CCD astrolabe solar diameter measurements in 1999–2000

Abstract

Solar diameter observations are presented covering a period close to the maximum of the solar activity cycle 23, from July 1999 to December 2000, at the Observatório Nacional, Rio de Janeiro. The observations were made at 5635 Å, with a CCD Solar Astrolabe, thus timing the limb crossing through a fixed zenith distance. The formal precision of a single measurement is typically 018. The results represent daily means from 4625 independent measurements. The series obtained has 225 homogeneously distributed points with a mean precision of 012. The average value obtained for the solar semi-diameter within the analyzed period is 95901 ± 002. The average of the observations, made under a wide range of different geometric, instrumental and environmental conditions, aims to provide results representing global variations of the solar radius. From this data, variations with amplitude equal to or larger than 002 can be investigated. No statistically significant linear trend is obtained for the examined series. There is, however, some indication of an increasing trend for 1999 followed by a stabilization during 2000. Periodic variations are obtained with amplitudes between 002 and 007. Periods related to the solar rotation and the sunspot characteristic lifetime can be recognized.