The question of whether light travels instantaneously or has a finite speed puzzled scientists for centuries. Ancient philosophers such as Aristotle believed that light’s motion was immediate, while others, including Empedocles, speculated that it might take time to travel. The first convincing measurement of the speed of light came in 1676, when the Danish astronomer Ole Rømer made a remarkable observation using Jupiter’s moon Io.
Rømer noticed that the timing of Io’s eclipses by Jupiter varied depending on Earth’s position in its orbit. When Earth was moving away from Jupiter, the eclipses appeared slightly delayed, and when moving towards Jupiter, they occurred earlier than expected. From these discrepancies, Rømer concluded that light must have a finite speed. Although his estimate was imprecise by modern standards, it was the first empirical evidence that light does not move instantaneously.
In 1728, the English astronomer James Bradley provided further proof through his discovery of stellar aberration. As Earth moves around the Sun, the apparent positions of stars shift slightly, an effect that can only be explained if light travels at a finite speed. These early astronomical measurements established the concept of a measurable light velocity but lacked precision.
The first successful laboratory measurements came much later. In 1849, the French physicist Hippolyte Fizeau used a rotating toothed wheel and a distant mirror to measure the time it took light to travel to the mirror and back. A year later, Léon Foucault improved the method by using a rotating mirror, obtaining a more accurate value close to the modern figure of approximately 300,000 kilometres per second.
In the 1860s, James Clerk Maxwell’s electromagnetic theory provided the theoretical foundation. His equations predicted that electromagnetic waves travel through space at a fixed speed equal to the measured speed of light, showing that light itself is an electromagnetic wave. This discovery suggested that the speed of light was not only finite but constant – a universal property of nature that would later become the cornerstone of Einstein’s theory of relativity.