Made of little things.

import { BackSide, Color, Mesh, ShaderMaterial, SphereGeometry, Vector3 } from 'three';
import { PALETTE } from './palette.ts';

/** Gradient sky dome that follows the camera and orients to the local "up" on the planet. */
export function createSky(): { mesh: Mesh; update(cameraPos: Vector3, up: Vector3, sunDir: Vector3): void } {
  const material = new ShaderMaterial({
    side: BackSide,
    depthWrite: false,
    fog: false,
    uniforms: {
      uUp: { value: new Vector3(0, 1, 0) },
      uSun: { value: new Vector3(0, 1, 0) },
      uTop: { value: new Color(PALETTE.skyTop) },
      uHorizon: { value: new Color(PALETTE.skyHorizon) },
      uGlow: { value: new Color(PALETTE.skyGlow) },
    },
    vertexShader: /* glsl */ `
      varying vec3 vDir;
      void main() {
        vDir = normalize(position);
        gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
      }`,
    fragmentShader: /* glsl */ `
      uniform vec3 uUp; uniform vec3 uSun; uniform vec3 uTop; uniform vec3 uHorizon; uniform vec3 uGlow;
      varying vec3 vDir;
      void main() {
        float h = dot(normalize(vDir), uUp);
        vec3 col = mix(uHorizon, uTop, smoothstep(-0.05, 0.55, h));
        col = mix(col, uHorizon * 0.92, smoothstep(0.0, -0.4, h));
        float sun = max(dot(normalize(vDir), uSun), 0.0);
        col += uGlow * pow(sun, 8.0) * 0.35 + vec3(1.0, 0.97, 0.88) * smoothstep(0.9975, 0.999, sun);
        gl_FragColor = vec4(col, 1.0);
      }`,
  });
  const mesh = new Mesh(new SphereGeometry(2800, 24, 16), material);
  mesh.frustumCulled = false;
  mesh.renderOrder = -1;
  return {
    mesh,
    update(cameraPos, up, sunDir) {
      mesh.position.copy(cameraPos);
      material.uniforms.uUp!.value.copy(up);
      material.uniforms.uSun!.value.copy(sunDir);
    },
  };
}