Measure every moon and dwarf planet against Horizons from 1950 to 2100, and say on its card how far it strays

The clock reaches AD 1 to AD 3000, but only the planets' cards named a span their elements hold
over; the 25 moons and the four SBDB dwarf planets gave a source and an epoch, though
BodyRecord.orbitSource is documented as "the span they hold over". And the worst offsets the ETL
stated came from twelve New Year's Days: Nereid's year is 360 days, so all twelve fell far from
its periapsis, where a mean ellipse is furthest out. The one date the ETL checked, 2025-01-01, saw
Nereid at 2.6 degrees; it reaches 11.19.

For each moon and each SBDB dwarf planet the ETL now fetches Horizons' ICRF vectors from 1950 to
2100, every other day (daily for Nereid, at an eccentricity of 0.75, and Hyperion, whose row's
eccentricity is a quarter of its real one: every other day gave it 22.14, daily 22.23), and
measures how far the mean elements stray, at the same TDB dates. The card appends it: "JPL SBDB
osculating elements, epoch 2026 Jun 9, within 7.2 degrees of Horizons from 1950 to 2100". Worst
offsets on the real catalogue: the Moon 2.62 (2010 March 27), Phoebe 2.58 (1969, where a comment
claimed "within 2.0"), Phobos 1.26, Mimas 7.43, Iapetus 10.34, Nereid 11.19 (2039 Nov 1),
Hyperion 22.23 (2055 Feb 26), Ceres 7.12 (1953); Io 0.07, Titan 0.06, Eris 0.06.

build.ts recomputes each from the same Horizons positions and fails if an orbit other than
Standish's names no span, if a card states less than it strays, or if a body passes its ceiling:
3 degrees, and Hyperion 23, Nereid 12, Iapetus 11, Mimas 8 and Ceres 8, each explained. The
2025-01-01 check stays for reading errors, its comment no longer passing one date's offsets off as
worst ones. The Sun's note says the moons' and those four's elements were checked from 1950 to
2100, and the date field's description that each card says how far its orbit strays over that span.
In the running app Ceres's, Phobos's and Nereid's cards end "within 7.2", "1.3" and "11.2 degrees
of Horizons from 1950 to 2100".

The CLOCK_WINDOW comment also had the calendars the wrong way at AD 1: proleptic Gregorian dates
are two days behind the Julian calendar there, level from AD 200 to 300, and ten days ahead by
1582. It now says so, and names Ceres's drift where it named Phobos's, which its orbit now carries.

Controls: the ETL measuring nothing fails ("Ceres's orbit ... names no span it holds over"),
rounding the stated figure down fails on Ceres (7.1 against 7.12), and Nereid held to the general
ceiling fails at 11.19; the note and the date field without the span fail their named tests.

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

tools/etl/build.ts

@@ -9,7 +9,8 @@ import { ExoplanetRecord } from '../../src/app/shared/models/exoplanet.model';
 import { StarRecord, SUN_STAR_ID } from '../../src/app/shared/models/star.model';
 import { fetchDeepSky } from './fetchDeepSky';
 import { fetchExoplanets } from './fetchExoplanets';
-import { fetchSolarSystem, FREELY_SPINNING_MOONS } from './fetchSolarSystem';
+import { fetchSolarSystem, FREELY_SPINNING_MOONS, offsetFromTrackDeg } from './fetchSolarSystem';
+import { TrackPoint } from './lib/horizons';
 import { BYTES_PER_STAR_META, BYTES_PER_STAR_POSITION, decodeStarCatalog, encodeStarCatalog } from '../../src/app/shared/models/star-catalog';
 import { fetchStars } from './fetchStars';
 import { describeSources } from './sources/registry';
@@ -152,28 +153,38 @@ const KM_PER_AU = 149597870.7;
 const DEG_TO_RAD = Math.PI / 180;
 
 /**
- * The moons whose table row cannot come within that on this one date, each for a reason no mean
- * ellipse carries, with a ceiling just above its offset here. What each reaches elsewhere is
- * larger, and neither this check nor twelve New Year's Days sampled from 1980 to 2100 see it:
- *
- * - Hyperion, 9.4 degrees here and 22.2 at worst, sampled every other day from 1980 to 2100: held
- *   in a 4:3 resonance by Titan; the row's eccentricity, 0.0232, is less than a quarter of the
- *   0.105 JPL's current table gives.
- * - Iapetus, 9.6 here and 10.1 at worst: the row sits 9.4 degrees behind Horizons at its own epoch,
- *   2000 Jan 1.5, and keeps that offset; its plane agrees with Horizons' to 0.07 degrees and its
- *   period to 0.001 per cent, so the fault is in the row's longitude, which this has no second
- *   source to correct.
- * - Nereid, 2.6 here and 11.2 at worst, sampled daily: an eccentricity of 0.75, the largest here,
- *   which a mean ellipse follows least well near periapsis, where the true anomaly runs ten times
- *   faster than the mean. Its 360-day year put all twelve New Year's Days, under 2.6, far from
- *   periapsis; sampled daily it is past 2.6 on 92 days in 2010-2020 and on 295 in 2040-2050, each
- *   near a periapsis, and under 0.4 on most days.
- *
- * Mimas's swing of 44 degrees either way, the libration of its resonance with Tethys, which also
- * needed a ceiling here once, is now in its orbit; see `orbitFromW` in `fetchSolarSystem.ts`.
+ * The moons whose table row cannot come within that on this one date, each with a ceiling just
+ * above its offset here; see {@link TRACK_OFFSET_CEILINGS_DEG} for what they reach from 1950 to 2100.
  */
 const MOON_OFFSET_CEILINGS_DEG: Record<string, number> = { hyperion: 21, iapetus: 11, nereid: 3 };
 
