Transits of Venus 2004 and 2012 and solar diameter from ground. Methods, results and perspectives from mercury transit of 2016

The variation of the solar diameter in time and in position angle has implication in astrophysics and in general relativity, as the long series of studies attest. The transits of venus in 2004 and 2012 have been carefully studied because of the rarity of the phenomenon, its historical importance, the opportunity to probe the venus' atmosphere and the diameter of the sun. Observation to the milliarcsecond level of precision have been realized also from satellite images. The results of the solar diameter measurements made with the observations in athens (2004) and at the hariou solar observing station in china (2012) are here presented along with the perspective for the transit of mercury of may9, 2016.