September 25, 1644, in Aarhus (Denmark) – September 19, 1710, in Copenhagen (Denmark)
In 1662, he graduated from Aarhus Kahedralskole, then moved to Copenhagen and enrolled at the University of Copenhagen.
In 1672, thanks to the support of Jean Picard (an astronomer who made the first measurement of a meridian in France, which provided a very accurate measurement of the Earth's radius), he went to France to enter the Paris Academy of Sciences, which had become created 6 years earlier.
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.
As a royal mathematician, he introduced the first national system of weights and measures in Denmark on May 1, 1683.
Under the influence of Rømer, the Gregorian calendar was introduced in Denmark in 1701. That same year, he devised a temperature measurement scale called “Rømer Degree”, which is now in disuse.
Almost all of his manuscripts were lost in the terrible fire that destroyed the Copenhagen Observatory on October 20, 1728.
He was appointed director of the Copenhagen Observatory (Denmark), where he built the first meridian telescope with the aim of determining longitude from the fact that when the same star passes through the local meridian at two different points on Earth, the difference temporal can be translated into degrees of longitude.
In addition, he invented an ingenious micrometer, widely used at the end of the 17th century in the observation of eclipses, with which the image of the Sun or the Moon could be increased or decreased until they were between two threads located near the eyepiece.
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