Ole Rømer estimated the speed of light in 1676. He was the first.

See this clip featuring Neil deGrasse Tyson. He begins by telling us that Galileo was the first one to attempt to measure the speed of light. He only concluded that light was either infinitely fast or it was faster than we could measure.

He then mentions a Danish astronomer named Ole Rømer. Wikipedia:

Rømer's determination of the speed of light was the demonstration in 1676 that light has an apprehensible, measurable speed and so does not travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer,[note 1] who was working at the Royal Observatory in Paris at the time.

By timing the eclipses of Jupiter's moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun.[1] Using modern orbits, this would imply a speed of light of 226,663 kilometres per second,[2] 24.4% lower than the true value of 299,792 km/s.[3] In his calculations Rømer used the idea and observations that the apparent time between eclipses would be greater while the Earth is moving further from Jupiter and lesser while moving closer.

Rømer's theory was controversial at the time that he announced it and he never convinced the director of the Paris Observatory, Giovanni Domenico Cassini, to fully accept it. However, it quickly gained support among other natural philosophers of the period such as Christiaan Huygens and Isaac Newton. It was finally confirmed nearly two decades after Rømer's death, with the explanation in 1729 of stellar aberration by the English astronomer James Bradley.

Remarkable. It shows the value of having the right conceptual ontology for thinking about the world. Which ontology are we talking about? I assume it's the one shared by Galileo, Huygens, Newton, and Rømer, among others. What aspect of this ontology allowed for, made possible, these observations? Surely it is bound up in the mechanical properties of natural phenomena. I'm guessing (I've not thought it through) that it would have been impossible to state these mechanical properties without decimal-point arithmetic. This is Rank 3 in the account of cultural evolution that David Hays and I elaborated in a series of papers (along with Hays's book on technology) in the 1990s.