Today is the 163rd anniversary of one of the most spectacularly confirmed predictions ever in the history of science. On this day in 1846 the German astronomer Johann Galle discovered the planet Neptune, there where the French astronomer, Urbain Le Verrier, said it would be. To understand why this was so spectacular we have to go back to 1781 and William Herschel’s discovery of another planet, Uranus. Uranus was the first planet to be added to the seven ‘wanderers’ that had been known since antiquity. In order to determine the orbit of a planet it is not enough to simply discover it, one has to observe it systematically over many years or even decades carefully measuring and recording its positions. In the decades following Herschel’s discovery this is exactly what happened to Uranus; however in the course of time it became clear that the orbit of Uranus displayed several perturbations (irregularities) that were not compatible with its theoretical orbit as determined through Newton’s law of gravity. This meant that either Newton was wrong or that some unknown gravitational factor was affecting the orbit of Uranus. Both Le Verrier and the British astronomer John Couch Adams determined that the perturbations must be the result of a relatively large planet and calculated the theoretical orbit of it. Armed with Le Verrier’s calculations Galle and Heinrich d’Arrest discovered the planet Neptune only one degree away from the position predicted by Le Verrier on the day that they had received the information. This was a stunning confirmation of Newton’s theory and remains till this day one of the greatest triumphs of science.