TITLE: TERRA — a world of noise

PROMPT:
Create a self-contained HTML page (all CSS and JS inline, no external fonts, images, CDNs, libraries or external SVG files) called "TERRA — a world of noise". Art direction: planetary science, deep space blue, warm sunlight. Palette: ground #05070c, ink #e6ecf2, dim #75818f, hairline rgba(230,236,242,.13), sun #ffd27a. Layout: a header with a shaded sphere logo mark and a monospaced specification line; then a hero with a sun-coloured kicker, a huge light-weight headline with one bold word, and a paragraph; then a stage panel — a bordered box split into a 480×480 canvas on the left, rendered round with a soft blue glow, and a 250px column of two instrument boxes on the right (Planet: radius, land fraction, highest point, deepest ocean, rotation speed and the seed; Sun: declination, sub-solar point, the terminator and the day length); then a control band of three sliders (Sea level, Rotation, Sun angle) and three buttons (New world / Graticule / Clouds); then a two-column section pairing an essay on why noise is not the same as random with a note on drawing a sphere honestly; and a footer. The world is generated by fractal value noise on the surface of a sphere: an equirectangular height field of five hundred and twelve by two hundred and fifty-six samples is built from four octaves of hash-based three-dimensional value noise with a further three octaves of detail, and the projection is orthographic, so every pixel inside the disc is a ray from the centre out to the surface, the surface point is rotated by the planet's longitude and latitude, and the height is looked up in the field with bilinear interpolation. Lighting is a Lambertian term against the sun direction, so the terminator is a true great circle, and there is limb darkening and an atmospheric halo. Interaction: dragging the globe rotates it; the Sea level slider raises or lowers the coastline and rebuilds the field; Rotation sets the spin rate; Sun angle moves the terminator; New world reseeds the noise; Graticule draws the meridians and parallels; Clouds adds a second noise layer. Responsive: the stage and the two-column section stack under 900px.

DESCRIPTION:
A planet made of nothing but noise and a threshold. Four octaves of value noise decide where the continents are and a fifth decides what kind of place each one is, and the coastlines are simply wherever the height field crosses the sea level — so moving the sea level slider drowns the lowlands and raises new islands. The projection is orthographic, so the far side is never drawn and the limb is a true circle, and the terminator moves as a real great circle when you turn the sun.

MAJOR VISUAL TECHNIQUES:
- Fractal value noise from a hash-based three-dimensional lattice
- Equirectangular height field precomputed and sampled with bilinear interpolation
- Orthographic projection with the far side correctly hidden
- Lambertian terminator, limb darkening and an atmospheric halo
- Coastline produced by thresholding the field at the sea level
- Second noise layer for cloud cover, and a drawn graticule

INTENDED INTERACTION MODEL:
Spin the world. Drag to turn it, raise and lower the sea, move the sun, add the graticule and the clouds, and reseed the whole planet when you want somewhere else to live.