Annibale Riccò and the catoptric proof of the Earth’s curvature

Phenomena as common as the daily rising or setting of the Sun can reveal, upon careful examination, something surprising and unexpected, which has been attracting scientific attention belatedly and had to wait long before being explained. From systematic observation of sunrises over the marine horizon, the Italian astronomer Annibale Riccò (1844–1919) unexpectedly claimed to have recognized a new and novel evidence of the Earth’s roundness, scrutinizing distorted images of the Sun reflected off waters. Others had previously come to a similar conclusion, but it was Riccò’s pioneering work that brought this “catoptric” proof of the Earth’s curvature to the scientific limelight of his time. The interesting debate ensued gave rise to more articulated interpretations of Riccò’s observations, including recovery of olden “dioptric” explanations. The catoptric model was eventually abandoned, as it does not adequately take into account the effects of atmospheric refraction and above all the observed phenomenon is not genuinely “reflective.” The argument is nevertheless correct in principle and retains historical and pedagogical interest.