Neptune wis the first planet foond bi mathematical prediction rather than bi empirical observation. Unexpectit chynges in the orbit o Uranus led Alexis Bouvard tae deduce that its orbit wis subject tae gravitational perturbation bi an unkent planet. Neptune wis subsequently observed on 23 September 1846 bi Johann Galle within a degree o the poseetion predictit bi Urbain Le Verrier, an its lairgest muin, Triton, wis discovered shortly thareefter, tho none o the planet's remainin 13 muins wur locatit telescopically till the 20t century. Neptune haes been visited bi anly ane spacecraft, Voyager 2, which flew bi the planet on 25 August 1989.