How to work the table

Getting around

Keyboard

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

The physics

The simulation runs in astronomical units, years and solar masses. In those units the gravitational constant is exactly G = 4π², so a body at one astronomical unit from a one-solar-mass star completes an orbit in exactly one year. That is not a coincidence — it is how the units were chosen.

Integration

cannon-es carries the bodies. The world's own gravity is switched off and replaced with a pairwise Newtonian force, F = G m₁m₂/r², applied to every pair before each substep. The step size is held between 0.0005 and 0.002 years and the frame is cut into at most 60 substeps, so the readout in the top bar always tells you the Δt actually in use. Push the time rate to the top of the slider and the integrator coarsens; pull it back and it tightens.

Ghost rails

In physics mode each body grows a dashed rail: where it will actually go. That rail is integrated forward with a fourth-order Runge–Kutta solver over the same force law, about six tenths of an orbit ahead. The solid ring behind it is the ideal Kepler ellipse. The gap between the two is the perturbation, and the Forces tab reports its largest value in astronomical units.

Collisions

Each body carries a collision sphere the size of the sphere you can see, which is exaggerated for the small worlds. When two of them touch, the lighter body is absorbed: momentum is conserved, the masses add, and the survivor's sphere grows. The event log in the left rail records it.

Force lines

The Forces tab picks any three bodies and draws the pull between them, labelled in newtons using the real constant G = 6.674×10⁻¹¹. The Sun on the Earth is about 3.5×10²² newtons — the same number you would get on paper.

The ten bodies

Orbital elements are the J2000 set: semi-major axis, eccentricity, inclination to the ecliptic, and the period that follows from them.

Moon counts are the confirmed totals as of late 2025, which is why Saturn reads 274 and Jupiter 97. Pluto is included as a dwarf planet, and so is Ceres — the largest object in the asteroid belt, holding about 35% of its mass and roughly a quarter of its own mass in water ice.

What is real and what is drawn

• Real: orbital elements, masses, radii, rotation periods, moon counts, temperatures, atmospheric compositions, and every force in newtons.
• Real: the N-body integration, the ghost rails, the collision merges and the perturbation figures.
• Drawn: the Size: visual mode compresses radii by a power law so a 939 km dwarf planet is still visible next to a 139,822 km gas giant. Switch to Size: true for the true ratios, magnified 38 times — the Sun then swells to fill the inner system, which is what it really does.
• Drawn: Dist: compressed takes the square root of each distance so Neptune and Pluto fit on screen. Dist: linear is the honest spacing, and it is mostly empty.
• Drawn: collision spheres match the drawn radii, so they are generous for small bodies. Real planets almost never collide.

Where the numbers come from

Planetary facts are the published NASA planetary fact sheets and mission results — diameters, masses, rotation and orbital periods, atmospheric composition, temperature ranges and moon counts. Orbital elements are the standard J2000 set. Nothing on this site is invented, and where a figure is uncertain the panel says so.

Interface icons by Lucide, used under the ISC licence. See credits for the full list. Back to the sandbox.