diff --git a/src/app/features/body-detail/body-detail.model.ts b/src/app/features/body-detail/body-detail.model.ts
index eed591b..bc02d0f 100644
--- a/src/app/features/body-detail/body-detail.model.ts
+++ b/src/app/features/body-detail/body-detail.model.ts
@@ -33,9 +33,8 @@ export interface BodyDetailViewModel {
   /** True when a real photograph is being shown rather than the derived surface. */
   hasPhotography: boolean;
   /**
-   * Sidereal orbital period. Measured where the archive published one; otherwise derived from the
-   * semi-major axis for heliocentric orbits, where the central mass is known exactly. Undefined
-   * when neither applies — see `heliocentricPeriodDays`.
+   * Sidereal orbital period. For a solar-system body, 360 degrees over JPL's published mean
+   * motion; for an exoplanet, the archive's period where it published one, and undefined where not.
    */
   orbitalPeriodDays?: number;
   /**
@@ -44,4 +43,6 @@ export interface BodyDetailViewModel {
    * derived surface as a photograph.
    */
   orbitalPeriodSource?: 'measured' | 'derived';
+  /** Where the orbit comes from and the span it holds over; see `BodyRecord.orbitSource`. */
+  orbitSource?: string;
 }
diff --git a/src/app/features/body-detail/body-readouts.ts b/src/app/features/body-detail/body-readouts.ts
index 67611fc..9953ade 100644
--- a/src/app/features/body-detail/body-readouts.ts
+++ b/src/app/features/body-detail/body-readouts.ts
@@ -65,7 +65,8 @@ export function bodyReadouts(body: BodyDetailViewModel): BodyReadouts {
     derived.push({ label: 'Bulk density', value: formatDensity(body.appearance.bulkDensityGramsPerCm3) });
   }
 
-  return { kindLabel: KIND_LABELS[body.kind], measured, derived, provenance: provenanceFor(body) };
+  const provenance = body.orbitSource ? `${provenanceFor(body)} Orbit: ${body.orbitSource}.` : provenanceFor(body);
+  return { kindLabel: KIND_LABELS[body.kind], measured, derived, provenance };
 }
 
 /**
diff --git a/src/app/features/body-detail/body-view-model.spec.ts b/src/app/features/body-detail/body-view-model.spec.ts
index 4792435..91a1e69 100644
--- a/src/app/features/body-detail/body-view-model.spec.ts
+++ b/src/app/features/body-detail/body-view-model.spec.ts
@@ -3,7 +3,8 @@ import { describe, expect, it } from 'vitest';
 import { BodyRecord, OrbitalElements } from '../../shared/models/body.model';
 import { ExoplanetRecord } from '../../shared/models/exoplanet.model';
 import { StarRecord, SUN_STAR_ID } from '../../shared/models/star.model';
-import { buildBodyViewModel, heliocentricPeriodDays } from './body-view-model';
+import { bodyReadouts } from './body-readouts';
+import { buildBodyViewModel } from './body-view-model';
 
 const orbit = (overrides: Partial<OrbitalElements> = {}): OrbitalElements => ({
   semiMajorAxisAu: 1,
@@ -51,43 +52,24 @@ const luna: BodyRecord = {
   orbitSource: 'JPL SSD satellite mean elements, epoch 2000 Jan 1',
 };
 
-describe('heliocentricPeriodDays', () => {
-  it('recovers a known period from the semi-major axis alone', () => {
-    // P² = a³ in these units, so Earth must come back a year.
-    expect(heliocentricPeriodDays(earth)).toBeCloseTo(365.25, 1);
-  });
-
-  it('scales as the three-halves power', () => {
-    const jupiter: BodyRecord = {
-      ...earth,
-      id: 'jupiter',
-      name: 'Jupiter',
-      orbit: orbit({ semiMajorAxisAu: 5.2044 }),
-    };
-    // Jupiter's real sidereal period is 4332.6 days.
-    expect(heliocentricPeriodDays(jupiter)).toBeCloseTo(4335, -1);
-  });
-
-  it('refuses to compute a period for a moon', () => {
-    // A moon's elements are relative to its planet, whose mass is not in the catalogue — the
-    // same arithmetic would be wrong by the ratio of that planet's mass to the Sun's.
-    expect(heliocentricPeriodDays(luna)).toBeUndefined();
-  });
-});
-
 describe('buildBodyViewModel', () => {
   const catalogues = { bodies: [earth, luna], exoplanets: [] as ExoplanetRecord[], stars: [sun] };
 
