Ole Rømer, Danish astronomer and instrument maker (b. 1644)
Ole Christensen Rømer (Danish: [ˈoːlə ˈʁœˀmɐ]; 25 September 1644 – 19 September 1710) was a Danish astronomer who, in 1676, made the first quantitative measurements of the speed of light.

Rømer also invented the modern thermometer showing the temperature between two fixed points, namely the points at which water respectively boils and freezes.

In scientific literature, alternative spellings such as "Roemer", "Römer", or "Romer" are common.