Edit a world × The universe is paused while you edit. Velocity uses play units per time unit. Changes can be undone.
A universe, at your fingertips. × Launch: drag from an empty spot in the direction you want your new body to travel. The arrow is its starting velocity; the dotted preview assumes other bodies stay in place, so it is only a guide.
Orbit: choose the Orbit tool and click anywhere outside the biggest body. We give your new body a sideways push for an approximate circular orbit. Other planets still tug on it. The Add orbiting planet button offers a keyboard-friendly alternative.
Explore: use Pan to drag the view, the mouse wheel or +/− to zoom, and Fit to bring everything back. Inspect a body to see its mass and speed or remove it. Space pauses/resumes, and Escape cancels a drag.
Save / Load: name a universe to save it in this browser. Use the same website address to find it later. Export a JSON file for a backup or another device; import it here to continue. Browser clearing/private browsing can remove local saves. Loading pauses the universe, and Reset returns to the loaded snapshot. Named saving is manual; autosave recovery keeps the latest snapshot every five seconds and when leaving the page, if browser storage is available. Recovery is offered on your next visit and is separate from named saves.
History: Undo/Redo keeps up to 20 edits. Rewind keeps up to 60 simulation units at quarter-unit intervals; releasing the slider removes the later branch. Undo can restore that pre-rewind universe. History buffers reset on load and are not stored in files. Editing, undo, redo and rewind pause the universe.
Edit and follow: choose a body below the scene or click it with Inspect. Edit its name, mass, velocity and color; named bodies have labels. Follow centers the camera on it. Pan or Fit returns to free view. A merge keeps the larger body’s identity; following the smaller body stops.
Guided lessons: open Guided challenges near the title. Each lesson shows a checklist, a primary next-action button, live measurements and an optional hint. Hints for escape velocity adapt to the explorer’s current position. Follow the checklist and press play yourself; guidance does not award completion or change the physics. Saved challenges resume with guidance inferred from their saved bodies and progress.
Challenges: open Challenges to begin. Goals, progress and restrictions appear below the scene. Gravity/rule changes or disallowed edits turn a challenge into practice; restart to try again.
Fields and graphs: field arrows use the same softened gravity as the simulation. Their length/brightness is logarithmic, not a linear ruler. Speed is measured in the world frame; energy is the whole system’s kinetic plus softened potential energy. Distance uses your chosen reference, defaulting to the heaviest other body. Edits and discontinuities reset traces. CSV contains the available measurements.
Compare: start two independent copies, change their rules, and run them on a shared simulation clock. Both spatial views and measurement plots share scales. Reset both repeats the initial states with your selected rules; Apply B is an undoable replacement of the main scene. Comparison buffers are temporary.
Sharing: the snapshot is carried in the link fragment. Opening is a deliberate paused import. The link contains the shared snapshot, not local saves or history. Link decoding has size and schema limits; use file export if a snapshot is too large. New advanced challenge metadata needs v1.4 or later.
Sensitive twins: the orange dashed circles are an independent reference run with a 0.01-unit offset in the first body’s starting x-position. They exert no force on the main system. Both runs use matching softened gravity and adaptive integration. Growth of separation is illustrative sensitivity, not a formal proof of chaos; close encounters also magnify numerical error.
Model: every body attracts every other body with softened inverse-square gravity (softening length 6). A fixed 1/240-unit leapfrog step runs independently of screen refresh; adaptive substeps reduce fast-body tunneling up to a capped budget. Extreme speeds or dense close encounters can still produce numerical error. Merges conserve mass and momentum, losing kinetic energy; bounces are elastic along the collision normal. Discs have constant-density radii; stars are visually glowing massive bodies. No relativity, fragmentation, tides, real solar-system scales, or fixed anchors. Maximum 100 bodies. Trails show past positions, not predicted future paths.