Article

Which reference frame for the sun?

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Abstract

Among the quantities in use in astronomy there is the value of the diameter of the sun which is at the distance of one astronomical unit. From this quantity the ephemerides are established and the dates of contacts for the solar eclipses calculated. In parallel with the astrophysical studies of the sun, astrometric observations may lead to improved values for this particular reference. In this paper after reviewing the values used in the ephemerides from Auswers's time and the modern ones, the variations of the diameter of the sun are given as deduced from the observations performed at CERGA (near Grasse, France). Tab. 1 gives the annual values for the observer Laclare and Tab. 2 shows the differences between various measurements made by several authors. The differences are larger than the mean square error. It appears that the values of the diameter of the sun employed in the ephemerides and different in the case of eclipses calculations must be changed. But discrepancies must be explained; in the view of having a better survey of the solar diameter a prism with a variable angle is under experiment at CERGA.

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