Ask a fundamental astronomical question, and the chance of an accurate answer is small. Throughout this account of gravitating systems, nearly each section ends on an incomplete note. We are just beginning to understand the richness and subtlety implicit in a system whose components interact with a force as simple as an inverse square. On all scales, from the sun and its planets to cosmic clusters of galaxies, flock insistent but unanswered questions.

For how long will the solar system be stable? Secular perturbations that grow as the planets follow their courses may eventually end in a resonance that forces ejection. Analytic theory gives us some reassurance, but not a definite answer. Computer simulations do not have the accuracy needed. And if we think of our solar system itself as an analog computer, then the calculation has not yet been done. All we know for certain is that the orbits of planets remaining today have avoided ejection for billions of years. Someday the sun may not rise tomorrow.

The uncertainties of planets orbiting the sun are relatively tame compared to the few-body problem whose masses nearly are equal. More opportunities for resonance flourish. A star hardly knows which way to go. Are computer experiments the only method for predicting the outcome from given initial conditions?