* Uranus's Anomalies: In the early 1800s, astronomers noticed that Uranus's orbital path didn't quite match the predictions based on Newton's laws of gravity. There were small but consistent discrepancies, suggesting the influence of another, unseen celestial body.
* Mathematical Predictions: Urbain Le Verrier, a French mathematician, and John Couch Adams, an English mathematician, independently calculated the position of this hypothetical object based on Uranus's deviations. Their calculations were remarkably similar, predicting an unseen planet in a specific region of the sky.
* Confirmation by Observation: In 1846, Johann Gottfried Galle, a German astronomer, used Le Verrier's calculations to search the predicted area of the sky. He successfully located a faint blue object, later confirmed to be the planet Neptune, within a degree of Le Verrier's prediction.
It's important to note that:
* Both Le Verrier and Adams deserve credit: While Le Verrier's calculations were slightly more precise and used by Galle for the discovery, Adams made his calculations earlier and independently. The "discovery" was a collaborative effort.
* The discovery of Neptune was a significant achievement: It was the first planet to be discovered mathematically before being observed, a testament to the power of scientific inquiry and the accuracy of Newton's laws of gravity.
Therefore, astronomers didn't "know" where to look in the sense of having a specific address. They used mathematical calculations and observed anomalies in Uranus's orbit to deduce the existence and general location of a new planet, which was then confirmed through observation.
