September 25, 1644 – Birth of Ole Rømer, the first to determine the speed of light

During the reign of Louis XIV, his minister Colbert, was convinced of the importance of France becoming the first scientific power and, with apparently unlimited funds, he managed to get prestigious scientists such as Christiaan Huygens, Giovanni Doménico Cassini, and Jean Picard, to join the project. Louis XIV appointed Ole Rømer guardian of the Dauphin and made him part of the construction of the magnificent fountains of Versailles.

In 1676, when Rømer and Cassini made observations of Jupiter's first satellite at the Paris observatory, they saw that eclipses with their moons differed from their predicted positions; specifically, they were anticipated when the Earth was approaching the planet and were delayed when it was moving away. Rømer assumed that this was due to the finite time it took light 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 light 22 minutes to cross the diameter of Earth's orbit (although modern estimates have managed to fit the approximation to 17 minutes). He later estimated that the time It takes 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 about 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 a 75% error concerning its real value because of the inaccurate knowledge of the planetary distances in those times). In this way, he became the first to make the first quantitative measurements of the speed of light.

Under the influence of Rømer, the Gregorian calendar was introduced in Denmark in 1701 and, that same year, he devised a temperature measurement scale called "Rømer Degree", which is now out of use.

He was appointed director of the Copenhagen Observatory (Denmark), where he built the first meridian telescope to determine longitude based on the fact that when the same body passes through the meridian of the place at two different points on Earth, the difference temporal can be translated into degrees of longitude.

If you want to know more about this scientist, click on the following link: Ole Christensen Rømer