Mercury's Perihelion

Mercury's orbit isn't a fixed ellipse — its closest point to the Sun, the perihelion, slowly swings around. Newtonian gravity, adding up the tugs of all the other planets, explained almost all of it but left a stubborn 43 arcseconds per century unaccounted for, a puzzle for decades.^[1] When Einstein computed the orbit in the curved spacetime near the Sun, out fell exactly that extra 43 arcseconds. It was general relativity's first success — and unlike a prediction, the answer was already sitting in the data, waiting.