Speed of light

Ole Rømer announced to the Paris Académie des Sciences in 1676 that an upcoming eclipse would occur ten minutes late. For a young, relatively unknown Danish astronomer to predict the heavens would misbehave was audacious—but on November 9, 1676, astronomers across Paris watched the eclipse of Jupiter's moon Io arrive exactly when Rømer said it would, not when orbital mechanics said it should. The universe had revealed that light takes time to travel.

Rømer had not set out to measure light's speed. He was compiling observations of Io's orbit at the Paris Observatory, hoping to create accurate tables for navigation. Galileo had discovered Jupiter's moons in 1610, and their regular eclipses behind the giant planet offered potential as a celestial clock. But Rømer noticed something peculiar: the intervals between eclipses grew shorter when Earth approached Jupiter in its orbit and longer when Earth receded. The discrepancy accumulated to about 22 minutes over the course of a year.

The dominant view, argued vigorously by Descartes, held that light traveled infinitely fast. If true, the interval between eclipses should remain constant regardless of Earth's position. Rømer's insight was that the anomaly arose because light had to travel farther when the planets were on opposite sides of the Sun. The 22-minute variation (the correct value is about 16.7 minutes) corresponded to light crossing the diameter of Earth's orbit—roughly 186 million miles.

The adjacent possible for this measurement had been assembling since Galileo's telescopic observations. Precise timekeeping had improved with Huygens's pendulum clocks. Systematic astronomical records over years allowed patterns to emerge. The Paris Observatory, founded in 1667, concentrated instrumentation and observers. What was missing was a mind that could look at timing anomalies and see, not orbital irregularities, but the finite speed of light itself.

Rømer never published an explicit velocity. Working backwards from his 22-minute figure yields approximately 227,000 kilometers per second—about 24% below the true value of 299,792 km/s. The error came from imprecise knowledge of Earth's orbital diameter and the eclipse timing itself. But the qualitative conclusion—that light travels at a finite, measurable speed—transformed physics.

The measurement techniques evolved convergently over two centuries. James Bradley confirmed Rømer's result in 1729 through stellar aberration—the apparent shift in star positions due to Earth's motion combined with light's finite speed. In 1849, Hippolyte Fizeau performed the first terrestrial measurement using a cogwheel that chopped a light beam into pulses sent to a mirror 8.6 kilometers away; he obtained 313,000 km/s, about 4.5% high. In 1862, Léon Foucault used a rotating mirror to achieve 298,000 km/s—within 0.6% of the modern value. Albert Michelson refined the technique from 1877 to 1930, ultimately approaching the correct value within 0.05%.

Each measurement method arose independently, suited to the technology of its era. Astronomical observations could detect light's cosmic delay across millions of miles. Mechanical devices could time terrestrial round-trips across kilometers. The convergence of different approaches on the same constant confirmed that something fundamental about the universe was being measured.

In 1905, Einstein made the speed of light the cornerstone of special relativity—not merely a measured constant but the universal speed limit, the same for all observers regardless of their motion. The number Rømer first glimpsed through Jupiter's moons became the c in E=mc², the conversion factor between mass and energy. Today, the meter itself is defined in terms of light speed: the distance light travels in 1/299,792,458 of a second. What began as an anomaly in eclipse timing ended as a foundational constant of physical law.