Where is the Equinox ?

Abstract

Within the work being carried out at Heidelberg on the establishment of the new fundamental reference coordinate system, the FK5, the determination of the location of celestial equator and the equinox form an important part. The plane of the celestial equator defined by the axis of rotation of the Earth and the plane of the ecliptic defined by the motion of the Earth about the Sun are both in motion due to various causes. The intersection of the equator and the ecliptic, the dynamical equinox, is therefore in motion. Great efforts have been made in the past to determine the location and motion of the dynamical equinox by means of observations of Sun, Moon and planets in such a manner that the dynamical equinox can serve as the origin of the right ascension system of a fundamental catalogue. The results have not been satisfactory, and we have some important evidence that the catalogue equinox of the FK4 is not identical with the “dynamical equinox”. Moreover, is has turned out that the difference α(DYN) - α(FK4) = E(T) depends on the epoch of observation T. Duncombe et al. (1974) have drawn attention to the possible confusion between the catalogue equinox and dynamical equinox; they mention the difference between two Earth longitude systems, one established by the SAO using star positions on the FK4 and the other one established by the JPL using planetary positions measured from the dynamical equinox. This is undoubtedly one legitimate explanation of the difference, even if other sources of errors may also have contributed.