who discovered the speed of light

The speed of light wasn’t “discovered” in one go by a single person, but the key breakthrough in measuring it is usually credited to the Danish astronomer Ole Rømer in 1676.

Quick Scoop

• Ancient philosophers debated whether light’s speed was infinite, but had no way to test it.
• In the early 1600s, Galileo tried timing lantern flashes between hills; he could only conclude that light, if not instantaneous, was extremely fast.
• Ole Rømer, in 1676, was the first to show convincingly that light has a finite, measurable speed using Jupiter’s moon Io.
• Christiaan Huygens used Rømer’s data to calculate an actual numerical value for the speed of light (in the right ballpark, though not exact).
• In the 1800s, Fizeau and Foucault did precise lab measurements with rotating toothed wheels and mirrors, refining the value.
• By the late 1800s to early 1900s, Maxwell’s electromagnetism and Einstein’s relativity turned the speed of light into a fundamental constant of nature, not just an astronomical curiosity.

Who “discovered” the speed of light?

If you’re looking for one main name for “who discovered the speed of light” in the sense of proving it has a finite speed and estimating it:

• Ole Rømer (1676) is the key figure:
  • He observed eclipses of Jupiter’s moon Io.
  • He noticed the timing of these eclipses shifted depending on whether Earth was moving toward or away from Jupiter in its orbit.
  • From the changing delay, he inferred that light takes time to travel — so its speed is finite — and estimated a value that’s within the right order of magnitude.

Because of this, most historians of science say Rømer was the first to measure the speed of light in a quantitative way and thus effectively “discovered” it as a finite, measurable speed. Information gathered from public forums or data available on the internet and portrayed here.