Neptune was found with a pencil before a telescope

Uranus kept drifting from where Newton's laws said it should be, so in 1846 French astronomer Urbain Le Verrier calculated the position of an unseen planet whose gravity would explain the wobble. He mailed the coordinates to Berlin Observatory's Johann Galle, who pointed his telescope at that exact spot and found Neptune within one degree, the same night. It remains the only planet discovered by prediction rather than by scanning the sky first.

— Urbain Le Verrier and Johann Galle, Discovery of Neptune — Announced to the Académie des sciences and Berlin Observatory, 1846