Solar diameter observations at Observatorio Nacional in 1998-1999

In January 1997, at the Observatorio Nacional (ON), nio de Janeiro, Brazil, digital CCD observations of the solar diameter began with a Danjon astrolabe adapted for solar observations and equipped with a variable angle reflecting prism. The prism enables observations of the Sun several times per day and all year round at the southern latitude phi = -22 degrees 54' As of June 1999, about 9000 observations have been made. The mean value of the apparent semi-diameter is 959." 13 +/- 0." 01. Here we report on the results obtained between July 1998 and June 1999. In 162 days: 4276 observations were made. For this period the mean apparent semi-diameter is 959." 07 +/- 0." 02. No significant difference between the apparent semi-diameters is found as observed on the East and West; sides of the meridian. The results of the observations are updated every month and available in electronic form in title SIMBAD database and at the World Wide Web page of the ON, Solar Radius Data: http://obsn.on.br/radius/.