Johann Galle and Heinrich d’Arrest observe Neptune using the calculations of Urbain Le Verrier while John Couch Adams independently makes similar mathematical predictions. Discovery of the furthest known planet is a substantial validation of celestial mechanics.
A 1989 image shows Neptune as viewed from the Voyager 2 space probe.
Jet Propulsion Laboratory and National Aeronautics & Space Administration
William Sheehan & Richard Baum, “Neptune’s Discovery: British Astronomers Had Neptune in the Bag, but Then They Let It Slip Away…,” Astronomy, 27 October 2002
William Sheehan, “Secret Documents Rewrite the Discovery of Neptune,” Sky and Telescope, 23 July 2003
William Smart, “John Couch Adams and the Discovery of Neptune,” Popular Astronomy, pages 301-311, June 1947
William Sheehan & Steven Thurber, “John Couch Adams’s Asperger Syndrome and the British Non-discovery of Neptune,” Notes and Records of the Royal Society of the History of Science, 3 July 2007
William Ashworth, “Johann Galle,” Linda Hall Library, 9 June 2017
William Ashworth, “Urbain Le Verrier,” Linda Hall Library, 11 March 2020
“Observations of Le Verrier’s Planet, In the Meridian,” Monthly Notices of the Royal Astronomical Society, pages 154-157, 1846
John Adams, “On the Perturbations of Uranus”, Appendices to Various Nautical Almanacs, pages 265-293, 1851