Under Newton, gravity's pull is exactly 1/r² and a bound orbit is a closed ellipse — the perihelion returns to the same place every lap. General Relativity adds a faint 1/r³ whisper, and the ellipse never quite closes: the perihelion creeps forward forever. For Mercury that creep is the famous 43″ a century.
Toggle Newtonian: the gold ellipse closes — the perihelion (gold dot) stays put, advance 0″. Toggle Einstein: the blue rosette opens and the perihelion marches around. Crank GR strength to exaggerate the creep, or pick a different planet — closer, more eccentric orbits precess more. The self-test proves the 43″ from the closed form 6πGM/(c²a(1−e²)).