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The measurement of the Sun's diameter has been first tackled by the Greek astronomers from a geometric point of view. Their estimation of ~1800", although incorrect, was not truly called into question for several centuries. The first…
The solar diameter changes or not? Whatever will be the answer the methods used for its measurements are more and more challenging, and facing new astrophysical and optical problems since the required space resolution is of astrometric…
The uncertainty of measurement of solar diameter is depending on the observational time scale. Full-disc images of SDO/HMI and the images from ground observations in Huairou Solar Observing Station have been analyzed to get the values of…
The size of the diameter of the Sun has been debated for a very long time. First tackled by the Greek astronomers from a geometric point of view, an estimate, although incorrect, has been determined, not truly called into question for…
Solar astrometry deals with the accurate measumerent of the solar diameter, and in general with the measurement of the shape of the Sun. During the last decades several techniques have been developed to monitor the radius and the irradiance…
The variation of the solar diameter is the subject of hot debates due to the possible effect on Earth climate and also due to different interpretations of long period solar variabilities, including the total solar irradiance. We shortly…
The modern methods of measurement of the solar diameter and oblateness are reviewed. Either ground-based or balloon-borne and satellite measurements are considered. The importance of solar astrometry for General Relativity is emphasized,…
Two methods are used to observationally determine the solar radius: One is the observation of the intensity profile at the limb, the other one uses f-mode frequencies to derive a 'seismic' solar radius which is then corrected to optical…
The unprecedented precision of broadband stellar photometry achieved with the planet-hunting missions CoRoT and \textit{Kepler} initiated a new era in examining the magnetically-driven brightness variations of hundreds of thousands of…
The Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory observed the transits of Mercury on 2003 May 7 and 2006 November 8. Contact times between Mercury and the solar limb have been used since the 17th century to…
The questions whether the Sun shrinks with the solar activity and what causes this have been a subject of debate. Helioseismology provides means to measure with high precision the radial displacement of subsurface layers, co-called "seismic…
The Total Solar Irradiance varies over a solar cycle of 11 years and maybe over cycles with longer period. Is the solar diameter variable over time too? We introduce a new method to perform high resolution astrometry of the solar diameter…
The high accuracy reached by solar limb observations, by helioseismic measurements and by Standard Solar Models (SSMs) calculations suggests that general relativity corrections are included when discussing the solar radius. The Allen value…
Why does the Sun have a radius around 696000~km? We will see in this article that dimensional arguments can be used to understand the size of the Sun and of a few other things along the way. These arguments are not new and can be found…
The Sun has long been considered a constant star, to the extent that its total irradiance was termed the solar constant. It required radiometers in space to detect the small variations in solar irradiance on timescales of the solar rotation…
The Sun emits radiation at several wavelengths of the electromagnetic spectrum. In the optical band, the solar radius is 695,700 km and this is what defines the photosphere, the visible surface of the Sun. However, as the altitude…
Several group in the World followed the solar diameter with dedicated instruments, namely solar astrolabes, since 1974. Their data have been gathered in several observing stations connected in the R2S3 (Reseau de Suivi au Sol du Rayon…
A sunset over the sea surface offers the possibility to chronometrate a solar transit across the horizon. The vertical solar diameter is proportional to the duration of the sunset, the cosine of the azimuth and the cosine of the latitude of…
The Sun is an extraordinary workbench, from which several fundamental astronomical parameters can be measured with high precision. Among these parameters, the solar radius $R_{\odot}$ plays an important role in several aspects, such as in…
The balloon-borne Solar Disk Sextant (SDS) experiment has measured the angular size of the Sun on seven occasions spanning the years 1992 to 2011. The solar half-diameter -- observed in a 100-nm wide passband centred at 615 nm -- is found…