Why Mercury's Orbit DriftsNewton says a lone planet retraces the same ellipse forever. Mercury doesn't: its closest point to the Sun creeps around by a tiny angle each lap, and a sliver of that creep — 43 arc-seconds per century — Newton could never explain. Einstein's gravity adds a whisper of extra pull close in, and the ellipse turns into a slowly rotating rosette.General relativity tweaks the force law with a small 1/r⁴ term. That breaks the exact 1/r² symmetry that keeps Kepler orbits closed, so the orbit no longer shuts — each pass the perihelion advances by 6πGM / c²a(1−e²). Here the effect is dialled way up so you can watch it.