-  it('marks a period computed from the semi-major axis as derived', () => {
+  it('gives a planet the sidereal year its published mean motion goes round in', () => {
     const model = buildBodyViewModel('earth', catalogues);
-    expect(model?.orbitalPeriodSource).toBe('derived');
-    expect(model?.orbitalPeriodDays).toBeCloseTo(365.25, 1);
+    expect(model?.orbitalPeriodSource).toBe('measured');
+    expect(model?.orbitalPeriodDays).toBeCloseTo(365.2564, 4);
   });
 
-  it('leaves a moon without a period rather than inventing one', () => {
+  it('gives a moon its period too, from the same mean motion that carries it round', () => {
+    // The card used to refuse, while the scene turned the Moon round the Earth all the same.
     const model = buildBodyViewModel('luna', catalogues);
-    expect(model?.orbitalPeriodDays).toBeUndefined();
-    expect(model?.orbitalPeriodSource).toBeUndefined();
+    expect(model?.orbitalPeriodSource).toBe('measured');
+    expect(model?.orbitalPeriodDays).toBeCloseTo(27.32166, 5);
+  });
+
+  it('says where the orbit comes from, in the card’s provenance', () => {
+    expect(bodyReadouts(buildBodyViewModel('luna', catalogues)!).provenance).toContain('Orbit: JPL SSD satellite mean elements, epoch 2000 Jan 1.');
   });
 
   it('marks a published exoplanet period as measured, not derived', () => {
diff --git a/src/app/features/body-detail/body-view-model.ts b/src/app/features/body-detail/body-view-model.ts
index becca17..9df6c18 100644
--- a/src/app/features/body-detail/body-view-model.ts
+++ b/src/app/features/body-detail/body-view-model.ts
@@ -4,7 +4,7 @@ import { luminositySolar } from '../../shared/astro/stellar';
 import { bodyTexturePath } from '../../shared/rendering/texture-catalog';
 import { BodyRecord } from '../../shared/models/body.model';
 import { ExoplanetRecord } from '../../shared/models/exoplanet.model';
-import { StarRecord, SUN_STAR_ID } from '../../shared/models/star.model';
+import { StarRecord } from '../../shared/models/star.model';
 import { BodyDetailViewModel } from './body-detail.model';
 
 /** Everything the view model is assembled from — the three catalogues, already loaded. */
@@ -41,7 +41,10 @@ export function buildBodyViewModel(id: string, catalogues: BodyCatalogues): Body
   const body = catalogues.bodies.find((candidate) => candidate.id === id);
   if (body) {
     const hostStar = catalogues.stars.find((star) => star.id === body.systemStarId);
-    const periodDays = heliocentricPeriodDays(body);
+    // The period the map draws, moons included: JPL's own mean motion, which is also what
+    // carries the body round the scene. Europa's card had no period at all while the scene
+    // turned it round Jupiter in 3.55 days.
+    const periodDays = 360 / body.rates.meanMotionDegPerDay;
     return {
       id: body.id,
       name: body.name,
@@ -53,7 +56,8 @@ export function buildBodyViewModel(id: string, catalogues: BodyCatalogues): Body
       appearance: appearanceForBody(body, catalogues.bodies, luminosityOf(hostStar)),
       hasPhotography: bodyTexturePath(body.id) !== undefined,
       orbitalPeriodDays: periodDays,
-      orbitalPeriodSource: periodDays === undefined ? undefined : 'derived',
+      orbitalPeriodSource: 'measured',
+      orbitSource: body.orbitSource,
     };
   }
 
