Observed Variations of the Solar Diameter in 1998/2000

Abstract

The results from 9112 independent measurements of the solar diameter, made between the years 1998 and 2000, at the Observatorio Nacional Rio de Janeiro station, are examined. After the removal of isolated observations and correction of systematic effects, the series of results presents a well-defined normal distribution with standard deviation of 566 milli-arc-seconds (mas). The east and west results, which were taken under quite different atmospheric and instrumental conditions, have been treated separately and the smoothed series correlate at the level of 80%. The solar figure as charted from these data diverges from a perfect sphere by a mid-heliolatitude depletion at the level of 50 mas, and an overall flatness, to the effect of the polar radius being shorter than the equatorial radius by 13±4 mas. Finally, a significant periodic variation around 515 days is found for the observed solar diameter, with amplitude of 60 mas.