Ole Christensen Rømer was a Danish astronomer who, in 1676, made the first measurement of the speed of light and discovery that light travels at a finite speed. Rømer also invented the modern thermometer showing the temperature between two fixed points, namely the points at which water respectively boils and freezes.
Ole Rømer, portrait by Jacob Coning from c. 1700
Rundetårn ("round tower") in Copenhagen, on top of which the university had its observatory from the mid 17th century until the mid 19th century when it was moved to new premises. The current observatory there was built in the 20th century to serve amateurs.
Ole Rømer at work
The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second. According to the special theory of relativity, c is the upper limit for the speed at which conventional matter or energy can travel through space.
One of the last and most accurate time of flight measurements, Michelson, Pease and Pearson's 1930–1935 experiment used a rotating mirror and a one-mile (1.6 km) long vacuum chamber which the light beam traversed 10 times. It achieved accuracy of ±11 km/s.
Hendrik Lorentz (right) with Albert Einstein (1921)
