diff --git a/src/app/features/scene/index.ts b/src/app/features/scene/index.ts
index 3cc42486..55b00e86 100644
--- a/src/app/features/scene/index.ts
+++ b/src/app/features/scene/index.ts
@@ -2,7 +2,6 @@ import type { Scene, SceneOptions } from './Scene';
 import { BackgroundSceneConfig, Capabilities, SceneTier, resolveTier } from './tier';
 import { getSkyState, subscribe } from './skyState';
 import type { SkyState } from './skyState';
-import { ridgeSkyFor, terrainSkyFor } from './skyForState';
 
 export type { Scene, SceneStats } from './Scene';
 export type { SceneStyle, SceneTier } from './tier';
@@ -23,6 +22,8 @@ async function create(
 ): Promise<Scene | null> {
   let createScene: (o: SceneOptions) => Scene | null;
   try {
+    // Loaded here, with the scenes, so the main chunk does not carry the terrain sky or the baked tables.
+    const { ridgeSkyFor, terrainSkyFor } = await import('./skyForState');
     if (config?.style === 'raymarch') createScene = (await import('./Scene')).createScene;
     else if (config?.style === 'terrain') {
       const { createTerrainScene } = await import('./terrain/Terrain');
diff --git a/src/app/features/scene/skyForState.test.ts b/src/app/features/scene/skyForState.test.ts
index 1e09021d..20bc023a 100644
--- a/src/app/features/scene/skyForState.test.ts
+++ b/src/app/features/scene/skyForState.test.ts
@@ -72,31 +72,43 @@ describe('terrainSkyFor', () => {
 });
 
 describe('the terrain sky lies inside skyBackdrop', () => {
-  // A grid offset from the one skyBackdrop samples, so a miss between its samples shows.
+  // Not the generator's grid: a golden-angle spiral over the upper sphere (no shared rows or columns)
+  // plus a 0.05 degree grid across the sun, five times finer than the generator's.
+  const luma = (c: readonly number[]) => {
+    const [r, g, b] = c.map((v) => Math.min(1, Math.max(0, v)));
+    return 0.2126 * r + 0.7152 * g + 0.0722 * b;
+  };
   const sweep = (state: 'day' | 'dusk' | 'night') => {
     const sky = chooseSky(terrainSkyFor(state));
+    const rad = Math.PI / 180;
+    const dirAt = (el: number, az: number): [number, number, number] => [
+      Math.cos(el * rad) * Math.sin(az * rad),
+      Math.sin(el * rad),
+      -Math.cos(el * rad) * Math.cos(az * rad),
+    ];
     const out: number[] = [];
-    for (let el = 1.3; el <= 90; el += 4.1) {
-      for (let az = 0.7; az < 360; az += 4.3) {
-        const e = (el * Math.PI) / 180;
-        const a = (az * Math.PI) / 180;
-        const c = skyColour(sky, [
-          Math.cos(e) * Math.sin(a),
-          Math.sin(e),
-          -Math.cos(e) * Math.cos(a),
-        ]);
-        const [r, g, b] = c.map((v) => Math.min(1, Math.max(0, v)));
-        out.push(0.2126 * r + 0.7152 * g + 0.0722 * b);
+    const N = 20000;
+    for (let i = 0; i < N; i += 1) {
+      const y = (i + 0.5) / N; // sin(elevation): uniform over the upper hemisphere
+      out.push(luma(skyColour(sky, dirAt(Math.asin(y) / rad, (i * 137.50776405) % 360))));
+    }
+    for (let dEl = -3; dEl <= 3; dEl += 0.05) {
+      for (let dAz = -3; dAz <= 3; dAz += 0.05) {
+        out.push(luma(skyColour(sky, dirAt(Math.max(0, sky.elevation + dEl), sky.azimuth + dAz))));
       }
     }
     return out;
   };
 
