diff --git a/src/app/features/scene/skyState.test.ts b/src/app/features/scene/skyState.test.ts
new file mode 100644
index 00000000..71d35fd0
--- /dev/null
+++ b/src/app/features/scene/skyState.test.ts
@@ -0,0 +1,185 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { baked } from '../../../sky/baked';
+import { AA, luminance } from '../../../sky/skyTokens.js';
+import {
+  SKY_STATES,
+  SKY_STATE_KEY,
+  skyBackdrop,
+  type SkyBackdrop,
+  type SkyState,
+} from './skyState';
+
+type Mod = typeof import('./skyState');
+
+function fakeStorage(initial: Record<string, string> = {}) {
+  const data = new Map(Object.entries(initial));
+  return {
+    data,
+    getItem: (k: string) => data.get(k) ?? null,
+    setItem: (k: string, v: string) => {
+      data.set(k, v);
+    },
+  };
+}
+const fresh = (): Promise<Mod> => {
+  vi.resetModules();
+  return import('./skyState');
+};
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+describe('sky state', () => {
+  let storage: ReturnType<typeof fakeStorage>;
+  beforeEach(() => {
+    storage = fakeStorage();
+    vi.stubGlobal('localStorage', storage);
+  });
+
+  it('defaults to auto when nothing is stored', async () => {
+    expect((await fresh()).getSkyState()).toBe('auto');
+  });
+
+  it.each(['night', 'day', 'dusk', 'auto'] as const)('restores a stored %s', async (s) => {
+    storage.data.set(SKY_STATE_KEY, s);
+    expect((await fresh()).getSkyState()).toBe(s);
+  });
+
+  it.each(['', 'noon', 'NIGHT', '"night"', 'null'])(
+    'falls back to auto on stored %j',
+    async (v) => {
+      storage.data.set(SKY_STATE_KEY, v);
+      expect((await fresh()).getSkyState()).toBe('auto');
+    }
+  );
+
+  it('falls back to auto when storage throws', async () => {
+    vi.stubGlobal('localStorage', {
+      getItem: () => {
+        throw new Error('denied');
+      },
+      setItem: () => {
+        throw new Error('denied');
+      },
+    });
+    const m = await fresh();
+    expect(m.getSkyState()).toBe('auto');
+    m.setSkyState('night');
+    expect(m.getSkyState()).toBe('night');
+  });
+
+  it('persists a set state under one key and reads it back after a reload', async () => {
+    (await fresh()).setSkyState('dusk');
+    expect([...storage.data.entries()]).toEqual([[SKY_STATE_KEY, 'dusk']]);
+    expect((await fresh()).getSkyState()).toBe('dusk');
+  });
+
+  it('ignores an invalid state', async () => {
+    const m = await fresh();
+    m.setSkyState('night');
+    m.setSkyState('noon' as SkyState);
+    expect(m.getSkyState()).toBe('night');
+    expect(storage.data.get(SKY_STATE_KEY)).toBe('night');
+  });
+
+  it('notifies subscribers on change only, and stops after unsubscribe', async () => {
+    const m = await fresh();
+    const cb = vi.fn();
+    const off = m.subscribe(cb);
+    m.setSkyState('day');
+    m.setSkyState('day');
+    expect(cb.mock.calls).toEqual([['day']]);
+    off();
+    m.setSkyState('night');
+    expect(cb).toHaveBeenCalledTimes(1);
+  });
+
+  it('tells a late subscriber nothing until the next change', async () => {
+    const m = await fresh();
+    m.setSkyState('day');
+    const cb = vi.fn();
+    m.subscribe(cb);
+    expect(cb).not.toHaveBeenCalled();
+  });
+});
+
+describe('skyBackdrop', () => {
+  it('orders the fixed skies night < dusk < day by their lightest sky', () => {
+    expect(skyBackdrop('night').lightest).toBeLessThan(skyBackdrop('dusk').lightest);
+    expect(skyBackdrop('dusk').lightest).toBeLessThan(skyBackdrop('day').lightest);
+  });
+
+  it('is the luminance of the baked gradients for day and night', () => {
+    const lum = (g: readonly string[]) => g.map(luminance);
+    expect(skyBackdrop('day').lightest).toBe(Math.max(...lum(baked.noonday.gradient)));
+    expect(skyBackdrop('night').darkest).toBe(Math.min(...lum(baked.midnight.gradient)));
+  });
+
+  it('bounds every fixed sky when auto', () => {
+    const a = skyBackdrop('auto');
+    (['day', 'dusk', 'night'] as const).forEach((s) => {
+      expect(a.lightest).toBeGreaterThanOrEqual(skyBackdrop(s).lightest);
+      expect(a.darkest).toBeLessThanOrEqual(skyBackdrop(s).darkest);
+    });
+  });
+
+  it('never has darkest above lightest', () => {
+    SKY_STATES.forEach((s) =>
+      expect(skyBackdrop(s).darkest).toBeLessThanOrEqual(skyBackdrop(s).lightest)
+    );
+  });
+});
+
+// Login text and link, per theme, from the card variables in pages/auth/styles.css.ts (the
+// fallback pair serves Light, Silver, Dark and Butter) and the sky themes' baked bubble text and
+// link. Light ink is measured against the lightest sky it can sit on, dark ink against the darkest.
