Rømer went to Paris in 1672, where he spent nine years working at the Royal Observatory. While there he noticed that the time elapsed between eclipses of Jupiter's moons by Jupiter became shorter as Earth moved closer to Jupiter and became longer as Earth and Jupiter drew farther apart. He correctly deduced that this phenomenon was caused by the time needed for light to cross the increased distance between the two planets and in 1676 announced that, according to his observations, the speed of light was 140,000 miles (225,000 km) per second. (Modern measurements have established a figure of 186,282 miles, or 299,792 km, per second.)

In 1679 Rømer went on a scientific mission to England, where he met Sir Isaac Newton and the astronomers John Flamsteed and Edmond Halley. Upon his return to Denmark in 1681, he was appointed royal mathematician and professor of astronomy at the University of Copenhagen. At the university observatory he set up an instrument with altitude and azimuth circles and a telescope, which accurately measured the position of celestial objects. He also held several public offices, including that of mayor of Copenhagen in 1705.