I built a gravitational N-body simulator whose rules are derived entirely from established physical laws, with no hand‑tuned parameters. Each physical constant is linked to a documented source via an automated provenance audit. The simulator initializes with NASA JPL DE440 ephemeris data and is validated against real Solar System observations. We derive an explicit predictability horizon that accounts for numerical integration error, chaotic dynamics, and measurement uncertainty, reproducing the known long‑term predictability limit of the Solar System. The work also provides a reproducible computational framework that integrates provenance auditing, numerical validation, and predictability analysis for gravitational simulations.