-  it.each(['day', 'dusk', 'night'] as const)('for %s', (state) => {
+  it.each(['day', 'dusk', 'night'] as const)('for %s, and no looser than 5e-4 from it', (state) => {
     const { darkest, lightest } = skyBackdrop(state);
     const l = sweep(state);
     expect(Math.min(...l)).toBeGreaterThanOrEqual(darkest - EPS);
     expect(Math.max(...l)).toBeLessThanOrEqual(lightest + EPS);
+    const top = Math.max(Math.max(...l), ridgeDrawn(state)?.lightest ?? 0);
+    expect(lightest - top).toBeLessThan(5e-4);
+    const bottom = Math.min(Math.min(...l), ridgeDrawn(state)?.darkest ?? 1);
+    expect(bottom - darkest).toBeLessThan(5e-4);
   });
 
   it('and the terrain dusk reaches the sun glow the ridge dusk does not', () => {
@@ -116,3 +128,92 @@ describe('both scenes of a state lie inside skyBackdrop', () => {
     });
   });
 });
+
+describe('skyBackdrop holds the union of what the scenes draw', () => {
+  // An independent extremum: not terrainDrawn's grid and compass search but alternating golden-section
+  // line searches in elevation and azimuth from 60 starts (a golden-angle spiral and scatter about the sun),
+  // the bracket shrinking to 1e-3 degrees. A plateau (the shader clamps) cannot stall it: each line search
+  // always moves to the better end of its bracket.
+  const extremes = (state: 'day' | 'dusk' | 'night') => {
+    const sky = chooseSky(terrainSkyFor(state));
+    const rad = Math.PI / 180;
+    const f = (el: number, az: number) => {
+      const e = Math.min(90, Math.max(0, el)) * rad;
+      const a = az * rad;
+      const [r, g, b] = skyColour(sky, [
+        Math.cos(e) * Math.sin(a),
+        Math.sin(e),
+        -Math.cos(e) * Math.cos(a),
+      ]).map((v) => Math.min(1, Math.max(0, v)));
+      return 0.2126 * r + 0.7152 * g + 0.0722 * b;
+    };
+    const P = 0.6180339887;
+    const line = (g: (x: number) => number, mid: number, w: number) => {
+      let a = mid - w;
+      let b = mid + w;
+      for (let k = 0; k < 40; k += 1) {
+        const c = b - P * (b - a);
+        const d = a + P * (b - a);
+        if (g(c) > g(d)) b = d;
+        else a = c;
+      }
+      return (a + b) / 2;
+    };
+    const search = (sign: 1 | -1) => {
+      let best = -Infinity;
+      for (let i = 0; i < 60; i += 1) {
+        let el =
+          i % 2
+            ? sky.elevation + (((i * 7919) % 1600) - 800) / 100
+            : Math.asin((i + 0.5) / 60) / rad;
+        let az =
+          i % 2 ? sky.azimuth + (((i * 104729) % 1600) - 800) / 100 : (i * 137.50776405) % 360;
+        for (let w = 8; w > 1e-3; w /= 2) {
+          const [el0, az0] = [el, az];
+          el = line((x) => sign * f(Math.max(0, x), az0), el0, w);
+          const el1 = el;
+          az = line((x) => sign * f(el1, x), az0, w);
+          best = Math.max(best, sign * f(el, az));
+        }
+      }
+      return sign * best;
+    };
+    return { darkest: search(-1), lightest: search(1) };
+  };
+
+  const computed = (state: 'day' | 'dusk' | 'night') => {
+    const r = [ridgeDrawn(state), terrainDrawn(state)].filter((x) => x !== undefined) as {
+      darkest: number;
+      lightest: number;
+    }[];
+    const ind = extremes(state);
+    const ridge = ridgeDrawn(state);
+    return {
+      darkest: Math.min(...r.map((x) => x.darkest), ind.darkest, ridge?.darkest ?? 1),
+      lightest: Math.max(...r.map((x) => x.lightest), ind.lightest, ridge?.lightest ?? 0),
+    };
+  };
+
+  // The constants are the swept extremes rounded outward to six places, so each lies in [extreme, extreme + 1e-6):
+  // a constant 1e-6 low is under the extreme, one 2e-5 high is over the bound. SEARCH is the searches' own error.
+  const SEARCH = 1e-8;
+  it.each(['day', 'dusk', 'night'] as const)(
+    '%s: the constant is the swept extreme rounded outward, within 1e-6 above it',
+    (state) => {
+      const c = computed(state);
+      const b = skyBackdrop(state);
+      expect(b.darkest).toBeLessThanOrEqual(c.darkest);
+      expect(b.lightest).toBeGreaterThanOrEqual(c.lightest);
+      expect(c.darkest - b.darkest).toBeLessThan(1e-6 + SEARCH);
+      expect(b.lightest - c.lightest).toBeLessThan(1e-6 + SEARCH);
+    }
+  );
+
+  it('auto is the union of the three fixed states', () => {
+    const all = (['day', 'dusk', 'night'] as const).map(skyBackdrop);
+    expect(skyBackdrop('auto')).toEqual({
+      darkest: Math.min(...all.map((x) => x.darkest)),
+      lightest: Math.max(...all.map((x) => x.lightest)),
+    });
+  });
+});
diff --git a/src/app/features/scene/skyForState.ts b/src/app/features/scene/skyForState.ts
index aa3a7803..aedc6861 100644
--- a/src/app/features/scene/skyForState.ts
+++ b/src/app/features/scene/skyForState.ts
@@ -70,6 +70,10 @@ export function ridgeDrawn(state: SkyState): Range | undefined {
 }
 
