On zenith-distance variations of the Borowiec astrolabe

Abstract

Systematic differences between zenith distances of two groups of stars observed successively during the same night occur in observations carried out with the Danjon Astrolabe in Borowiec. Four suggestions for the determination of an additional parameter correlated in the best way with the observed variations of zenith distance are made. The best correlation was found with the time span after sunset and a less good correlation with temperature. Some differences between the results of observations reduced with classical methods and rereduced with consideration of the empirical 'after sunset' function are given.