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4 Discussion

The annual mean right ascension differences in the sense observed- ephemeris, presented in Table 1, show a positive and significative general trend throughout all the observational period. As it is stated above, the orientation of the local reference of the astrolabe is based on the Earth rotation parameters provided by IERS which are refered to the International Celestial Reference System (ICRS) (Arias et al. 1995). Beside some eventual systematic effect of instrumental and/or personal bias origin, the systematic differences in right ascension of Table 1 could be partially due to an offset of the ICRS equinox and the dinamical equinox of the DE200/LE200 solar ephemeris. It has been found that observed positions of the Sun in the FK5 system, show also a positive trend for the differences in right ascension FK5-DE200/LE200 (Kolesnik 1995, Da Rocha Poppe 1998). This might be a confirmation of our results if one considers that the ICRS origin of right ascension is consistent with the FK5 origin (Arias et al. 1995). According to Hohenkerk et al. (1992) an effort was made to make the origin of DE200/LE200 ephemerides agree with the FK5 equinox, but it was recognized that there is some difference, which will be determined more accurately with time.

The annual mean values of the solar semidiameter of Table 1 represent the averages of the results of the east and west observations at $30\hbox{$^\circ$}$ and $60\hbox{$^\circ$}$ zenith distances. The dispersion of the semidiameter measurements derived separately from the east and west observations, is $0\hbox{$.\!\!^{\prime\prime}$}42$ and $0\hbox{$.\!\!^{\prime\prime}$}43$ respectively for $30\hbox{$^\circ$}$,and $0\hbox{$.\!\!^{\prime\prime}$}55$ and $0\hbox{$.\!\!^{\prime\prime}$}45$ for $60\hbox{$^\circ$}$. The higher dispersion of the east results at $60\hbox{$^\circ$}$ is probably due to the local topography which should affect more the observations made at larger zenith distances. The high mountains of Los Andes range are at the east and rather close to Cerro Calán Observatory.

According to Table 1, a systematic decrease of the semidiameter is observed throughout the observational period except for 1997. This variation has a similar trend as the variation of sunspot number during the same period. An eventual solar origin of the Sun semidiameter variations observed with the astrolabe of Santiago is under research and for a preliminary discussion see Noël (1997).

Acknowledgements

The project of the Danjon astrolabe at Santiago is a joint research program between the European Southern Observatory (ESO) and Universidad de Chile. The solar program of Santiago is partially supported by Fondo Nacional de Ciencia y Tecnología (FONDECYT), Santiago, Chile, under research project No. 1970312.


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