← Back to Articles
Physics & Measurement
How Do We Even Measure the Speed of Light?
Sep 2026•7 min read•By Chetraj Jaishi
PhysicsSpeed of LightMeasurementHistory of ScienceOptics
“From Rømer's moons-of-Jupiter timing in 1676 to Michelson's rotating mirror, to modern laser interferometry — tracing the ingenious experiments that pinned down one of nature's fundamental constants.”
The speed of light is 299,792,458 m/s exactly — not approximately. Since 1983, the metre is defined in terms of c, so the number is fixed by definition. But how did we actually measure it before that? The story is a sequence of increasingly clever experiments spanning three centuries.
Rømer's observation (1676). Danish astronomer Ole Rømer noticed that the orbital period of Io (Jupiter's moon) appeared shorter when Earth moved toward Jupiter and longer when it moved away. He correctly attributed this to the finite travel time of light across the changing Earth-Jupiter distance. His estimate: ~220,000 km/s. Wrong by 26%, but the first proof that light has a finite speed.
Bradley's stellar aberration (1729). James Bradley observed that the apparent position of stars shifts slightly depending on Earth's velocity direction — like rain appearing to tilt when you walk through it. This stellar aberration gave c ≈ 301,000 km/s (off by ~0.4%), and crucially proved that light travels at a finite speed without needing a solar system baseline.
Fizeau's rotating cog wheel (1849). Hippolyte Fizeau sent a light beam through a gap in a rotating toothed wheel, reflected it off a mirror 8.6 km away, and adjusted the wheel speed until returning light passed through the next gap instead of being blocked by a tooth. From wheel rotation rate and gap geometry: c ≈ 313,300 km/s.
Foucault's rotating mirror (1862). Léon Foucault replaced the cog wheel with a rotating mirror. Light bounced off it, traveled to a distant fixed mirror, and returned to the rotating mirror — which had rotated slightly in the meantime, deflecting the return beam by a measurable angle. Result: c ≈ 298,000 km/s, accurate to within 0.6%.
Michelson's refinements (1879–1927). Albert Michelson spent decades refining the rotating mirror method, using longer baselines (up to 35 km between Mount Wilson and Mount San Antonio) and more precise angle measurements. By 1927 he had c = 299,796 ± 4 km/s.
Modern laser interferometry. By the 1970s, laser frequency could be measured to extraordinary precision using frequency chains. The wavelength of a stabilized laser was measured interferometrically, giving c = wavelength × frequency. By 1975 BIPM defined c = 299,792,458 m/s. In 1983 the metre was redefined as the distance light travels in 1/299,792,458 of a second — locking c by definition.
Why it matters. Every GPS satellite correction, every relativistic calculation in particle accelerators, every radar distance measurement depends on knowing c precisely. The history of measuring it is the history of building ever more precise timing and distance standards.