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The measurement of the solar diameter is introduced in the wider framework ofsolar variability and of the influences of the Sun upon the Earth's climate.Ancient eclipses and planetary transits would permit to extend the knowledge ofthe solar irradiance back to three centuries, through the parameterW=dLogR/dLogL. The method of Baily's beads timing during eclipses is discussed,and a significant improvement with respect to the last 40 years has beenobtained by reconstructing the Limb Darkening Function's inflexion point fromtheir light curve and the corresponding lunar valleys' profiles. The case ofthe Jan 15, 2010 annular eclipse has been studied in detail, as well as thelast two transits of Venus. The atlas of Baily's beads, realized with worldwidecontributions by IOTA members is presented along with the solar diameter duringthe eclipse of 2006. The transition between the photographic atlas of the lunarlimb (Watts, 1963) and the laser-altimeter map made by the Kaguya lunar probein 2009 has been followed. The other method for the accurate measurement of thesolar diameter alternative to the PICARD / PICARD-sol mission is the drift-scanmethod used either by the solar astrolabes either by larger telescopes. Theobservatories of Locarno and Paris have started an observational program of theSun with this method with encouraging results. For the first time an imagemotion of the whole Sun has been detected at frequencies of 1/100 Hz. This maystart explain the puzzling results of the observational campaigns made inGreenwich and Rome from 1850 to 1955. The meridian line of Santa Maria degliAngeli in Rome is a giant pinhole telescope and it permits to introducedidactically almost all the arguments of classical astrometry here presented.The support to the PICARD mission continues with the analyses of the transit ofVenus and the total eclipse of 2012.

High precision ground-based measurements of solar diameter in support of PICARD mission