EDP Sciences
Free access
Volume 374, Number 2, August I 2001
Page(s) 697 - 702
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20010749

A&A 374, 697-702 (2001)
DOI: 10.1051/0004-6361:20010749

Visual and CCD astrolabe observations of the solar radius

F. Noël

Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile

(Received 13 February 2001 / Accepted 2 May 2001 )

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 fits applied to both data sets give a standard deviation of $\pm0\farcs365$ and $\pm0\farcs360$ 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 $-0\farcs060\pm0\farcs013$ according to the CCD observations of Rio de Janeiro and of $+0\farcs213\pm0\farcs018$ according to the visual observations of Santiago. We show here that this discrepancy could be due to systematic effects, probably of instrumental origin, that produce significant 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 (Noël 1997, 2001).

Key words: astrometry -- Sun: activity -- Sun: fundamental parameters

© ESO 2001