The Site Where Albert Michelson Measured the Speed of Light 145 Years Ago

For more than 175 years, the United States Naval Academy has educated future officers along the scenic shores of Maryland’s Severn River, just before it converges with the Chesapeake Bay. In the late 1870s, a young physics teacher, Albert A. Michelson, meticulously aligned optical equipment against that backdrop.

There, on the school’s seawall, he measured the speed of light far more accurately than anyone had before.

That seawall is gone now; as the Academy grew, it filled in much of the shoreline to make room for new buildings. Today, the original path of Michelson’s experiment is marked by round aluminum disks set into the terrace between Michelson Hall and Chauvenet Hall (named for mathematician William Chauvenet, who helped found the Naval Academy). Michelson Hall houses the school’s science wing.

“When I was a kid coming to the Academy with my dad, I would walk that path,” says Naval Academy Provost Samara Firebaugh. Years later, as an electrical engineering professor, she would take her students there, and today, she still points out the markers to visitors and shares Michelson’s story.

The site now has newfound fame: In a ceremony on April 12, APS President Young-Kee Kim and CEO Jon Bagger formally recognized it as an APS Historic Site, an honor given to a select number of places each year where important events in physics history took place. The Naval Academy was chosen because of Michelson’s enduring contribution to physics.

Michelson’s family immigrated to the United States from Prussia in 1856, opening a dry goods store in a California mining town, and later in Nevada. His parents sent him to school in San Francisco, where he excelled in science and became fascinated with optics.

“Albert’s brothers and sisters were swept into the drama of Western life,” wrote Michelson’s daughter Dorothy Michelson Livingston in The Master of Light: A Biography of Albert A. Michelson, but Albert “had acquired a different set of values.”

Michelson sought to continue his science education after high school. He saw his chance when the paper announced a statewide competition for a coveted spot at the U.S. Naval Academy, which had a strong optics program. Michelson tied for first place, but the appointment went to a co-winner.

With the support of his U.S. representative, Michelson appealed to President Ulysses S. Grant for a special appointment to the Academy. President Grant initially declined, having already made the 10 appointments he was allotted, but later reconsidered and awarded Michelson an 11th appointment.

At the Naval Academy, Michelson led his class in optics and thermodynamics but lagged in seamanship. Around his graduation, the Naval Academy’s superintendent, Rear Admiral John Lorimer Worden, told Michelson, “If in the future you’d give less attention to those scientific things and more to your naval gunnery, there might come a time when you would know enough to be of some service to your country,” according to a 1931 New York Times article.

After a two-year tour, Michelson returned to the Academy as a chemistry and physics teacher. In the fall of 1877, his supervisor suggested Michelson start an advanced physics class by demonstrating Léon Foucault’s 1862 speed of light measurement. Michelson reluctantly agreed — demonstrations were a new teaching method at the Academy — and studied the original experiment.

To find the speed of light, Foucault had measured how much a rapidly rotating mirror displaced a reflected beam of light, and then he had correlated the displacement to the mirror’s angular velocity. The result was solid, but Michelson saw room for improvement.

“The objection to Foucault’s method is that the displacement, a quantity which enters directly in the formula, is very small, and therefore difficult to measure accurately,” Michelson wrote in a paper published in The Scientific Monthly. By increasing the displacement, Michelson realized he could measure it more precisely. That meant he could more accurately determine the speed of light — a fundamental measurement for navigation at sea and for science broadly.

With a few months and scavenged parts, Michelson built a proof-of-concept apparatus. He spent the next year refining a large-scale design that, in 1879, yielded a 115 mm displacement (Foucault’s was less than 1 mm) and a value of 299,940 km/s for the speed of light. The most accurate value of its time, Michelson’s measurement was within 0.05% of the currently accepted speed of light.

This work kicked off his groundbreaking career in optics, during which Michelson experimentally challenged the existence of ether, invented the interferometer bearing his name, and, in 1907, received the Nobel Prize in Physics, becoming the first American laureate in science. He resigned from the Navy in 1881 to pursue physics but always stayed connected. “Father’s devotion to the Navy took second place only in relation to his romantic feeling about light,” wrote Dorothy Michelson Livingston in an article for the U.S. Naval Institute Proceedings.

Each year, the Naval Academy honors Michelson by inviting a distinguished researcher to share their expertise with midshipmen during the Michelson Memorial Lecture. The event is a campus-wide reminder that although technology landscapes and seawalls may shift, clever instrument design and careful measurement have lasting impact.