1644-1710. As a student living in Rasmus Bartholin's house, Rømer was given the task of preparing an edition of Tycho Brahe's works, for which reason he accompanied the French astronomer Jean Picard to Paris in 1672.
Here, in 1676, he demonstrated that certain irregularities in the timing of the eclipses of the moons of Jupiter could be explained as the result of light's "hesitating" or its finite velocity, which has since emerged as the most fundamental natural constant in physics.
After his return to Denmark in 1681 he reorganised the observatory in the Round Tower at the same time as undertaking various public duties which included surveying the Danish road network, establishing a unified system of weights and measures, and introducing the Gregorian calendar in 1700.
Long before Celcius, Rømer was using the freezing and boiling points of water as natural fixed points on thermometers. In his private observatories, Rømer first developed the transit instrument and then the meridian circle, which 100 years later became astronomy's most accurate instrument for measuring position.
Apart from a notebook called Adversaria and three days' observations from 1706, all Rømer's scientific notes were lost in the Fire of Copenhagen in 1728.
Olaf Pedersen, Gyldendal Leksikon