In 1845, Urbain Le Verrier, and John Couch Adams calculated the existence of Neptune by observing the disturbance of Uranus's orbital motion. This slight perturbation hints to them that a celestial body with a strong gravitational field is hidden somewhere near Uranus. An eight planet was then predicted to have existed. Neptune was finally discovered by Johann Galle on 23 September 1846 using the Fraunhofer telescope. Verrier's and Adams's calculation was just 1 degree off the real position of Neptune!

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Source: https://www.nasa.gov/feature/175-years-ago-astronomers-discover-neptune-the-eighth-planet