Visual and CCD astrolabe observations of the solar radius

Abstract

Here we discuss and compare two sets of solar semidiameter measurements made with Danjon astrolabes during the period 1997{1999. One is based on 10 522 CCD observations made at Rio de Janeiro, Brazil; the other one is based on 956 visual observations made at Santiago, Chile. Linear ts applied to both data sets give a standard deviation of0: 00 365 and0: 360 for each daily mean value of the solar radius for Rio de Janeiro and Santiago respectively. During the period of this analysis, the apparent solar radius had an annual variation of 0: 060 0: 00 013 according to the CCD observations of Rio de Janeiro and of +0: 00 213 0: 00 018 according to the visual observations of Santiago. We show here that this discrepancy could be due to systematic eects, probably of instrumental origin, that produce signicant internal inconsistencies in the results of Rio de Janeiro. The drift of the apparent solar radius observed with the astrolabe of Santiago during 1997{1999 is consistent with former results obtained with this instrument since 1990. These results show that the apparent semidiameter of the Sun varies in phase with solar activity (Noel 1997, 2001).