Abstract. Visual astrometric observations of the Sun covering the second and first part of solar cycles 22 and 23 respectively, have been carried out during the last 13 years with the Danjon astrolabe of Santiago, Chile. These observations give, among other solar parameters, an absolute value of the Sun's apparent radius. We report here the results obtained from 4092 homogeneous radius measurements at 30• and 60• zenith distances. The data set shows at both zenith distances a significant radius variation in phase with magnetic activity. Moreover, the observations at 30• , which are less affected by atmospheric noise, give a significantly higher correlation coefficient between radius variation and sunspot numbers. Other investigations of solar radius variations during the last decades based on different observing techniques, as well as two analyses of historical data, are commented. Most of them show also positive correlations between radius variation and solar activity. With the noted exception of Calern, France, the results obtained at other astrolabe stations during recent years are in agreement with Santiago. The discrepancy between Calern and Santiago and its probable cause are discussed.

Abstract. During the last 25 years the Danjon astrolabe has been used extensively for measuring the apparent solar diameter. Here we discuss some aspects of the results obtained with the CCD astrolabes of Antalya (Turkey) and Rio de Janeiro (Brazil), and with the visual astrolabes of CERGA (France) and Santiago (Chile). The most extended databases obtained with these astrolabes are those of CERGA and probably Santiago. However, concerning observed variations in time of the solar radius, the results of both stations are discrepant. While the CERGA results show a marginal radius variation in opposing phase with solar magnetic activity, those of Santiago give a significant variation in time, but in phase with solar activity. According to Chollet & Sinceac (1999) the apparent solar radius variation is identical for all astrolabe measures (except Santiago) and anticorrelated with the magnetic cycle of the Sun. However, we show here that according to published results, the astrolabes of Antalya and Santiago give similar radius variation. With respect to the results of Rio de Janeiro, they show strong internal inconsitencies due probably to instrumental modifications made during its observational campaign. After removing these inconsistencies, the results of Rio de Janeiro become almost identical to those of Santiago. The results of these astrolabes, Antalya, Rio de Janeiro and Santiago, are in agreement with recent results obtained with different observational techniques, which show also that the apparent solar radius would vary in phase with solar magnetic activity.

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 fits applied to both data sets give a standard deviation of ±0. 365 and ±0. 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. 013 according to the CCD observations of Rio de Janeiro and of +0. 213 ± 0. 018 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(Noël , 2001).

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Abstract. Observations of the Sun with a modified Danjon astrolabe at 30• and 60• zenith distances are beeing carried out since 1990 at Santiago, Chile. Here are presented the results in right ascension, parameter Y and apparent semidiameter obtained during the period 1995-1997. These results and those obtained in former years are available in electronic form 1 . The differences astrolabe minus ephemeris in alpha and semidiameter are briefly discussed.