Ole Rømer (1644 - 1710) was a Danish astronomer, engineer and commissioner of police, and the first person to measure a fundamental physical constant, namely the speed of light, in 1676. "Hesitation of Light” was a sensation, which was not immediately accepted in the scientific community.
Ole Rømer was born in Aarhus, and he graduated from Aarhus Katedralskole and the University of Copenhagen. He was involved in a publication of Tycho Brahe's observation journals, and assisted in locating Brahe’s observatory on the island of Hven. His work helped interpret Brahe’s observations more accurately. Although Ole Rømer accomplished quite a lot during his lifetime - he introduced the Gregorian calendar in Denmark, and directed the Round Tower Observatory - his determination of "the hesitation of light" is the achievement, he is remembered for.