摘要
Astrometry method is useful in measuring the longitude and latitude variations of an astronomical station. But owing to the limitation of optical technique, progress in this field has not been significaut since a long time ago. The 500,000 measurements of the worldwide 136 optical instruments during 1962-1982 were reduced once again by the author recently in the BIH in order to redetermine the Earth Rotation Parameters for the twenty years. With such a large quantity of data and a new reduction method, the obtained local corrections and their time derivatives (group unknowns G and G) of a star group in time and latitude measurement should be a better source for studying
