Solar Radius Determination From Sodism/Picard and Hmi/Sdoobservations of the Decrease of the Spectral Solar Radiance During the 2012 June Venus Transit

On 2012 June 5–6, the transit of Venus provided a rare opportunity to determine the radius of the Sun using solar imagers observing a well-defined object, namely, the planet and its atmosphere, partially occulting the Sun. A new method has been developed to estimate the solar radius during a planetary transit. It is based on the estimation of the spectral solar radiance decrease in a region around the contact between the planet and the Sun at the beginning of the ingress and at the end of the egress. The extrapolation to zero of the radiance decrease versus the Sun-to-Venus apparent angular distance allows estimation of the solar radius at the time of first and fourth contacts. This method presents the advantage of being almost independent on the plate scale, the distortion, the refraction by the planetary atmosphere, and on the point-spread function of the imager. It has been applied to two space solar visible imagers, SODISM/PICARD and HMI/ SDO . The found results are mutually consistent, despite their different error budgets: 959 85 ± 0 19 (1σ) for SODISM at 607.1 nm and 959 90 ± 0 06 (1σ) for HMI at 617.3 nm.