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

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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>
This commit is contained in:
Senrokai
2026-09-29 19:57:16 +02:00
co-authored by Claude Opus 5.5
parent 2d4b8a98d5
commit a281f22f34
9 changed files with 167 additions and 67 deletions
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tools/etl/fetchSolarSystem.ts
+39 -8
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@@ -2,7 +2,9 @@ import { writeFileSync } from 'node:fs';
import { BodyRecord, OrbitalElements } from '../../src/app/shared/models/body.model';
import { SUN_STAR_ID } from '../../src/app/shared/models/star.model';
import { fetchHorizonsBody } from './lib/horizons';
import { fetchHorizonsBody, fetchHorizonsTrack, TRACK_START_YEAR, TRACK_STOP_YEAR, TrackPoint } from './lib/horizons';
import { eclipticToEquatorial, laplacePlaneToEquatorial } from '../../src/app/shared/astro/coordinates';
import { meanElementsAt, positionAtEpoch } from '../../src/app/shared/astro/kepler';
import { MeanOrbit, parsePlanetMeanElements, parseSatelliteMeanElements, parseSmallBodyElements } from '../../src/app/shared/astro/mean-elements';
import { fetchPlanetMeanElementsText, fetchSatelliteMeanElementsHtml, fetchSmallBodyAnswer } from './lib/mean-elements';
import { MIN_PERIODIC_TERM_DEG, orbitalTermsOfPrimeMeridian, parsePckRotationalElements, SUN_ROTATIONAL_ELEMENTS } from '../../src/app/shared/astro/rotational-elements';
@@ -52,6 +54,14 @@ interface BodySpec {
* given. See `orbitalTermsOfPrimeMeridian`.
*/
orbitFromW?: { angleRateDegPerCentury?: number };
/**
* Days between the Horizons positions the orbit is checked against from 1950 to 2100; 2 unless
* the error changes faster than that. Nereid, at an eccentricity of 0.75, sweeps through its
* periapsis, where the mean ellipse is furthest out, in days; Hyperion, on a row whose
* eccentricity is a quarter of its real one, peaks within a day too (22.23 degrees sampled daily
* where every other day gave 22.14).
*/
trackStepDays?: number;
}
/** S5 in pck00011.tpc, 316.45 + 506.2 T: the libration of Mimas and Tethys in their 4:2 resonance. */
@@ -114,13 +124,13 @@ const BODY_SPECS: BodySpec[] = [
{ 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.
// 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: 'hyperion', name: 'Hyperion', kind: 'moon', horizonsCommand: '607', center: '500@699', parentBodyId: 'saturn', spinsFreely: true, measuredEccentricity: 0.105, trackStepDays: 1 },
{ 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
// its source misstated the mean motions of retrograde moons. On the row's figure Phoebe was
// 25 degrees from Horizons by 2025 and 100 by 2075; on the current period, within 2.0 from 1980
// to 2100.
// 25 degrees from Horizons by 2025 and 100 by 2075; on the current period, within 2.6 from 1950
// to 2100 (2.58 in 1969).
{ id: 'phoebe', name: 'Phoebe', kind: 'moon', horizonsCommand: '609', center: '500@699', parentBodyId: 'saturn', spinsFreely: true, periodDays: 550.30391 },
{ id: 'miranda', name: 'Miranda', horizonsCommand: '705', ...URANUS_MOON },
{ id: 'ariel', name: 'Ariel', horizonsCommand: '701', ...URANUS_MOON },
@@ -129,7 +139,7 @@ const BODY_SPECS: BodySpec[] = [
{ id: 'oberon', name: 'Oberon', horizonsCommand: '704', ...URANUS_MOON },
{ id: 'triton', name: 'Triton', kind: 'moon', horizonsCommand: '801', center: '500@899', parentBodyId: 'neptune' },
// Nereid's eccentric orbit, 0.75, cannot hold a face to Neptune; its page states no spin.
{ id: 'nereid', name: 'Nereid', kind: 'moon', horizonsCommand: '802', center: '500@899', parentBodyId: 'neptune', spinsFreely: true },
{ id: 'nereid', name: 'Nereid', kind: 'moon', horizonsCommand: '802', center: '500@899', parentBodyId: 'neptune', spinsFreely: true, trackStepDays: 1 },
{ id: 'proteus', name: 'Proteus', kind: 'moon', horizonsCommand: '808', center: '500@899', parentBodyId: 'neptune' },
// Pluto's section prints its epoch as 2000 Jan 1.0; JPL's current table gives Charon's as
// 2000-01-01.5, and read at 1.0 Charon sat 27.8 to 28.2 degrees — half a day of its motion is
@@ -148,10 +158,11 @@ export const FREELY_SPINNING_MOONS = new Set(BODY_SPECS.filter((spec) => spec.sp
* point and which face is where. Horizons' osculating elements for the same date come back
* alongside, for `build.ts` to check the mean ones against.
