Alex asked: Care to elaborate why Mercury is difficult to model? Is Mercury hard to
model because of its proximity to an enormous gravitational mass?
I think you have it right, close to the Sun. Explosions are constantly happening on the sun, shooting matter up into space. Each time this happens Mercury's orbit is perturbed. We had a similar problem with the Space Shuttle. The darn thing was only 200 miles up, so every little anomaly in the Earth's shape affected it's orbit. NASA's gravitational models had many terms, but certainly did not model all the features that could materially affect the orbit. The moon is far enough away from the earth that simple gravitation models, maybe even point models, do pretty well. We really couldn't propagate the Shuttle's orbit ahead more than a few days with any accuracy, and our ability was degraded further by the effect of the jets that controlled the attitude. Tim