Danish astronomer Ole Rømer born on Sept 24, 1644
Ole Rømer, Danish astronomer, engineer, and civil servant, was born September 25, 1644 in Aarhus, Denmark–Norway.
His proof on the speed of light and instrument making regarding the meridian circle, Rota Meridiana are among his best known works.
Ole Rømer
Issued by Denmark in 1944
Designer: Johannes Britze
UNESCO’s page on Ole Rømer and his observatory is an excellent beginning for anyone looking to read more about this ground breaking scientist. Here is a small snippet from the site:
Ole Christensen Rømer (1644–1710) studied astronomy in Copenhagen, 1662–1671. Jean Picard (1620–1682) invited him to Paris, where he worked from 1672 to 1681 at the new observatory of the Académie Royale des Sciences (micrometer 1572, mechanical models for planetary orbits, including a Jovilabium, 1677, a Lunarium 1680).
Ole Christensen Rømer (1644–1710) is known for his spectacular discovery in Paris, in 1676, that the speed of light is finite, which he measured with astonishing precision — by observing the Jupiter moons: he saw that light arrives from Jupiter with a delay depending on the distance to Jupiter.
Already Galilei had proposed a method, based on the times of the eclipses of the four moons of Jupiter, using the Jovian system as a cosmic clock in order to solve the longitude problem.As a professor of mathematics in Copenhagen from 1681 he unified weights and measures.
His most important instrumental achievement was the development of the transit instrument … and the meridian circle (1704), an innovative accurate measuring instrument for star positions. Rømer wanted to measure the parallax precisely in order to prove Copernicus’ (1473–1543) heliocentric world view.
Rømer was also a very careful observer, who discussed instrumental errors long before this was normal like in the 19th century. Especially, Rømer derived a measurement error formula for his meridian instrument in his Adversaria (1705). Today this measurement error formula is attributed to Tobias Mayer (1723–1762), who found it 51 years later, i.e. in 1756, without knowing Rømer’s derivation.
https://web.astronomicalheritage.net/show-entity?identity=139&idsubentity=1
The University of Copenhagen has an solid section on Rømer as well: https://nbi.ku.dk/english/www/roemer/roemer
A few SODs to enjoy. A short read and a longer one. So grab a coffee and enjoy.