Measurements and variations of the solar diameter

Abstract

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.

A Virial Theorem Investigation of Magnetic Variations in the Sun

“…This agrees precisely with the Solar Disk Sextant (SDS) measurements of Egidi et al (2006). The long-running CERGA astrolabe measurements (Laclare et al 1996) give essentially the same result, .…”

“…As in the case of the ∼80 yr cycle, however, the results vary. The diameter appears to have changed either in phase with surface activity (Ulrich & Bertello 1995;Basu 1998;Emilio et al 2000;Noël 2004) or in antiphase with surface activity (Gilliland 1981;Sofia et al 1994;Laclare et al 1996;Li & Sofia 2001;Reis Neto et al 2003;Thuillier et al 2005;Egidi et al 2006). Some studies have detected no significant change at all (LaBonte & Howard 1981;Brown & Christensen-Dalsgaard 1998;Wittmann 2003;Kuhn et al 2004;Badache-Damiani & Rozelot 2006).…”

Apparent solar diameter and variability

“…This has enabled a careful study of the radius variations with different inclinations. It has been first investigated by Philippe Delache (1988) and more recently by Laclare et al (1996): the coherence is good between the radius variations at polar and equatorial inclinations (third point).…”

“…The most homogeneous time series obtained from an astrolabe is due to F. Laclare from Observatoire de la Côte d'Azur, France (Laclare et al 1996) but other measurements are available from astrolabes established in Belgrade (Ribes et al 1988) or Brazil (Leister and Benevides-Soares 1990). The diameter can also be a by-product of measurements of solar oblateness or more generally of the limb figure (obtained for example from solar images from space).…”

“…The second timing is proceeded when the reverse occurs. The radius is half the difference between the zenith distances computed at the times of the two crossings (Laclare et al 1996) and is represented on figure la.…”