From Galileo, we wonder about the instantaneous character of the speed of light. However, the Parisian astronomers, who were studying the motions of the Jovian satellites in detail, had noted that the eclipses of one of them, Io, by the planetary shadow took place later as the Earth moved away from Jupiter and earlier when the planet approached the Earth. Cassini suggested that this effect could be due to the finite velocity of the light as it travelled from Jupiter to the Earth. However, the effect was not evident for the other satellites of Jupiter, since it was masked by the irregularities of their motions. Consequently, he lost faith in his own explanation. But it was successfully proved by the academician Olaus Rømer (1644-1710), who presented it to the Academy and then published it in the Journal des Sçavans of December 1676.

caption : Demonstration of Rømer - copyright : Observatoire de Paris