Sept. 23, 1846: Johann Galle discovers Neptune

Today in the history of astronomy, mathematical predictions reveal a hidden planet. | Published: September 23, 2026 Voyager 2 shot this image of Neptune on Aug. 31, 1989. Credit: NASA/JPL-Caltech/Kevin M. Gill For decades following the discovery of Uranus in 1781, astronomers were puzzled by the irregularities in the planet’s orbit. Newton’s laws could not explain the perturbations, suggesting another planet was out there, affecting Uranus’ path. Two astronomers, Urbain Le Verrier in Paris and John Couch Adams in Cambridge, England, independently computed its likely location at approximately the same time, a complicated process of reverse-engineering the motions of one planet from the motions of another. Le Verrier sent his calculations to the Berlin Observatory, where Johann Galle used them to successfully observe Neptune on Sept. 23, 1846. The newly discovered planet was only 1 degree away from where Le Verrier had predicted it would be, and was the first planet to be predicted by mathematical work. A series of ignored communications and delayed analyses of observations meant that Adams’ predictions didn’t receive the same notice. Eventually, British nationalistic fervor brought attention to them, and today Le Verrier and Adams often share credit for the discovery. Elisa Neckar is senior production editor of Astronomy magazine, and the editor of the Today in the History of Astronomy feature on Astronomy.com.