  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 several centuries. The French school of astronomy, under the
impetus of Mouton and Picard in the XVIIth century can be considered as a
pioneer in this issue. It was followed by the German school at the end of the
XIXth 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 the
second half of the XIXth century leading to controversial results mainly due to
the difficulty to disentangle between the solar and atmospheric effects.
Dedicated space measurements yield to a very faint dependence of the solar
diameter with time. New studies over the entire radiation spectrum lead to a
clear relationship between the solar diameter and the wavelength, reflecting
the height at which the lines are formed. Thus the absolute value of the solar
diameter, which is a reference for many astrophysical applications, must be
stated according to the wavelength. Furthermore, notable features of the Near
Sub-Surface Layer (NSSL), called the leptocline, can be established in relation
to the solar limb variations, mainly through the shape asphericities
coefficients. The exact relationship has not been established yet, but recent
studies encourage further in-depth investigations of the solar subsurface
dynamics, both observationally and by numerical MHD simulations.
