On 23 September 1846, Neptune was discovered.
French mathematician Urbain Le Verrier had calculated the position of an unseen planet from irregularities in Uranus’s orbit.
Johann Galle at the Berlin Observatory, following Le Verrier’s predictions, found the planet within about 1° of the predicted spot. This was the first planet discovered by mathematics rather than by looking through a telescope.
Le Verrier himself died on the same date in 1877.