Sept. 25, 1644: The birth of Ole Roemer

Today in the history of astronomy, the first person to measure the speed of light is born. | Published: September 25, 2026 A 1735 copperplate engraving published after Ole Roemer's death shows him at work in his home observatory in Copenhagen. Credit: Horrebows Basis Astronomiæ/Public domain via Wikimedia Commons On Sept. 25, 1644, Ole Roemer was born in Aarhus, Denmark. Beginning in 1662, he studied at the University of Copenhagen with physicist Erasmus Bartholin, who tasked him with editing the manuscripts of Tycho Brahe; then, after a 1672 move to Paris, he spent eight years at the Paris Observatory and tutored the crown prince of France in astronomy. But in 1676 came the scientific achievement for which he’d be best known: In his extensive measurements of the motions of the Jupiter moon Io, Roemer noticed discrepancies over the course of a year. He realized this was due to differences in the amount of time it took for light to reach Earth when Jupiter was nearer or farther away. He became the first to quantitatively measure the speed of light, calculating it at around 220,000 kilometers per second and countering the beliefs of the day that the speed of light was infinite. (Today we know the speed of light to be 299,792.458 km per second.) Roemer returned to Denmark in 1681 and went on to a long career in the sciences, public works, and law, before passing away in 1710. Elisa Neckar is senior production editor of Astronomy magazine, and the editor of the Today in the History of Astronomy feature on Astronomy.com.