Detection of the Rossiter-McLaughlin effect in the 2012 june 6 venus transit

Eclipsing bodies on stars produce radial velocity variations on the photospheric stellar lines known as the Rossiter-McLaughlin (RM) effect. The body occults a small area of the stellar disc and, due to the rotation of the star, the stellar line profiles are distorted according to the projected location of the body on to the stellar disc. The effect originally observed in eclipsing binaries was also shown to be produced by extrasolar planets transits. Here we report the detection of the RM effect in the Sun due to the Venus transit of 2012 June 6. We used the integrated sunlight as reflected by the Moon at night time to record part of the transit by means of the high-precision HARPS spectrograph at the 3.6-m La Silla European Southern Observatory (ESO) telescope. The observations show that the partial Venus eclipse of the solar disc in correspondence of the passage in front of the receding hemisphere produced a modulation in the radial velocity with a negative amplitude of ≈-1 m s^-1, in agreement with the theoretical model. The radial velocity change is comparable to the solar jitter and more than a factor of 2 smaller than previously detected in extrasolar hot Neptunes. This detection, facilitated by an amplification factor of 3.5 of the Venus radius due to proximity, anticipates the study of transits of Earth-size bodies in solar-type stars by means of a high-resolution spectrograph attached to a 40-m class telescope.