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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, anestimate, although incorrect, has been determined, not truly called intoquestion for several centuries. The French school of astronomy, under theimpetus of Mouton and Picard in the XVIIth century can be considered as apioneer in this issue. It was followed by the German school at the end of theXIXth century whose works led to a canonical value established at 959".63(second of arc). A number of ground-based observations has been made in thesecond half of the XIXth century leading to controversial results mainly due tothe difficulty to disentangle between the solar and atmospheric effects.Dedicated space measurements yield to a very faint dependence of the solardiameter with time. New studies over the entire radiation spectrum lead to aclear relationship between the solar diameter and the wavelength, reflectingthe height at which the lines are formed. Thus the absolute value of the solardiameter, which is a reference for many astrophysical applications, must bestated according to the wavelength. Furthermore, notable features of the NearSub-Surface Layer (NSSL), called the leptocline, can be established in relationto the solar limb variations, mainly through the shape asphericitiescoefficients. The exact relationship has not been established yet, but recentstudies encourage further in-depth investigations of the solar subsurfacedynamics, both observationally and by numerical MHD simulations.

A brief history of the solar diameter measurements: a critical quality assessment of the existing data