The speed of light was first measured with any accuracy by the Danish astronomer Ole Rømer in 1676. Rømer was studying the motion of Jupiter’s moon, Io, and noticed that the timing of its orbits appeared to change depending on Earth’s position relative to Jupiter.
At certain points in Earth’s orbit, the moons appeared to move slower, and at other points, faster. Rømer realized this discrepancy was due to the time it took for light to travel from Jupiter to Earth. He proposed that light had a finite speed and that the apparent delay was due to the time light took to reach Earth from Jupiter.
Rømer estimated that light took about 22 minutes to travel a distance equivalent to Earth’s diameter. This was a rough measurement, but it was the first solid indication that light had a finite speed.
Later, in the 19th century, other scientists, such as Albert Michelson in the late 1800s, refined the measurement using more precise methods. Michelson’s work, which won him the Nobel Prize in Physics in 1907, provided an accurate value for the speed of light—approximately 299,792 kilometers per second (186,282 miles per second)—a figure that remains accepted today.