heinrich d'arrest, a student at the berlin observatory, suggested that a newly drawn map of the sky in the region of le verrier's predicted area could be compared with the current sky. this map was needed to look for the change of position of a planet, compared to a fixed star. neptune was discovered the same night on 23 september 1846. it was found 1° from where le verrier had thought it would be. it was about 10° from adams' prediction. challis later found out that he had seen the planet twice in august. he did not recognize it at the time because of his careless work approach. neptune became the first planet to be discovered by mathematical calculations instead of a telescope.