  The possibility to measure the solar diameter using the transits of Mercury
has been exploited to investigate the past three centuries of its evolution and
to calibrate these measurements made with satellites. This measurement
basically consists to compare the ephemerides of the internal contact timings
with the observed timings. The transits of Venus of 2004 and 2012 gave the
possibility to apply this method, involving a planet with atmosphere, with the
refraction of solar light through it creating a luminous arc all around the
disk of the planet. The observations of the 2012 transit made to measure the
solar diameter participate to the project Venus Twilight Experiment to study
the aureole appearing around it near the ingress/egress phases.
