A Review of Secchi's Contribution to Marine Optics and the Foundation of Secchi Disk Science

Angelo Secchi was an astrophysicist and a pioneer in the use of spectroscopy in that field and also contributed to meteorology and geodesy. His only contribution to oceanography was on the physics of the "Secchi disk," a simple device that provides a measure of water transparency. While the Secchi disk was widely adopted after his death, the cruise report Secchi wrote in 1865 has unfortunately remained ignored, despite addressing many crucial questions in marine optics, such as how the angle of the sun, the disk's color and directional reflectance, the disk's diameter, the ship's shadow, and cloudiness influence the transparency measurements. This article aims to correct these oversights by calling attention to the scientific contributions of Angelo Secchi to optical oceanography. It shows how his training as an empirical physicist, his meticulousness, and his dedication enabled him to establish Secchi disk science.