MEASUREMENTS AND VARIATIONS OF THE SOLAR DIAMETER

As a consequence of an astrometry program, conducted since 1975 on a s olar astrolabe at the Calern Observatory (Observatoire de la Cote d'Az ur), we have obtained a data set of apparent solar diameters which enc ompasses periods greater than one solar cycle. From a set of more than 5000 visual observations, made by the same observer between 1975 and 1994, the mean value of the semi-diameter was measured at 959.42 '' +/ - 0.01 ''. Also, a set of CCD measurements made with the same instrume nt between 1989 and 1994 yields the mean value 959.40 '' +/- 0.01 ''. Both results obtained by raw measurements are consistent but significa ntly differ from values obtained by other methods and on other instrum ents. We discuss some systematic effects that can affect our visual me asurements and their precision. Taking account of a zenith distance ef fect provides for the semi-diameter a mean value closer to the value o f the ephemeris. Our observations also reveal deviations around the me an diameter in the royal zones and for high heliographic latitudes; th eir amplitudes reaching as much as 0.08 ''. Finally, semi-diameter var iations appear in our series; their origin is unknown but they may pos sibly be related to observed variations of magnetic activity or other solar parameters.