Analysis of Solar Astrolabe Measurements during 20 Years

Abstract

Recent observations of the Sun made between 1974 and 1995 at two observatories were examined to determine the constant and/or linear terms to the equinox and equator of the FK5 reference frame, the mean obliquity of the ecliptic, the mean longitude of the Sun, the mean eccentricity of the Earth's orbit, and the mean longitude of perihelion. The VSOP82 theory was used to reduce the data. The global solution of the weighted least-squares adjustment shows that the equinox of the FK5 requires a correction of +0072 ± 0005 at the mean epoch 1987.24. The FK5 and dynamical equinox agree closely at J2000.0 (-0040 ± 0020), but an anomalous negative secular variation with respect to the dynamical equinox was detected: -0881 ± 0116 century-1. The FK5 equator requires a correction of +0088 ± 0016, and there is no indication of a time rate of change. The corrections to the mean longitude of the Sun (-0020 ± 0010) and to the mean obliquity of the ecliptic (-0041 ± 0016) do appear to be statistically significant, although only marginally. The time rates of change for these quantities are not significant on the system to which the observations are referred. In spite of the short time span used in this analysis, the strong correlation between constant and linear terms was completely eliminated with the complete covering of the orbit by the data sets of both sites.