On September 23, 1846, astronomers Johann Galle and Heinrich d’Arrest observed Neptune, confirming the existence of the distant planet for the first time.

The discovery was especially remarkable because astronomers located Neptune through mathematics before they saw it through a telescope. Astronomers had noticed that Uranus was not moving exactly as expected and believed the gravity of another, unseen planet might be affecting its orbit.

Using calculations that predicted where the mysterious planet should be found, Galle and d’Arrest searched the night sky and identified Neptune near the predicted location. The discovery became one of the most significant achievements in the history of astronomy and showed how mathematics could help scientists uncover objects far beyond what could be seen with the naked eye.