Analysis of the Sun's observations with prismatic Astrolabe

Extract

A regular program of observations of the Sun with prismatic astrolabes is under way since 1974 in both, the “Abrahão de Moraes” (φ = −23°0.1′) and “Caussols” (φ = +43°44.9′) observatories. The zenith distance of 2968 transits of the solar limbs observed during the period from 1988 January to 1994 April, in both centers, are analysed. The primary goal of this programme is to determine corrections ΔE and ΔA to the FK5 equinox and equator, as well as to the Earth's orbital constants. Secondly, we present here possible variations of the solar radius, which are of interest because of their astrophysical significance. The results of the analysis of the observations made with these instruments depend on both the prism angles used during the observation and the latitude of the site (Leister 1989, Poppe 1994). With the use of the different reflector prisms, instead of the refracting equilateral, it was possible to observe at differents zenith distances, allowing to sweep, at the brazilian observatory, the apparent orbit of the Sun during a period of about 10 months per year, with two prisms. The instrument of the french observatory works with 11 differents zenith distances, permitting an increase of about 10 months per year in the observational period either.