Ole Rømer and
Speed of Light
While his 17 -century contemporaries were debating the nature of light,
Ole Rømer was busy measuring its velocity. is little-known Danish
scientist was the first to determine that light moves at a finite speed.
n the 1670s, light was a popular topic of scientific inquiry. Natural philosophers did
not know what light was made of, but they knew it when they saw it. In England
at that time, Royal Society members Isaac Newton and Robert Hooke were bitterly
debating whether light was a stream of particles or an ethereal wave.
Meanwhile, elsewhere in Europe, another aspect of light was just beginning to be
explored. A Danish astronomer working with decades of careful solar-system observations
published his discovery that light—whatever its form—travels at a finite, measurable speed.
Although we take that fact for granted today, it was a groundbreaking concept in the 17
century. e prevailing view was that light did not travel at all; it simply existed.
Ole Christensen Rømer, a Dane educated at the University of Copenhagen, used the
movements of Jupiter’s moons to show that that wasn’t the case. Although Rømer arrived at
a highly imprecise figure—and some say that he only placed a lower limit on the velocity
at which light can travel—he laid the groundwork for a major paradigm shift in the way
scientists think about light and its properties.
Rømer wasn’t aiming to make a scientific breakthrough that would reverberate through
the ages. He and his co-workers had a far more pedestrian goal in mind: to measure European longitudes more accurately. e discovery of the velocity of light was more or less a
by-product of the e ort to create better maps.