Paris 1676: The Discovery of the Velocity of Light and the Roles of Rømer and Cassini

Abstract

It is often claimed that the 1676 discoveries at the Paris Observatory of a new irregularity in the orbit of Jupiter’s first satellite and of the velocity of light were not due to Rømer alone but that Cassini played a major role or even that Rømer took over these ideas from Cassini. These claims indirectly accuse Rømer and Cassini of dishonesty. We investigate the foundations of these allegations through an analysis of apparent contradictions in the primary, historical sources. The minutes from a meeting of the Academy on 22 August 1676 are especially important. Our analysis shows no reason to believe that Cassini took part in the discoveries, but his criticism incited Rømer to argue his case better. We conclude that the discoveries were due to Rømer alone and that the accusations against the two scientists of dishonesty are ill founded.