+const inks = [
+  { theme: 'Light/Silver/Dark/Butter', text: '#b7c7d9', link: '#99c7ff' },
+  { theme: 'Midnight Sky', text: baked.midnight.bubbles.OtherText, link: baked.midnight.link },
+  { theme: 'Noonday Sky', text: baked.noonday.bubbles.OtherText, link: baked.noonday.link },
+];
+// WCAG contrast of an ink against a backdrop of the given luminance, with the ink's polarity
+// deciding which end of the sky is its worst case.
+const ratio = (ink: string, b: SkyBackdrop) => {
+  const li = luminance(ink);
+  const bg = li > 0.18 ? b.lightest : b.darkest;
+  const [hi, lo] = li > bg ? [li, bg] : [bg, li];
+  return (hi + 0.05) / (lo + 0.05);
+};
+
+describe('login ink against the sky backdrop', () => {
+  const rows = SKY_STATES.flatMap((state) =>
+    inks.flatMap(({ theme, text, link }) =>
+      (
+        [
+          ['text', text],
+          ['link', link],
+        ] as const
+      ).map(([role, ink]) => ({ state, theme, role, ratio: ratio(ink, skyBackdrop(state)) }))
+    )
+  );
+
+  it('prints the measured ratios', () => {
+    // eslint-disable-next-line no-console
+    console.table(rows.map((r) => ({ ...r, ratio: r.ratio.toFixed(2), ok: r.ratio >= AA })));
+    expect(rows).toHaveLength(SKY_STATES.length * inks.length * 2);
+  });
+
+  // Bare sky does not hold either ink in most states: the card behind the login form stays opaque
+  // (pages/auth/styles.css.ts) for that reason. These are the cells that clear AA; every other
+  // cell is asserted to fail, so a sky or theme change that fixes one turns it red until listed.
+  const clears = new Set([
+    'day|Noonday Sky|text',
+    'day|Noonday Sky|link',
+    'night|Light/Silver/Dark/Butter|text',
+    'night|Light/Silver/Dark/Butter|link',
+    'night|Midnight Sky|text',
+  ]);
+  describe.each(rows)('$theme $role in $state', (r) => {
+    const holds = clears.has(`${r.state}|${r.theme}|${r.role}`);
+    (holds ? it : it.fails)(holds ? 'is at least AA' : 'is below AA (known)', () => {
+      expect(r.ratio).toBeGreaterThanOrEqual(AA);
+    });
+  });
+});
diff --git a/src/app/features/scene/skyState.ts b/src/app/features/scene/skyState.ts
new file mode 100644
index 00000000..60e0471f
--- /dev/null
+++ b/src/app/features/scene/skyState.ts
@@ -0,0 +1,84 @@
+// Which sky the signed-out page is drawn against. The state is one value, kept in one
+// 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 { baked } from '../../../sky/baked';
+import { luminance } from '../../../sky/skyTokens.js';
+import { skyColours } from './ridge/skyColour';
+
+export type SkyState = 'auto' | 'day' | 'dusk' | 'night';
+
+export const SKY_STATES: readonly SkyState[] = ['auto', 'day', 'dusk', 'night'];
+export const SKY_STATE_KEY = 'ratatoskr.skyState';
+
+const isSkyState = (v: unknown): v is SkyState => SKY_STATES.includes(v as SkyState);
+
+function load(): SkyState {
+  try {
+    const stored = localStorage.getItem(SKY_STATE_KEY);
+    return isSkyState(stored) ? stored : 'auto';
+  } catch {
+    return 'auto';
+  }
+}
+
+let current: SkyState | undefined;
+const listeners = new Set<(s: SkyState) => void>();
+
+export const getSkyState = (): SkyState => {
+  current ??= load();
+  return current;
+};
+
+export function setSkyState(s: SkyState): void {
+  if (!isSkyState(s)) return;
+  const changed = s !== getSkyState();
+  current = s;
+  try {
+    localStorage.setItem(SKY_STATE_KEY, s);
+  } catch {
+    // Storage is refused (private mode, quota): the choice holds for this page load only.
+  }
+  if (changed) [...listeners].forEach((cb) => cb(s));
+}
+
+export function subscribe(cb: (s: SkyState) => void): () => void {
+  listeners.add(cb);
+  return () => {
+    listeners.delete(cb);
+  };
+}
+
+/**
+ * Relative luminance (WCAG, 0..1) of the lightest and darkest sky the state can show behind the
+ * page. Light ink is worst against `lightest`, dark ink against `darkest`. 'auto' follows the
+ * clock, so it can show any of the three fixed skies and bounds all of them.
+ */
+export interface SkyBackdrop {
+  readonly darkest: number;
+  readonly lightest: number;
+}
+
+const hex = (c: readonly number[]) => `#${c.map((v) => v.toString(16).padStart(2, '0')).join('')}`;
+const bound = (colours: readonly string[]): SkyBackdrop => {
+  const l = colours.map(luminance);
+  return { darkest: Math.min(...l), lightest: Math.max(...l) };
+};
+
+const FIXED: Record<Exclude<SkyState, 'auto'>, SkyBackdrop> = (() => {
+  const dusk = skyColours();
+  return {
+    day: bound(baked.noonday.gradient),
+    dusk: bound([dusk.top, dusk.mid, dusk.horizon].map(hex)),
+    night: bound(baked.midnight.gradient),
+  };
+})();
+
+export function skyBackdrop(state: SkyState): SkyBackdrop {
+  if (state !== 'auto') return FIXED[state];
+  const all = Object.values(FIXED);
+  return {
+    darkest: Math.min(...all.map((b) => b.darkest)),
+    lightest: Math.max(...all.map((b) => b.lightest)),
+  };
+}