SOLAR DIAMETER OBSERVATIONS WITH THE ASTROLABE AT OBSERVATORIO NACIONAL - RIO-DE-JANEIRO

Observations of the solar diameter are reported from the astrolabe sta tion at Observatorio Nacional. All the observations presented here wer e taken with the 1986-CERGA prototype of Variable Prism, which enables many diameter measurements per day and to observe all year round. The 1996 DANOF software for digital CCD image acquisition and data proces sing has been used. We adopted the recording of 46 CCD frames with dir ect and reflected images of the limbs along the Sun's transit through the almucantar. The independent observable is the accurate UT datation of each frame. Here the first campaign results, from January to July/ 1997, are presented. An asymmetry between the east and west transit is noticeable. The west transit leads to a larger value for the mean sem i-diameter and exhibits a larger standard deviation. No clear dependen ce is found upon the daily results, either on time or on observational parameters. The main outcome of this campaign remains the average sol ar semi-diameter, from 1149 transits (863 East and 286 West): obtained as 959 ''.21 +/- 0 ''.03.