In the mid-1840s, astronomers noticed a strange wobble in the path of the planet Uranus as it revolved around the sun.
Some theorized that such a wobble could only be caused by another large gravitational field pulling on Uranus. They did a bunch of complex math applying Isaac Newton's theories of gravity, pointed their telescopes at a spot in the sky where the missing planet should be, and boom—that's how humans confirmed the existence of Neptune.
