Short vs Long Time Series : Example of the Solar Diameter

Abstract

The significance of a time series obviously changes with its length and accuracy; data about the Sun are no exception. In this short paper, we compare the 23-year long series of visual observations of the solar semi-diameter at Calern Observatory with the activity cycle to find an apparent opposing phase. The comparison of the sub-series of the same, carried out during 26-month long measurements of SOHO/MDI (April 1996, June 1998), reveals an apparent phase with the same activity cycle, in line with most of the recent results. Consequently, the importance of long and coherent ground measurements of the solar diameter has to be stressed.