Astronomers didn't know Neptune existed before they could see it. It was discovered in 1846 by Urbain Le Verrier and John Couch Adams, who both independently predicted its existence based on gravitational anomalies in the orbit of Uranus.

Here's how it worked:

1. Uranus's orbit was off: Astronomers had observed that Uranus's orbit didn't quite match the predictions of Newtonian gravity. The planet was moving in a way that suggested an unseen gravitational influence.

2. Mathematical predictions: Both Le Verrier and Adams independently calculated that the anomalies in Uranus's orbit could be explained by the presence of an unknown planet beyond it. They calculated the planet's location, mass, and orbit.

3. Observation: Based on their calculations, astronomers Johann Galle and Heinrich d'Arrest at the Berlin Observatory were able to find Neptune within a degree of Le Verrier's predicted position.

So, Neptune's existence was predicted mathematically, not observed directly, before it was finally seen through a telescope. This was a remarkable feat of scientific deduction and a significant milestone in our understanding of the solar system.