*/
export async function fetchSolarSystem(): Promise<{ bodies: BodyRecord[]; horizonsOrbits: Map<string, OrbitalElements> }> {
export async function fetchSolarSystem(): Promise<{ bodies: BodyRecord[]; horizonsOrbits: Map<string, OrbitalElements>; horizonsTracks: Map<string, TrackPoint[]> }> {
console.log(`Fetching ${BODY_SPECS.length} solar-system bodies from JPL (mean elements, Horizons, NAIF's PCK)...`);
const bodies: BodyRecord[] = [];
const horizonsOrbits = new Map<string, OrbitalElements>();
const horizonsTracks = new Map<string, TrackPoint[]>();
const planetElements = await fetchPlanetMeanElementsText();
const satelliteElements = await fetchSatelliteMeanElementsHtml();
const pck = await fetchPckText();
@@ -207,6 +218,18 @@ export async function fetchSolarSystem(): Promise<{ bodies: BodyRecord[]; horizo
rates: { ...corrected.rates, meanMotionDegPerDay: corrected.rates.meanMotionDegPerDay + fromW.meanMotionDegPerDay, meanAnomalyTerms: fromW.meanAnomalyTerms }
}
: corrected;
// Standish's fit states its own span, 3000 BC to AD 3000. The moons' table and the SBDB state
// none, and hold for far less: each card says how far its orbit stays from Horizons over the
// span it was measured, where the clock reaches AD 1 to AD 3000.
let orbitSource = mean.orbitSource;
if (parentName || smallBody) {
const stepDays = spec.trackStepDays ?? 2;
const track = await fetchHorizonsTrack(spec.horizonsCommand, spec.center, stepDays, `horizons-track-${spec.id}-${stepDays}d.txt`);
horizonsTracks.set(spec.id, track);
const worst = Math.max(...track.map((point) => offsetFromTrackDeg(mean, point)));
orbitSource += `, within ${(Math.ceil(worst * 10) / 10).toFixed(1)} degrees of Horizons from ${TRACK_START_YEAR} to ${TRACK_STOP_YEAR}`;
}
const radiusKm = smallBody?.radiusKm ?? spec.radiusKm ?? result.radiusKm;
if (radiusKm === undefined) {
console.warn(` no physical radius found for ${spec.name}; defaulting to 0.`);
@@ -238,7 +261,7 @@ export async function fetchSolarSystem(): Promise<{ bodies: BodyRecord[]; horizo
orbit: mean.orbit,
rates: mean.rates,
...(mean.laplacePole ? { laplacePole: mean.laplacePole } : {}),
orbitSource: mean.orbitSource,
orbitSource,
...(spec.measuredEccentricity !== undefined ? { measuredEccentricity: spec.measuredEccentricity } : {}),
...(spec.parentBodyId ? { parentBodyId: spec.parentBodyId } : {}),
...(parentGm !== undefined ? { massRatio: result.gmKm3PerS2! / parentGm } : {}),
@@ -251,7 +274,15 @@ export async function fetchSolarSystem(): Promise<{ bodies: BodyRecord[]; horizo
ensureDataDir();
writeFileSync(dataPath('bodies.json'), JSON.stringify(bodies, null, 2));
console.log(` wrote ${bodies.length} bodies.`);
return { bodies, horizonsOrbits };
return { bodies, horizonsOrbits, horizonsTracks };
}
/** Degrees between where a moon's or dwarf planet's mean elements put it and where Horizons has it. */
export function offsetFromTrackDeg(mean: Pick<MeanOrbit, 'orbit' | 'rates' | 'laplacePole'>, point: TrackPoint): number {
const own = positionAtEpoch(meanElementsAt(mean.orbit, mean.rates, point.jd));
const place = mean.laplacePole ? laplacePlaneToEquatorial(own, mean.laplacePole) : eclipticToEquatorial(own);
const cosine = (place.x * point.x + place.y * point.y + place.z * point.z) / (Math.hypot(place.x, place.y, place.z) * Math.hypot(point.x, point.y, point.z));
return (Math.acos(Math.min(1, Math.max(-1, cosine))) * 180) / Math.PI;
}
if (require.main === module) {