 const SAMPLE_DEG = 3;
+const SUN_DEG = 0.25;
+const SUN_SPAN_DEG = 8;
+const HORIZON_DEG = 0.5;
+const REFINE_FROM = 6;
 const linearLuminance = (c: Vec3) => {
   const [r, g, b] = c.map((v) => Math.min(1, Math.max(0, v)));
   return 0.2126 * r + 0.7152 * g + 0.0722 * b;
@@ -77,22 +81,64 @@ const linearLuminance = (c: Vec3) => {
 
 /**
  * The terrain sky's range: `skyColour` (the CPU twin of its sky shader) over the whole upper sphere, which the camera
- * sweeps as it turns, for the sky the scene picks for this state. Sampled every 3 degrees.
+ * sweeps as it turns, for the sky the scene picks for this state. A 3 degree grid, a 0.5 degree ring of the horizon, the
+ * zenith and a 0.25 degree patch around the sun find where the extremes are; each of the best few is then refined by a
+ * compass search whose step halves from 1 degree to under 0.001, because the sun's glow peaks (and the sky's darkest
+ * patch sits) between any fixed grid's samples.
  */
 export function terrainDrawn(state: SkyState): Range {
   const sky = chooseSky(terrainSkyFor(state));
-  let darkest = Infinity;
-  let lightest = -Infinity;
+  const rad = Math.PI / 180;
+  const lum = (el: number, az: number) => {
+    const e = Math.min(90, Math.max(0, el)) * rad;
+    const a = az * rad;
+    return linearLuminance(
+      skyColour(sky, [Math.cos(e) * Math.sin(a), Math.sin(e), -Math.cos(e) * Math.cos(a)])
+    );
+  };
+  const seen: { el: number; az: number; l: number }[] = [];
+  const at = (el: number, az: number) => seen.push({ el, az, l: lum(el, az) });
   for (let el = 0; el <= 90; el += SAMPLE_DEG) {
-    for (let az = 0; az < 360; az += SAMPLE_DEG) {
-      const e = (el * Math.PI) / 180;
-      const a = (az * Math.PI) / 180;
-      const l = linearLuminance(
-        skyColour(sky, [Math.cos(e) * Math.sin(a), Math.sin(e), -Math.cos(e) * Math.cos(a)])
-      );
-      darkest = Math.min(darkest, l);
-      lightest = Math.max(lightest, l);
+    for (let az = 0; az < 360; az += SAMPLE_DEG) at(el, az);
+  }
+  for (let az = 0; az < 360; az += HORIZON_DEG) at(0, az);
+  at(90, 0);
+  for (let dEl = -SUN_SPAN_DEG; dEl <= SUN_SPAN_DEG; dEl += SUN_DEG) {
+    for (let dAz = -SUN_SPAN_DEG; dAz <= SUN_SPAN_DEG; dAz += SUN_DEG) {
+      at(sky.elevation + dEl, sky.azimuth + dAz);
     }
   }
+  type Point = { el: number; az: number; l: number };
+  const around = (p: Point, step: number): Point[] =>
+    [
+      [step, 0],
+      [-step, 0],
+      [0, step],
+      [0, -step],
+    ].map(([dEl, dAz]) => {
+      const el = Math.min(90, Math.max(0, p.el + dEl));
+      return { el, az: p.az + dAz, l: lum(el, p.az + dAz) };
+    });
+  const refine = (from: Point, sign: 1 | -1) => {
+    let best = from;
+    for (let step = 1; step > 5e-7; step /= 2) {
+      let improved = true;
+      while (improved) {
+        const cur = best;
+        best = around(cur, step).reduce((b, c) => (sign * c.l > sign * b.l ? c : b), cur);
+        improved = best !== cur;
+      }
+    }
+    return best.l;
+  };
+  const byLight = [...seen].sort((p, q) => q.l - p.l);
+  const lightest = Math.max(
+    byLight[0].l,
+    ...byLight.slice(0, REFINE_FROM).map((p) => refine(p, 1))
+  );
+  const darkest = Math.min(
+    byLight[byLight.length - 1].l,
+    ...byLight.slice(-REFINE_FROM).map((p) => refine(p, -1))
+  );
   return { darkest, lightest };
 }
diff --git a/src/app/features/scene/skyState.ts b/src/app/features/scene/skyState.ts
index bc164d7a..dc2b6199 100644
--- a/src/app/features/scene/skyState.ts
+++ b/src/app/features/scene/skyState.ts
@@ -2,8 +2,6 @@
 // localStorage key; whoever draws the sky (the scene, the title bar's control) reads it here, and
 // `skyBackdrop` says how light or dark the sky behind the login text can be in each state, so the
 // text colours can be checked against it.
-import { ridgeDrawn, terrainDrawn } from './skyForState';
-
 export type SkyState = 'auto' | 'day' | 'dusk' | 'night';
 
