fix(sky): keep the Sun and Moon, settle outside the ICRF tables

- Pass the custom sky box to the Viewer instead of `skyBox: false`, which
  also stopped CesiumWidget from creating the Sun and the Moon. Guarded by
  scripts/qa-sky-sun-moon.mjs.
- Outside the IAU 2006 tables (before 1975, after ~2050) the preload
  resolves without data; remember that span and stay on TEME instead of
  preloading and rendering every frame.
- The anime style no longer cel-shades the far plane, where four bands
  turned the faint Milky Way into blotches.
- Paint the planets out after the star-removing opening and with the
  ring median, so no brighter plateau is left; faces rebuilt.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/sky/starrySky.js

@@ -174,6 +174,7 @@ export class StarrySky {
     this._rotationPrecise = false;
     this._icrfRequest = undefined;
     this._icrfUnavailable = false;
+    this._icrfMissing = undefined;
     this._loading = false;
     this._starData = undefined;
     this._faceImages = undefined;
@@ -237,7 +238,7 @@ export class StarrySky {
   _updateRotation(time) {
     // An approximate rotation is final only while nothing better can come.
     const settled =
-      this._rotationPrecise || this._icrfRequest || this._icrfUnavailable;
+      this._rotationPrecise || this._icrfRequest || !this._canPreloadIcrf(time);
     if (settled && isSkyRotationCurrent(this._rotationTime, time)) return;
     const { precise } = computeSkyRotation(time, this._rotation);
     this._rotationPrecise = precise;
@@ -245,9 +246,20 @@ export class StarrySky {
     if (!precise) this._preloadIcrf(time);
   }
 
-  /** Fetch Cesium's IAU 2006 tables around `time` once; retry on new spans. */
+  /** Whether loading Cesium's IAU 2006 tables could still help at `time`. */
+  _canPreloadIcrf(time) {
+    return (
+      !this._icrfUnavailable &&
+      !(
+        this._icrfMissing &&
+        Cesium.TimeInterval.contains(this._icrfMissing, time)
+      )
+    );
+  }
+
+  /** Fetch Cesium's IAU 2006 tables around `time` once per span. */
   _preloadIcrf(time) {
-    if (this._icrfRequest || this._icrfUnavailable) return;
+    if (this._icrfRequest || !this._canPreloadIcrf(time)) return;
     const interval = new Cesium.TimeInterval({
       start: Cesium.JulianDate.addDays(time, -1, new Cesium.JulianDate()),
       stop: Cesium.JulianDate.addDays(time, 1, new Cesium.JulianDate()),
@@ -257,7 +269,14 @@ export class StarrySky {
     )
       .then(
         () => {
+          // The tables cover roughly 1975–2050; outside them the preload
+          // resolves with nothing, and TEME stays for this span.
+          if (!Cesium.Transforms.computeIcrfToFixedMatrix(time)) {
+            this._icrfMissing = interval;
+            return;
+          }
           this._rotationTime = undefined;
+          if (!this._destroyed) this._requestRender();
         },
         (error) => {
           // TEME is within a fraction of a degree; do not retry every frame.
@@ -267,7 +286,6 @@ export class StarrySky {
       )
       .finally(() => {
         this._icrfRequest = undefined;
-        if (!this._destroyed) this._requestRender();
       });
   }
 

src/sky/starrySky.test.mjs

@@ -87,3 +87,31 @@ test('the sky draws nothing when hidden, outside 3D, or in a pick pass', () => {
   assert.equal(sky.destroy(), undefined);
   assert.equal(sky.isDestroyed(), true);
 });
+
+const frame = () => new Promise((resolve) => setImmediate(resolve));
+
+for (const iso of ['1970-01-01T00:00:00Z', '2060-01-01T00:00:00Z']) {
+  test(`a clock outside the IAU 2006 tables (${iso}) settles on TEME instead of re-rendering every frame`, async () => {
+    const time = Cesium.JulianDate.fromIso8601(iso);
+    let renders = 0;
+    let preloads = 0;
+    const original = Cesium.Transforms.preloadIcrfFixed;
+    Cesium.Transforms.preloadIcrfFixed = (interval) => {
+      preloads += 1;
+      return original(interval);
+    };
+    try {
+      const sky = new StarrySky({ requestRender: () => (renders += 1) });
+      for (let i = 0; i < 20; i += 1) {
+        sky._updateRotation(time); // what update() does once per frame
+        await frame();
+      }
+      assert.ok(
+        renders <= 1 && preloads <= 1,
+        `20 frames at a frozen clock -> ${preloads} preloads, ${renders} render requests (expected <= 1 each)`,
+      );
+    } finally {
+      Cesium.Transforms.preloadIcrfFixed = original;
+    }
+  });
+}