diff --git a/src/app/features/body-detail/body-detail-scene.component.spec.ts b/src/app/features/body-detail/body-detail-scene.component.spec.ts
index de7694d..194eea2 100644
--- a/src/app/features/body-detail/body-detail-scene.component.spec.ts
+++ b/src/app/features/body-detail/body-detail-scene.component.spec.ts
@@ -34,9 +34,11 @@ const SATURN: BodyRecord = {
   rates: {meanMotionDegPerDay: 0.033459683702669406, longitudeOfAscendingNodeDegPerDay: -0.000006848734291581108, argumentOfPeriapsisDegPerDay: 0.000021682266940451745},
   rotationalElements: {poleRaDeg: [40.589, -0.036, 0], poleDecDeg: [83.537, -0.004, 0], primeMeridianDeg: [38.9, 810.7939024, 0]}
 };
-// Eris as it is shipped for this page's purposes: no IAU model, so its page keeps its own light.
-const ERIS: BodyRecord = { ...EARTH, id: 'eris', name: 'Eris', kind: 'dwarf', radiusKm: 1163, rotationalElements: undefined };
-const BODIES = [EARTH, SATURN, ERIS];
+// Eris and Hyperion as they are shipped for this page's purposes: no IAU model, so their pages keep
+// their own light; Eris's day is measured, and Hyperion tumbles and has none.
+const ERIS: BodyRecord = { ...EARTH, id: 'eris', name: 'Eris', kind: 'dwarf', radiusKm: 1163, rotationalElements: undefined, rotationPeriodHours: 378.504 };
+const HYPERION: BodyRecord = { ...ERIS, id: 'hyperion', name: 'Hyperion', kind: 'moon', radiusKm: 135, parentBodyId: 'saturn', rotationPeriodHours: undefined };
+const BODIES = [EARTH, SATURN, ERIS, HYPERION];
 
 /** Stands in for the WebGPU engine: a scene, a camera, and the tick hook, driven by hand. */
 class FakeEngineService {
@@ -155,6 +157,30 @@ describe('BodyDetailSceneComponent', () => {
     expect(page.sunLight.position.distanceTo(new THREE.Vector3(4, 3, 5))).toBeLessThan(1e-9);
   });
 
+  it('puts the sphere back at rest when the next body shown does not turn: Hyperion after Earth', async () => {
+    await open('earth');
+    expect(page.planet.quaternion.angleTo(new THREE.Quaternion())).toBeGreaterThan(0.1);
+    route.next(convertToParamMap({ id: 'hyperion' }));
+    await flushAsync();
+    engine.tick(0.016);
+    engine.tick(0.016);
+    expect(page.planet.quaternion.angleTo(new THREE.Quaternion())).toBeLessThan(1e-9);
+  });
+
+  it('turns a body whose day is measured but not its pole at that day on the map’s clock: Eris a sixth of a turn in 63.084 hours', async () => {
+    await open('eris');
+    const start = page.planet.rotation.y;
+    // The clock stands (the page is opened at rate 0): so does Eris, where it used to turn for show.
+    engine.tick(1);
+    expect(page.planet.rotation.y).toBe(start);
+    time.setDate(new Date(Date.parse('2025-06-01T12:00Z') + (378.504 / 6) * 3600000));
+    engine.tick(0.016);
+    const turned = (((page.planet.rotation.y - start) / (2 * Math.PI)) % 1 + 1) % 1;
+    expect(turned).toBeCloseTo(1 / 6, 6);
+    // Pole up, as the system view turns it about its orbit's normal.
+    expect(new THREE.Vector3(0, 1, 0).applyQuaternion(page.planet.quaternion).y).toBeCloseTo(1, 12);
+  });
+
   it('says on its dock the date the body is drawn for, and nothing at the present, and offers the clock', async () => {
     await open('saturn', '2032-06-01T12:00Z', true);
     fixture.detectChanges();
diff --git a/src/app/features/body-detail/body-detail-scene.component.ts b/src/app/features/body-detail/body-detail-scene.component.ts
index 100c3e1..2e1486c 100644
--- a/src/app/features/body-detail/body-detail-scene.component.ts
+++ b/src/app/features/body-detail/body-detail-scene.component.ts
@@ -63,8 +63,9 @@ const SUN_LIGHT_POSITION = new THREE.Vector3(4, 3, 5);
         </div>
       }
       <!-- Search, what has been kept and the clock: there is no scene readout here, the info
-           panel is the reading, and the panel's own control is what keeps this body. The body is
-           drawn at the clock's date and turns at its rate, so both are shown and can be set here. -->
+           panel is the reading, and the panel's own control is what keeps this body. A solar-system
+           body is drawn at the clock's date and turns at its rate, so both are shown and can be set
+           here; an exoplanet, whose day no one has measured, turns for show whatever the clock says. -->
       <app-hud-dock [date]="date()" [clock]="true" (bookmarkChosen)="goToBookmark($event)" />
     </div>
   `
@@ -280,8 +281,10 @@ export class BodyDetailSceneComponent implements AfterViewInit, OnDestroy {
 
   /**
    * A body the IAU gives rotational elements for is turned as it is at the map's date, under its
-   * real Sun, at the rate the map's clock runs (see `bodyPageView`). Any other — an exoplanet, or
-   * Eris, Haumea or Makemake — turns slowly for show, as the page always turned them.
+   * real Sun, at the rate the map's clock runs (see `bodyPageView`). Eris, Haumea, Makemake and
+   * Nereid, whose day is measured but whose pole is not, turn pole up at that day on the same
+   * clock, as the system view turns them; Hyperion, which tumbles, is left still, as it is there.
+   * An exoplanet turns slowly for show, as the page always turned it.
    */
   private tick(deltaSeconds: number): void {
     this.controls?.update();
@@ -292,7 +295,11 @@ export class BodyDetailSceneComponent implements AfterViewInit, OnDestroy {
     const sunAzimuth = Math.atan2(SUN_LIGHT_POSITION.x, SUN_LIGHT_POSITION.z);
     if (this.body && bodyPageView(this.body, this.bodies, this.time.julianDate(), sunAzimuth, this.planet.quaternion, this.sunLight.position)) {
       this.sunLight.position.multiplyScalar(SUN_LIGHT_POSITION.length());
-    } else {
+    } else if (this.body?.rotationPeriodHours !== undefined) {
+      // Counted from the orbit's epoch, as `spinFor` counts: where the meridian starts is unknown.
+      const turns = ((this.time.julianDate() - this.body.orbit.epochJd) * 24) / this.body.rotationPeriodHours;
+      this.planet.rotation.set(0, (turns % 1) * 2 * Math.PI, 0);
+    } else if (!this.body) {
       this.planet.rotation.y += deltaSeconds * 0.08;
     }
     const sunSide = this.sunLight.position.y < 0 ? -1 : 1;