 export const SKY_STATES: readonly SkyState[] = ['auto', 'day', 'dusk', 'night'];
@@ -58,27 +56,16 @@ export interface SkyBackdrop {
 }
 
 // The union of every sky a scene draws for the state: the ridge sky and the terrain sky, each from
-// the function its scene uses. 'auto' follows the clock (and a URL), so it bounds all three fixed
-// states. Computed on first use: the terrain range samples its sky.
-const union = (ranges: readonly (SkyBackdrop | undefined)[]): SkyBackdrop => {
-  const r = ranges.filter((x): x is SkyBackdrop => !!x);
-  return {
-    darkest: Math.min(...r.map((x) => x.darkest)),
-    lightest: Math.max(...r.map((x) => x.lightest)),
-  };
+// the function its scene uses (`ridgeDrawn`, `terrainDrawn` in skyForState.ts). Held as constants so
+// this module, which the main chunk loads, does not pull in the terrain sky and the baked scattering
+// tables with it; skyForState.test.ts fails when they drift from the computed union.
+// Rounded outward to six places. 'auto' follows the clock (and a URL), so it
+// bounds all three fixed states.
+const BACKDROPS: Record<SkyState, SkyBackdrop> = {
+  auto: { darkest: 0.016727, lightest: 0.967912 },
+  day: { darkest: 0.237309, lightest: 0.928052 },
+  dusk: { darkest: 0.016727, lightest: 0.967912 },
+  night: { darkest: 0.058099, lightest: 0.077467 },
 };
 
-const FIXED_STATES = ['day', 'dusk', 'night'] as const;
-const cache = new Map<SkyState, SkyBackdrop>();
-
-export function skyBackdrop(state: SkyState): SkyBackdrop {
-  let b = cache.get(state);
-  if (!b) {
-    b =
-      state === 'auto'
-        ? union(FIXED_STATES.map(skyBackdrop))
-        : union([ridgeDrawn(state), terrainDrawn(state)]);
-    cache.set(state, b);
-  }
-  return b;
-}
+export const skyBackdrop = (state: SkyState): SkyBackdrop => BACKDROPS[state];