@@ -81,19 +85,3 @@ export function buildBodyViewModel(id: string, catalogues: BodyCatalogues): Body
     orbitalPeriodSource: exoplanet.periodDays === undefined ? undefined : 'measured',
   };
 }
-
-/**
- * Kepler's third law for a body orbiting the Sun: P² = a³ with P in years and a in AU, which
- * holds exactly in these units because the Sun's mass is the unit of mass.
- *
- * Only for heliocentric orbits. A moon's elements are relative to its parent planet, whose mass
- * the catalogue does not carry, so the same arithmetic there would be wrong by the ratio of the
- * planet's mass to the Sun's — a factor of a thousand for Jupiter.
- */
-export function heliocentricPeriodDays(body: BodyRecord): number | undefined {
-  if (body.parentBodyId !== undefined || body.systemStarId !== SUN_STAR_ID) {
-    return undefined;
-  }
-  const a = body.orbit.semiMajorAxisAu;
-  return a > 0 ? Math.pow(a, 1.5) * 365.25 : undefined;
-}
diff --git a/src/app/features/galaxy-system/galaxy-system-scene.component.spec.ts b/src/app/features/galaxy-system/galaxy-system-scene.component.spec.ts
index 89a5a7e..97142b2 100644
--- a/src/app/features/galaxy-system/galaxy-system-scene.component.spec.ts
+++ b/src/app/features/galaxy-system/galaxy-system-scene.component.spec.ts
@@ -831,6 +831,19 @@ describe('GalaxySystemSceneComponent camera-flight transitions', () => {
     expect(navigationStore.viewLevel()).toBe('system');
   });
 
+  it('says where a system’s orbits come from, and for the Sun how long they hold', async () => {
+    const note = (): string => (fixture.componentInstance as unknown as { hudNote: () => string }).hudNote();
+    navigationStore.selectStar(SUN.id);
+    await flushAsync();
+    await advanceFrames(engine, 2.5);
+    expect(note()).toBe('Orbits propagated from JPL mean elements, the planets’ fit for 3000 BC to AD 3000, to the current date.');
+
+    navigationStore.selectStar(ALPHA_CENTAURI.id);
+    await flushAsync();
+    await advanceFrames(engine, 5);
+    expect(note()).toBe('Orbits propagated from published elements to the current date.');
+  });
+
   it('performs the floating-origin recenter: the camera lands close to the AU-space origin, not out at parsec-scale coordinates', async () => {
     navigationStore.selectStar(ALPHA_CENTAURI.id);
     await flushAsync();
diff --git a/src/app/features/galaxy-system/galaxy-system-scene.component.ts b/src/app/features/galaxy-system/galaxy-system-scene.component.ts
index 78454f9..45ca160 100644
--- a/src/app/features/galaxy-system/galaxy-system-scene.component.ts
+++ b/src/app/features/galaxy-system/galaxy-system-scene.component.ts
@@ -1760,7 +1760,10 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
           ? [{ label: 'Luminosity', value: formatLuminosity(luminosity), derived: true }]
           : []),
       ]);
-      this.hudNote.set(this.time.atNow() ? 'Orbits propagated from published elements to the current date.' : 'Orbits propagated from published elements to the date on the clock.');
+      // Where the orbits come from, and for the Sun how far from the present they hold: each
+      // body's card names its own source and epoch.
+      const source = this.bodies.some((body) => body.systemStarId === star.id) ? 'JPL mean elements, the planets’ fit for 3000 BC to AD 3000,' : 'published elements';
+      this.hudNote.set(`Orbits propagated from ${source} to ${this.time.atNow() ? 'the current date' : 'the date on the clock'}.`);
       this.hudRange.set(
         formatAu(
           this.engine.visibleHalfHeight(