It wasn't just one scientist, but the combined observations and calculations of several that led to the prediction of Neptune's existence. Here's the breakdown:

1. The Anomalies of Uranus:

* Uranus's orbit: Astronomers noticed that Uranus wasn't moving precisely as predicted by Newton's laws of gravity. It seemed to be deviating from its calculated path.

* Possible explanations: This could be due to either errors in the calculations or an unseen object's gravitational influence.

2. The Mathematical Detective Work:

* Urbain Le Verrier (France) and John Couch Adams (England): These two mathematicians independently began to calculate the position of a possible unseen planet that could be responsible for the irregularities in Uranus's orbit.

* Adams's work: Adams completed his calculations first but faced difficulties in getting his findings published.

* Le Verrier's success: Le Verrier published his findings in 1846, prompting astronomers to search for the predicted planet.

3. The Discovery:

* Johann Gottfried Galle: Using Le Verrier's calculations, Galle at the Berlin Observatory located the planet within a degree of the predicted position.

* Neptune's confirmation: The discovery of Neptune confirmed the calculations of Le Verrier and Adams, demonstrating the power of mathematical prediction in astronomy.

So, it wasn't a single "aha" moment, but a process of:

* Observing Uranus's unusual orbital behavior.

* Applying the laws of gravity and mathematics to predict the location of an unseen planet.

* Observing the predicted location and confirming the existence of Neptune.