As a consequence of an astrometry program, conducted since 1975 on a solar astrolabe at the Calern Observatory (Observatoire de la C6te d'Azur), we have obtained a data set of apparent solar diameters which encompasses periods greater than one solar cycle. From a set of more than 5000 visual observations, made by the same observer between 1975 and 1994, the mean value of the semidiameter was measured at 959.42" -4-0.01". Also, a set of CCD measurements made with the same instrument between 1989 and 1994 yields the mean value 959.40" -4-0.01". Both results obtained by raw measurements are consistent but significantly differ from values obtained by other methods and on other instruments. We discuss some systematic effects that can affect our visual measurements and their precision. Taking account of a zenith distance effect provides for the semi-diameter a mean value closer to the value of the ephemeris. Our observations also reveal deviations around the mean diameter in the royal zones and for high heliographic latitudes; their amplitudes reaching as much as 0.08". Finally, semi-diameter variations appear in our series; their origin is unknown but they may possibly be related to observed variations of magnetic activity or other solar parameters.

No abstract

No abstract

Abstract. Observing the Solar disk is a challenge and, as for the past visual observations, we have many results depending on observers and/or instruments. This was due to the differences in visual perceptions of the Sun's limb, instrumental errors and atmospheric disturbances. After a long series of visual observations at Calern Observatory, Francis Laclare felt the need for more impersonal and automatic measurements of the Solar diameter. After a series of analog CCD measurements (1989)(1990)(1991)(1992)(1993)(1994)(1995), a digital data acquisition and processing was tested by the Paris Observatory group (F. Chollet and V. Sinceac) during the 1996 spring at Calern Observatory. Before starting a new continuous campaign of observations, to confirm eventual variations of the diameter and solar flatness, the aim is to find the best definition of the solar edge. The test campaign was spent comparing different solutions that were tried on two different astrolabes at Calern Observatory: The "classical" one, outfitted with eleven zerodur ceramic prisms (S astrolabe), that has been used for twenty years in the Laclare series and on the other hand an instrument equipped with a varying angle prism (V astrolabe) enabling many measurements (385 in 1996) for perfecting the know how.This article focusses on acquisition techniques and their feasibility. Two procedures were tried: The first one used alternately the direct and reflected images (separated using a revolving shutter in front of the objective) and the second one mathematically sorts out both components inside the computer (an image being a two-dimensionral array of numbers). According to the principle of the astrolabe, the measured quantity is the exact time crossing the parallel of altitude (defined by the prism angle) by the Sun's edge, i.e. the time of merging of the two images of the Sun in the focal plane of the telescope where the Send offprint requests to: F. Chollet CCD matrix stands. Here comes the definition of the Solar edge for one frame as the collection of the inflect points on the luminosity function along each of the 256 useful lines (the matrix is 512 by 512 pixels). This means that a numerical derivation is performed on every other line of the CCD video camera which has to stand as vertical as possible. Then, for every frame, and through the 256 points, a parabola is fitted, using the least squares method. The top of this parabola materializes the prospective characteristic point. The sets of such points associated with the corresponding times of acquisition, are collected for both images and the exact time of contact of the two images may be obtained. This time is also the time when the solar edge crosses the almucantar.The results for the semi-diameter obtained during 1996 campaign are derived from sixty measurements with the revolving mask and sixty seven without it, performed on the Solar Astrolabe. They give a mean value of 959. 39 ± 0. 03 with a scatter of 0. 29.It is interesting to remark that the values of the error bar and the scatter ob...

Abstract. In January 1997, at the Observatório Nacional (ON), Rio de Janeiro, Brazil, CCD observations of the solar diameter with a Danjon astrolabe equipped with a variable angle reflecting prism began. This prism is operated manually and enables to observe the Sun many times per day and all year round on the Southern latitude ϕ = −22• 54 . Thanks to this and owing to a large number of sunny days at Rio de Janeiro, a high density of observations was achieved -2900 East and 1730 West observations during the first 18 months.Results of the observations are available in electronic form in the SIMBAD database and at the World Wide Web page of the Observatório Nacional, Solar Radius Data: http://obsn.on.br/radius/.