In the absence of reliable clocks, the observation of the jovian satellites was one of the methods which enabled one to determine longitudes, an indispensable measurement for the development of maritime exchanges.
Through this work, the Paris Observatory astronomers Jean-Dominique Cassini and Ole Rømer found that the predictions in Cassini’s tables for the motion of the first jovian satellite Io did not agree with observation.
To explain this deviations, they postulated in 1676 they were due to the time that light akes to reach us, which is variable as it depends on the distance between Jupiter and the Earth.
He deduced from this that light took between 10 and 11 minutes to cross the radius of the Earth’s orbit (i.e. the distance of the Earth to the Sun) but he made no attempt to calculate its speed.
"Light needs time to cross that distance"
While this was Rømer’s hypothesis, , Cassini, followed by the Academy of Sciences, was not so quick to abandon other, competing hypotheses, such as the eccentricity of the satellite, or some irregularity in its smotion.
Christian Huygens agreed with Rømer and while in Paris, where he worked until 1681, was the first to calculated the speed of light. He estimated its value as 600 000 times that of the speed of sound (i.e. in today’s units, as 230 000 km per second), a result published in 1690 in his /Traité de la lumière/.