Give every body the period it is drawn going round in, and say where its orbit comes from

Audit #38: Europa's card listed its axis, eccentricity and inclination but no period, while the
scene turned it round Jupiter all the same: heliocentricPeriodDays refused every moon, since the
catalogue carried no planet masses.

Every solar-system body's period is now 360 over the JPL mean motion that carries it round the
scene, filed under Measured since that is JPL's published figure: Europa 3.55 d, the Moon 27.3 d,
Saturn 29.5 yr on the live cards, Earth 365.2564 d. heliocentricPeriodDays is gone. Exoplanets
keep the archive's period, or none.

The card's provenance line now ends with where the orbit comes from, "Orbit: JPL SSD satellite
mean elements, epoch 1997 Jan 16." for Europa, "Orbit: JPL approximate mean elements (Standish),
fit for 3000 BC to AD 3000." for a planet, and the Sun's system note says so too: "Orbits
propagated from JPL mean elements, the planets' fit for 3000 BC to AD 3000, to the current
date." Other systems keep "published elements". All three read in the running app.

Each has a test that fails without it (moons refused a period, provenance without the orbit, a
note without the source).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

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', () => {