It was in the year 1846 when Neptune was first discovered by Johann Gottfried Galle. The discovery is said to have come up because of some mathematical computations. One thing is for sure though, they are not the first ones who have observed the planet for the first time. This is credited to Galileo Galilei who has noted the appearance of Neptune in the sky two centuries before its “actual” discovery.
There aren’t a lot of details that are known about Neptune probably because it is far from Earth. The discovery may have stemmed from the fact that Uranus is slowly going out of its usual orbit because of the pull of Neptune.