Neptune discovered at Berlin Observatory; Lassell finds Triton

Background. By 1846, irregularities in Uranus's orbit could not be explained by Newton's law. In 1845, Urbain Le Verrier in Paris and John Couch Adams in Cambridge separately began calculations to determine the position of a perturbing planet. Le Verrier announced his final predicted position publicly on 31 August 1846, while Adams supposedly communicated his work to Challis in mid-September 1845, though the manner of that communication is disputed.

Discovery of Neptune. Le Verrier's letter reached Galle on 23 September 1846. Galle, assisted by student Heinrich Louis d'Arrest, observed the planet that evening using the Berlin Observatory's achromatic refractor. The planet was found within 1° of Le Verrier's predicted location near the boundary between Capricorn and Aquarius, confirming the mathematical predictions.

Discovery of Triton. William Lassell, an English merchant and astronomer based in Liverpool, discovered Triton, Neptune's largest moon, on 10 October 1846. This occurred seventeen days after the discovery of Neptune itself. Lassell used his self-built instrument to identify the satellite, which was the first Neptunian moon to be discovered.