Gravity
The simulator applies Newtonian point-mass acceleration: a = G·M·r̂/r². The code scales the displacement vector by G·M/r³ so direction and magnitude stay coupled.
Model notes
Physics stays in SI units.
Every body stores mass in kilograms, radius in meters, positions in meters, velocities in meters per second, and acceleration in meters per second squared.
Integration
Core and catalog scenes use fixed 15-second velocity-Verlet steps: half-kick velocity, drift position, recompute acceleration, then finish the second half-kick. Guided lessons can choose a different fixed step and compare explicit Euler.
Initial orbits
The core demonstration starts planets and Vesta at planar perihelion, Earth's Moon at perigee, and Mars's moons at periareion with vis-viva speeds. Jovian moons instead use sourced mean elements and reference-frame conversion. These states do not share a dated ephemeris.
Choose the model before interpreting the result.
Selected catalog experiments align source orbits and pinned planetary vectors to JD 2461200.5 TDB, then integrate with the fixed-Sun point-mass model. Unknown masses are test particles; unknown radii remain explicitly unavailable.
Guided lessons use controlled initial conditions to study numerical error, free barycentric motion, resonance, encounters, and opt-in head-on contact. The moving-Sun lesson differs from the default fixed-Sun core. Relativity, tides, and planetary oblateness are outside the model.
The A/B lab compares matched C measurements. A small energy change or a stable-looking orbit alone does not establish accurate orbital phase; use analytical references and timestep convergence where available. Display magnification never changes SI state.