Transits of Venus, Solar Diameter and Sky Transparency

The transits of Venus occur in couples each 105/122 years: the observed ones are 1639; 1761-1769; 1874-1882, and 2004-2012. The next couple will be in 2117-2125. We would need all four contacts to determine the solar diameter accurately. The black-drop phenomenon blurred always the internal contacts, so we developed a parabolic analysis of the chords drawn by the disk of Venus on the solar limb. The extrapolation to zero gives the contact timings. We tested this method with some high quality images obtained in 2004 and 2012, and we applied it to the observations of 2012 in visual band (Huairou Solar Observing Station, hazy weather) and H-alpha (Shen Zen Astronomical Observatory). To exclude a reduction of the measured diameter by the haze, we made two series of measures at the Clementine Gnomon (Rome) and at PHYSIS telescope (Rome), under various sky transparencies and with diffraction limited instruments. The haze, and the low altitudes above the horizon reduced the accuracy at all first contacts examined, without changing the solar diameter. Our measures obtained in China during the transit of 2012 yielded a photospheric radius R¤P=959.33”±0.06”, based on 76+75 images diffraction limited; compatible with the chromospheric radius measured at the base of the spiculae was R¤C=959.78”±0.11”, relying on 7+5 diffraction-limited series of images.