Ole Rømer was a Danish astronomer known for being the first to determine the speed of light quantitatively.
During the reign of Louis XIV, his minister Colbert was convinced of the importance of France becoming the leading scientific power and, with apparently unlimited funds, he managed to get prestigious scientists such as Christiaan Huygens, Giovanni Domenico Cassini and Picard himself, join the project. Louis XIV appointed Ole Rømer tutor to the Dauphin and made him a part of the construction of the magnificent fountains at Versailles.
In 1676, when Rømer and Cassini made observations of Jupiter's first satellite at the Paris observatory, they saw that the eclipses with their moons differed from the predicted positions; specifically, they anticipated when the Earth approached the planet and delayed when it moved away. Rømer surmised that this was due to the finite time that light took to travel the (continuously variable) distance between Jupiter and Earth. With his telescope he carefully studied Jupiter's satellite Io and estimated that it took 22 minutes for light to cross the diameter of the Earth's orbit (although modern estimates have managed to adjust the approximation to 17 minutes). that it takes for light to cross the distance that separates the Sun from the Earth was 8 m 13 s, while his disciple fixed it at 14 m 10 s.
Taking this into account, using the recent estimate that Cassini had made of the distance to Jupiter and making precise calculations, he determined that the speed of light was 225,000 km/s (it is currently known that he made an error of 75% with respect to its real value because of the inaccurate knowledge of planetary distances in those times). In this way he became the first to make the first quantitative measurements of the speed of light.
If you want to know more about this scientist, click on the following link: Ole Christensen Rømer