The precession of the perihelion of Mercury

Mercury's ellipse is not fixed in space, but precesses slowly. That means the place where it comes closest to the sun (its perihelion) slowly rotates, itself, around the sun. This has several causes, one of which is the force Jupiter exerts on Mercury. But the causes known, up to 1916, could only explain part of this "precession". An additional 42 seconds of arc each century remained to be explained. Einstein's modification of Newton's gravity provided just that. The effect is not confined to Mercury of course, but is largest there because the gravitational field of the sun is stronger, closer to the sun.