy and Petit, G., 2004. IERS Conventions (2003). IERS Technical Note 32. IERS Convention Center, Frankfurt am Main.