Print Hyperion's eccentricity as JPL measures it now, 0.105, not the archived row's 0.023

The card listed Hyperion's eccentricity under "Measured" as 0.023: the archived JPL satellite row
the orbit is drawn from gives 0.0232. JPL's current table (SAT441) gives 0.105, and Horizons'
osculating orbit ranges 0.074 to 0.132 from 1980 to 2100 (0.1099 on 2025-01-01). The row stays
the orbit: with 0.105 put into it, Hyperion is further from Horizons, not nearer (median 9.6
degrees against 7.8 over 1980-2100, as a reviewer measured), so only the card changes.

BodyRecord.measuredEccentricity carries the figure the card prints where it is not the orbit's
own; the ETL sets it for Hyperion, and buildBodyViewModel prints it. build.ts now checks every
card's eccentricity against Horizons' osculating one on 2025-01-01, within 0.03: measured at most
0.0151 (Phoebe, and the Moon, whose eccentricity swings) once Hyperion prints 0.105, where the
row put it 0.0867 out. The live Hyperion page reads "ECCENTRICITY 0.105".

Controls: the ETL with Hyperion on its row's figure fails ("Hyperion's card gives an eccentricity of
0.0232, where Horizons' osculating orbit has 0.1099"); the view model ignoring the field fails
"prints the eccentricity measured for a moon whose orbit keeps an older one".

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

tools/etl/build.ts

@@ -145,6 +145,8 @@ function validateMerge(stars: StarRecord[]): void {
  * 0.9.
  */
 const MAX_PLANET_OFFSET_DEG = 0.25;
+/** Measured on this catalogue: at most 0.0151 (Phoebe and the Moon) once Hyperion prints its current 0.105. */
+const MAX_ECCENTRICITY_OFFSET = 0.03;
 const MAX_MOON_OFFSET_DEG = 2.5;
 const KM_PER_AU = 149597870.7;
 const DEG_TO_RAD = Math.PI / 180;
@@ -259,6 +261,13 @@ function validateBodies(bodies: BodyRecord[], horizonsOrbits: Map<string, Orbita
       `${body.name}'s mean elements put it ${offset.toFixed(2)} degrees from where Horizons has it (at most ${ceiling} expected) — the elements were read wrongly.`
     );
     offsets.push(`${body.id} ${offset.toFixed(3)}`);
+    // The card prints this under "Measured". An osculating eccentricity swings about its mean — the
+    // Moon's by 0.015 here, Phoebe's by as much — but not by the 0.087 Hyperion's older row was out.
+    const printed = body.measuredEccentricity ?? body.orbit.eccentricity;
+    assertCondition(
+      Math.abs(printed - horizons!.eccentricity) <= MAX_ECCENTRICITY_OFFSET,
+      `${body.name}'s card gives an eccentricity of ${printed}, where Horizons' osculating orbit has ${horizons!.eccentricity.toFixed(4)} (at most ${MAX_ECCENTRICITY_OFFSET} apart expected).`
+    );
 
     // A radius of 0 is what a page whose radius no pattern reads comes out as — Charon's did.
     assertCondition(body.radiusKm > 0, `Body ${body.id} has no radius; its page states it in a form the ETL does not read.`);

tools/etl/fetchSolarSystem.ts

@@ -33,6 +33,8 @@ interface BodySpec {
   radiusKm?: number;
   /** A measured sidereal day, in hours, where a later measurement overturns the one its source gives. */
   rotationPeriodHours?: number;
+  /** The eccentricity for the card, where the row the orbit is drawn from gives an outdated one; see `BodyRecord.measuredEccentricity`. */
+  measuredEccentricity?: number;
   /** A moon that does not keep one face to its planet: its page's own spin, or none, is kept. */
   spinsFreely?: boolean;
   /** A moon heavy enough to move its planet round their barycentre visibly; see `BodyRecord.massRatio`. */
@@ -111,7 +113,8 @@ const BODY_SPECS: BodySpec[] = [
   { id: 'rhea', name: 'Rhea', kind: 'moon', horizonsCommand: '605', center: '500@699', parentBodyId: 'saturn' },
   { id: 'titan', name: 'Titan', kind: 'moon', horizonsCommand: '606', center: '500@699', parentBodyId: 'saturn' },
   // Hyperion tumbles ("Rotational period = Chaotic") and Phoebe, captured, turns in 9.27 hours.
-  { id: 'hyperion', name: 'Hyperion', kind: 'moon', horizonsCommand: '607', center: '500@699', parentBodyId: 'saturn', spinsFreely: true },
+  // Hyperion's eccentricity is 0.105 in JPL's current table (ssd.jpl.nasa.gov/sats/elem, SAT441).
+  { id: 'hyperion', name: 'Hyperion', kind: 'moon', horizonsCommand: '607', center: '500@699', parentBodyId: 'saturn', spinsFreely: true, measuredEccentricity: 0.105 },
   { id: 'iapetus', name: 'Iapetus', kind: 'moon', horizonsCommand: '608', center: '500@699', parentBodyId: 'saturn' },
   // Phoebe's row gives a mean motion of 0.6569114 degrees a day, a 548.02-day year, where its
   // Horizons page and JPL's current table (SAT441) give 550.30: the table's own note warns that
@@ -232,6 +235,7 @@ export async function fetchSolarSystem(): Promise<{ bodies: BodyRecord[]; horizo
       rates: mean.rates,
       ...(mean.laplacePole ? { laplacePole: mean.laplacePole } : {}),
       orbitSource: mean.orbitSource,
+      ...(spec.measuredEccentricity !== undefined ? { measuredEccentricity: spec.measuredEccentricity } : {}),
       ...(spec.parentBodyId ? { parentBodyId: spec.parentBodyId } : {}),
       ...(parentGm !== undefined ? { massRatio: result.gmKm3PerS2! / parentGm } : {}),
       ...(rotationPeriodHours !== undefined ? { rotationPeriodHours } : {}),