Experimental Determination of the Velocity of Light Made at the U.S. Naval Academy, Annapolis

In the late 1870s a young naval officer tackled the stubborn problem of measuring light’s exact speed. He found that earlier methods, like Foucault’s, produced deflections far too small for the precision astronomers required. His answer was a rapidly rotating mirror that enlarged the deflection hundreds offold.

A modest trial over a 500‑foot path with a 128‑rpm mirror gave a consistent result near 186,500 miles per second, though atmospheric blur limited clarity. A private benefactor then financed a larger arrangement along the Academy’s sea‑wall, extending the distance to 2,000 feet and installing a 1¼‑inch mirror reaching 250 revolutions per second together with an 8‑inch, 150‑foot focal‑length lens. At sunset the sharpened slit image steadied to within a hundredth of a millimeter, showing that Michelson’s technique could attain the ten‑thousandth‑part accuracy needed for celestial calculations.