Transits of Venus, Solar Diameter and Sky Transparency

The transit of Venus of 2012 observed at Huairou Solar Station was the second event of a couple occurring each 105/122 years: 8 June 2004 and 6 June 2012. The previous observed ones occurred in 1639; 1761-69 and 1874-82, the next ones will be on 10-11 December 2117and 8 December 2125, a whole century from now. The angular dimension of Venus at transit is nearly 6 times the one of Mercury and it allows a better determinations of the chords described with the solar limb at the contacts, through which we obtain the duration of the transit to the uppermost accuracy possible on the Earth, overcoming the black-drop phenomenon by chords’ extrapolation to zero lenght. We consider the influence of clouds’ veils on the measure of the solar diameter as new insights on the 2012 transit’s observations. The ingress and egress instants have been recorded with a Schmidt-Cassegrain telescope specifically prepared to this unique event, and located ontop the Huairou solar tower. The meteorological conditions at Huariou were hazy during the transit and we present an observational study conducted at the Meridian line of St. Maria degli Angeli in Rome, on the influence of the haze to the measure of the solar diameter. Being the transits of Mercury and Venus so rare, their observations have been tempted with all weather conditions, exactly as the first historical observations report, so our study can be exploited in re-analyzing all historical transits.