1 Introduction

Variations in time of the solar radius is a fairly important issue for Astrophysics. If these variations are real, they would constitute a serious challenge to stellar structure theory. According to Spruit (1994), the implications would be much more serious than a mere contradiction between, say, a dynamo theory and observations of the solar cycle. The suspicion that the solar radius could be variable is a motivation for measuring it, and the role of the results is to guide the theory, rather than to verify it (Ribes et al. 1991).

The Danjon astrolabe has been used extensively to measure the apparent solar radius in the last twenty five years. The astrolabe, developed at the end of the forties and the beginning of the fifties at the Paris Observatory by André Danjon (1960), works according to the method of equal altitudes (Débarbat & Guinot 1970). It was applied very succesfully to the research of the Earth rotation parameters. However, it was shown that it was also a powerful instrument for stellar astrometry (Anguita & Noël 1969; Fricke 1972). Good results also have been obtained for astrometric observations of planets (Débarbat 1968; Standish et al. 1981; Noël 1987). The excellent astrometric results obtained by the Danjon astrolabe were due partly to the intrinsic qualities of the instrument and partly to the method of equal altitudes (Danjon 1960).

Following the pioneering work of Dr. Francis Laclare at the Centre d'Études et de Recherches en Géodynamique et Astrométrie (CERGA), France, the Danjon astrolabe, after some modifications, also was applied to solar astrometry. Astrolabe observations of the Sun give, among other solar parameters, an absolute measurement of the apparent solar radius (Laclare 1983). The work of Laclare was followed by programs of solar observations with modified Danjon astrolabes in São Paulo and Rio de Janeiro, Brazil (Leister & Benevides Soares 1990; Jilinski et al. 1999), in San Fernando, Spain (Sánchez et al. 1993), in Antalya, Turkey (Golbasi et al. 2001) and in Santiago, Chile (Chollet & Noël 1993). All these astrolabes, except Santiago, are now equipped with CCD cameras.