Neptune was discovered with pen and paper before anyone saw it through a telescope.
Uranus was not moving quite as Newton's laws predicted. The French astronomer Urbain Le Verrier calculated where an unknown planet would have to be to explain the difference, and in 1846 a Berlin observatory found Neptune within one degree of his prediction.
Why is this true?
It was the first planet found by prediction rather than by chance, and one of the great triumphs of Newtonian physics. A British astronomer, John Couch Adams, had made similar calculations, which caused a long argument over credit.
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More about space
Space facts are mostly a lesson in scale. Days longer than years, storms wider than Earth, light that left its star before the pyramids existed — the numbers stop being intuitive very quickly. Every entry here comes from established astronomy and includes a short explanation of what causes the effect, so the fact holds up when someone asks the obvious follow-up question at the dinner table.