"That star is not on the map!"
These words of astronomy student Heinrich Louis d'Arrest rang through the dome of the Berlin Observatory on the night of September 23, 1846, and have reverberated through the institutions of astronomy ever since.

With a star map spread out on the table in front of him, d'Arrest was helping staff astronomer Johann Gottfried Galle test an extraordinary prediction made by French mathematician Urbain Jean Joseph Le Verrier. The Frenchman had hypothesized that Uranus, then the outermost-known planet from the sun, was moving off track because of the gravitational pull of a hitherto unseen planet. Just five days earlier he had written to Galle: "You will see, Sir, that I demonstrate ... [that] one can only account for the observations of Uranus by introducing the action of a new Planet, unknown up till now; and, what is remarkable, there is only one position in the ecliptic that can be attributed to this disturbing planet."

   
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