The story surrounding of Neptune's discovery provides an excitingillustration of what historians of mathematics do. Neptune was first sightedas a new planet on 23 September 1846 at the Berlin observatory. Thesensational news reached London a week later and the ensuing dispute createdone of the great (and ongoing) priority debates in the history of science.About a month after the initial observation, word of the new planet alsoarrived in America where the controversy captured both popular interest andscientific attention. A handful of nineteenth-century scientists who shareda vision for professionalizing science in America viewed the Neptune affairas an opportunity to establish the legitimacy of American science inresponse to perceived European scientific superiority. While Europeanadministrators of science quibbled over the priority question, the Harvardmathematician Benjamin Peirce — considered an upstart Americanscientist — dared to question the mathematical particulars of the discovery.Recent twentieth-century events and manuscript discoveries furtherilluminate the story of planetary controversy.