Urbain Le Verrier had calculated the position of an unseen planet from irregularities in Uranus's orbit, then mailed his predictions to Berlin Observatory. On the night of September 23-24, Johann Galle and student Heinrich d'Arrest located Neptune within about one degree of Le Verrier's forecast. The triumph of Newtonian mechanics sparked a bitter international priority dispute with England's John Couch Adams, who had made similar calculations but published later.
For decades astronomers had puzzled over small wobbles in Uranus's orbit that no known force could explain. Rather than assume Newton's laws were wrong, the French mathematician Urbain Le Verrier assumed an invisible planet was tugging at Uranus and set out to calculate exactly where it must be, using nothing but pen, paper, and gravitational theory.
He sent his predicted coordinates to Johann Gottfried Galle at the Berlin Observatory, who received the letter on September 23, 1846. That same night, Galle and his assistant Heinrich Louis d'Arrest pointed their telescope at the specified patch of sky in Aquarius and found a bluish disk exactly where the math said it should be, within about one degree of prediction. They had discovered Neptune without ever needing to scan the sky randomly.
The achievement ignited a nationalist firestorm. English astronomers pointed out that Cambridge's John Couch Adams had derived nearly identical predictions a year earlier but had failed to get anyone to search seriously. The dispute over credit dragged on for years, but the deeper story remained: a planet had been discovered by mathematics alone, arguably the greatest vindication of Newtonian physics in history.
Key people: Urbain Le Verrier, Johann Galle, Heinrich d'Arrest, John Couch Adams