Various Definitions of the Ecliptic
The precession and nutation model IAU 2000 is based on the ‘‘ecliptic in the inertial sense’’, whereas former precession models referred to the ‘‘ecliptic in the rotational sense’’. The kinematical relationships between these two ecliptics are analysed. The ‘‘conventional ecliptic’’ is found by an adjustment of the position vector of the Earth-Moon barycentre so that it is always lying in this plane which has only a secular rotation. It also contains the time derivative of the adjusted position vector relative to the conventional ecliptic, but not the one relative to an inertial system. The ‘‘true ecliptic’’, which is spanned by the adjusted position vector and its time derivative relative to an inertial system, rotates not only secularly, but also periodically with a half-year period. Splitting off the periodic parts leads to the ‘‘mean ecliptic’’, which rotates purely secularly, but contains only approximately the adjusted position vector and velocity vector of the Earth-Moon barycentre. The conventional ecliptic is the ‘‘ecliptic in the rotational sense’’, and the mean ecliptic is the ‘‘ecliptic in the inertial sense’’. The relative orientation between the conventional and mean ecliptics is derived in accordance with formulae given by Standish (1981).
KeywordsEcliptic ecliptic in the inertial sense ecliptic in the rotational sense
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