On 23rd September 1846 the planet Neptune - now the furthest discovered planet from the Sun again, since Pluto has been relegated to dwarf planet status - was officially discovered following an observation of a location calculated by Urban Le Verrier.
Officially discovered, because this was not the first time the planet had been observed. It had been seen in the past - Galileo observed it in 1612 and 1613 - but it had not been identified as such. Galileo mistook it for a star, and prior to the official discovery, it had been photographed, but not recognised until after the fact.
Image: By NASA (JPL image) [Public domain], via Wikimedia Commons (commons.wikimedia.org/wiki/File:Neptune_Full.jpg)