+/**
+ * How far a moon's or dwarf planet's orbit may stray from Horizons from 1950 to 2100, sampled every
+ * other day (Nereid and Hyperion daily). One date showed each at its best: twelve New Year's Days
+ * gave Nereid 2.6 degrees, and 2025-01-01 alone is all the check above sees. Each card says how
+ * far its own orbit strays over the span (`fetchSolarSystem`), and this holds that figure to
+ * account.
+ *
+ * Measured on this catalogue: at most 2.62 degrees (the Moon, 2010 March 27: no mean ellipse has
+ * its evection or variation; Phoebe reaches 2.58 in 1969, where "within 2.0" was once claimed for
+ * it). Five need their own:
+ *
+ * - Hyperion, 22.23 (2055 Feb 26): held in a 4:3 resonance by Titan; the row's eccentricity,
+ *   0.0232, is less than a quarter of the 0.105 JPL's current table gives.
+ * - Nereid, 11.19 (2039 Nov 1): an eccentricity of 0.75, the largest here, which a mean ellipse
+ *   follows least well near periapsis, where the true anomaly runs ten times faster than the mean;
+ *   its 360-day year kept every New Year's Day far from one.
+ * - Iapetus, 10.34: the row sits 9.4 degrees behind Horizons at its own epoch, 2000 Jan 1.5, and
+ *   keeps that offset; its plane agrees with Horizons' to 0.07 degrees and its period to 0.001 per
+ *   cent, so the fault is in the row's longitude, which this has no second source to correct.
+ * - Mimas, 7.43: its orbit carries the 44-degree libration of its resonance with Tethys (see
+ *   `orbitFromW` in `fetchSolarSystem.ts`), but not the rest of what Horizons integrates.
+ * - Ceres, 7.12 (1953): the SBDB's elements are osculating, exact at 2026 Jun 9 and drifting
+ *   either side; 1.9 by 2050, 5.3 by 2100, and 39 at 1600 on Horizons' own figures.
+ */
+const MAX_TRACK_OFFSET_DEG = 3;
+const TRACK_OFFSET_CEILINGS_DEG: Record<string, number> = { hyperion: 23, nereid: 12, iapetus: 11, mimas: 8, ceres: 8 };
+
 /**
  * The bodies the IAU WGCCRE 2015 report gives no rotational elements for: Hyperion tumbles, and
  * Nereid, Eris, Haumea and Makemake have no model. Every other body must carry them, or the
@@ -236,7 +247,7 @@ function angleBetweenDeg(a: { x: number; y: number; z: number }, b: { x: number;
   return (Math.acos(Math.min(1, Math.max(-1, cosine))) * 180) / Math.PI;
 }
 
-function validateBodies(bodies: BodyRecord[], horizonsOrbits: Map<string, OrbitalElements>): void {
+function validateBodies(bodies: BodyRecord[], horizonsOrbits: Map<string, OrbitalElements>, horizonsTracks: Map<string, TrackPoint[]>): void {
   assertCondition(bodies.length > 0, 'No solar-system bodies were produced.');
 
   const ids = new Set(bodies.map((body) => body.id));
@@ -261,6 +272,23 @@ 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} ${offset.toFixed(3)}`);
+
+    // Standish's fit names its own span; every other orbit is measured over 1950-2100, and says so.
+    const track = horizonsTracks.get(body.id);
+    assertCondition(
+      (track !== undefined) === !body.orbitSource.startsWith('JPL approximate mean elements (Standish)'),
+      `${body.name}'s orbit, "${body.orbitSource}", ${track ? 'names its own span' : 'names no span it holds over'}.`
+    );
+    if (track) {
+      const worst = Math.max(...track.map((point) => offsetFromTrackDeg(body, point)));
+      const trackCeiling = TRACK_OFFSET_CEILINGS_DEG[body.id] ?? MAX_TRACK_OFFSET_DEG;
+      const stated = Number(body.orbitSource.match(/within ([\d.]+) degrees of Horizons/)?.[1]);
+      assertCondition(
+        worst <= trackCeiling && stated >= worst,
+        `${body.name}'s mean elements put it up to ${worst.toFixed(2)} degrees from Horizons between 1950 and 2100 (at most ${trackCeiling} expected), and its card says "${body.orbitSource}".`
+      );
+      offsets.push(`${body.id} ${worst.toFixed(2)} at worst`);
+    }
     // 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;
@@ -440,7 +468,7 @@ async function build(): Promise<void> {
 
   const stars = await fetchStars();
   console.log();
-  const { bodies, horizonsOrbits } = await fetchSolarSystem();
+  const { bodies, horizonsOrbits, horizonsTracks } = await fetchSolarSystem();
   console.log();
   const exoplanets = await fetchExoplanets(stars);
   console.log();
@@ -450,7 +478,7 @@ async function build(): Promise<void> {
   console.log('Validating output...');
   validateStars(stars);
   validateMerge(stars);
-  validateBodies(bodies, horizonsOrbits);
+  validateBodies(bodies, horizonsOrbits, horizonsTracks);
   validateExoplanets(exoplanets, new Set(stars.map((star) => star.id)));
   validateDeepSky(deepSky);