1 · Look around

Drag anywhere on the dark to swing the camera around the Sun. Scroll or pinch to zoom, right-drag or two-finger drag to pan. Click any body — or any name in the body index — to open its file.

2 · Draft an orbit

Select a body and open the Draft tab. Four handles appear on its orbit ring:

• Perihelion and aphelion diamonds — drag them in or out to stretch the ellipse, or around the Sun to swing the whole orbit. The two radii set the semi-major axis and the eccentricity.
• Node — drag it around the ecliptic to rotate the orbital plane's longitude.
• Tilt lever — pull it up out of the ecliptic to incline the plane.

The Draft tab also carries sliders for the same five elements, and a reset for one orbit.

3 · Switch on gravity

Physics hands every body to a cannon-es rigid-body world with gravity switched off, and applies Newton's law between every pair instead: F = G·m₁·m₂/r². Nothing is scripted — the paths you see are integrated from the forces. Distances are in astronomical units, masses in solar masses and time in years, so G = 4π² exactly and Earth's year comes out at 365.25 days.

Bodies are spheres with a capture radius equal to the radius you see drawn, so a close pass can end in an impact: the smaller body is absorbed, momentum is conserved, and the event lands in the log.

4 · Read the perturbation

With physics on, each body grows a ghost rail: a dashed prediction of where it is going, integrated forward with a fourth-order Runge–Kutta solver over the same force model. Compare it with the solid ring, which is the ideal two-body ellipse. Where the two part company, that gap is the perturbation — the pull of everything else. The Forces tab reports the largest gap in astronomical units.

5 · Measure the pull

The Force lines toggle draws the three active pulls inside a trio you choose in the Forces tab. Each line is labelled with the real force in newtons, computed from the actual masses and the actual separation.

Keys

• Space play / pause
• ← → step one frame while stopped
• [ ] slower / faster
• 1–9 0 - select a body
• P physics on / off
• F force lines
• R ghost rails
• G grid
• L labels
• S size scale
• D distance scale
• U hide the panels
• H this guide
• Esc close

Honesty about scale

Two switches keep the scene readable. Size toggles between radii compressed for visibility and the true relative radii magnified a uniform ×38 — at true scale the Sun swells to a quarter of Earth's orbit, which is what the real solar system looks like. Dist toggles between true linear distances and a square-root compression that pulls the outer planets in. The physics always runs on true distances; only the drawing is compressed.

Planetary data is from NASA's planetary fact sheets and NASA/JPL solar system pages. Moon counts are the confirmed totals as of late 2025.