MERCURY'S ORBITAL PRECESSION AND THE GENERAL THEORY OF RELATIVITY
Mercury, like the other planets, traces an elliptical orbit as it travels around the sun, -- at least, approximately. The ellipse is not fixed. It gradually rotates, so that Mercury's point of closest approach to the Sun - called the perihelion of the orbit - drifts, or precesses about 0.001 degrees per century. This image will give you an idea:
Why the precession? Well, according to Sir Isaac Newton's theory of gravity, any two massive bodies attract each other. If we want to apply this "classical" view of gravity to Mercurry's motion, we must take into account the effects not only of the Sun, but also of other bodies relatively close to Mercury. Though the Sun is by far the overwhelming factor, other planets contribute non-negligible effects.
So we go scrambling around the solar system, calculating those small perturbations, and find that they should indeed cause Mercury's orbit to prcess. Yet there's a problem: our theoretical calculation for Mercury's rate of precession is 7% off from the true measured value.
This is too large and consistent an error to ignore. So where's our problem? What have we not accounted for out there? There's the Sun, the planets, the asteroids, .. the occasional comet, which is so comparatively small and rare that it contributes a negligible effect. What other significant "stuff" is floating around in the space that contains our solar system? ... Nothing!
Nothing but a bunch of space.
Wait a minute. What exactly is space, anyway?
Albert Einstein thought this might be worth wondering about. And he suggested that we might be narrow-minded in regarding space as a passive, static "nothing" in which masses rigidly float. Perhaps massive bodies and the space they occupy are not materially separate from each other. What if they interact? What if space is a sort of "fabric" - capable of change?
In Einstein's General Theory of Relativity, he interprets gravity as the manifestation of the interplay between massive bodies and space. He suggests that a mass doesn't merely float in an unperturbed vacuum; it actually _shapes_ the space around it. Warps, or curves it. And a mass experiencing the pull of gravity is simply following a natural path in a curved space.
Imagine that a star warps space the same way a heavy ball will warp a rubber sheet -- it creates a depression. Nearby objects (e.g., planets) follow trajectories in this dented space (i.e., 'orbits'):
This theory sounds pretty wild when you first encounter it, and it may be hard to regard it as more than entertaining science fiction. And yet, by applying this interpretation to the problem of Mercury's orbital precession, you make that 7% discrepancy entirely vanish.
(Here is more information on Albert Einstein.)