Harmonices Mundi

Kepler said the planets sing. This is the performance.

A single-page site: a WebGL2 orrery runs the six planets Kepler knew on their true elliptical orbits, and the Web Audio API gives each one a voice whose pitch follows its angular velocity, the way Kepler wrote it in Book V of Harmonices Mundi (1619). Scrolling reads a four-chapter essay. Each chapter takes over the camera and the clock.

Run it

npm install
npm run dev

Then open http://localhost:8080. npm run build writes dist/ for GitHub Pages; npm test runs the orbital-mechanics tests, including six that assert Kepler's intervals fall out of the eccentricities.

How it is built

Piece	Where	What it refuses to know
Ephemeris	src/assets/js/lib/ephemeris.js	Pixels, sound, the DOM. Pure math: JPL mean elements at J2000, Kepler's equation by Newton's method.
Clock	src/assets/js/lib/clock.js	What time is for. Owns simulated days and an eased rate.
Camera	src/assets/js/lib/camera.js	What it is looking at. Spherical orbit, eased pose, off-center projection so text can sit beside the Sun.
Choir	src/assets/js/lib/choir.js	Orbits. Six voices told a tempo, gliding pitch to match. AudioContext is injected.
Orrery	src/assets/js/lib/orrery.js	Time, AU, which planet is which. WebGL2 renderer: sky, Sun, orbit ribbons, trails, Kepler wedge.
Stage	src/assets/js/lib/stage.js	Maps AU to display units (r^0.6) and holds the palette.
Orrery controller	src/assets/js/controllers/orrery_controller.js	The one joint. Frame loop; passes messages between the pieces above.
Chapter controller	src/assets/js/controllers/chapter_controller.js	Who is listening. Announces a pose when it straddles the viewport midpoint.

Distances are compressed so Mercury and Saturn share a screen. Directions, timing, and the shape of every ellipse are true to the ephemeris.

Stack

Eleventy 3, Tailwind 4, Stimulus 3, per the house static site spec. No bundler; Stimulus loads from an import map. Fonts are Fraunces and JetBrains Mono from Google Fonts.

Deploy

Push to main. The workflow in .github/workflows/deploy.yml builds and publishes dist/ to GitHub Pages. pathPrefix in eleventy.config.js assumes the repository is named harmonices-mundi.