The Most Elegant and Powerful Experiment in the History of Science

175 years ago today, proving what had been assumed but was unproven.

On March 31, 1851, a crowd of curious Parisians gathered at the Pantheon to witness a historic scientific demonstration. In the centre of the building, directly beneath its towering dome, they found a deceptively simple piece of equipment: a 28-kilogram brass-coated lead sphere, suspended from the building’s dome by a 67-metre-long wire. Beneath this was placed a wooden platform covered in a thin layer of sand. Once the crowd had settled, the organizer of the event, 32-year-old amateur scientist Léon Foucault, stepped forward and held a candle to the string anchoring the metal sphere to the wall. A few seconds later the string burned through, releasing the sphere. As the crowd watched, the pendulum swung slowly across the hall, a spike at its base scribing a line in the sand with each pass. At first nothing changed, but as the minutes ticked past something extraordinary began to happen. Slowly, the line in the sand began to shift, creeping steadily clockwise around the wooden platform. Within an hour it had rotated more than 10 degrees, while by the next day it had completed a full circle, returning to its original starting point. The implications of this strange phenomenon were not lost on the astonished onlookers. With only the simplest of devices, Léon Foucault had conclusively demonstrated what many had long suspected but had been unable to prove: that the earth does indeed rotate around its axis. This is the story of one of the most elegant and powerful experiments in the history of science.