If we constructed the laws of physics merely by observing the planets we could have concluded that a mass rotated around an axis will move in epicycles, which is how the pre-Copernican astronomers explained the movements of Mars. Astronomy is merely the application of experimentally determined physics here on earth to the movement of celestial bodies.
You're missing an awful lot of astronomy if you think that orbital mechanics is the sum total of astronomical knowledge. There's no earth-bound laboratory that can create the hundreds of Teslas magnetic fields found in the sun; there's no earthbound laboratory where one can see high-mass effects like gravitational lensing; there's no earth bound laboratory that can explain or even observe the existence/absence of dark matter. Astronomy is much, much more than dynamics.
