v4: pin JPL Horizons topocentric reference for the Moon

Owner-supplied Horizons capture (DE441; Tokyo site, airless apparent
RA/Dec of date, azimuth/elevation and range, 2026-07-18 00:00..24:00
UTC, 6h steps) pinned as a reference test for the observer pipeline.

Measured residuals: RA -8 arcsec (constant), Dec ~+1 arcsec, Az/El
within 8 arcsec, range -30 km — the documented accuracy class of the
Meeus 60-term lunar series against a full numerical ephemeris. The
test additionally asserts that the equatorial and horizontal residuals
are the same sky-plane error, i.e. the topocentric geometry (parallax,
frames, sidereal time) contributes nothing beyond the underlying
series. Mars and Sun blocks will be appended when their captures
arrive.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0169Ya4WJEvmot5RKG4nFP5N

Author: claude

package.json

@@ -12,7 +12,7 @@
     "MIT-LICENSE"
   ],
   "scripts": {
-    "test": "node test/smoke.js && node test/v4/run.mjs && node test/v4/bodies.mjs && node test/v4/observer.mjs",
+    "test": "node test/smoke.js && node test/v4/run.mjs && node test/v4/bodies.mjs && node test/v4/observer.mjs && node test/v4/horizons-ref.mjs",
     "build": "rollup -c rollup.config.js",
     "edu:snippets": "node tools/snippets.js",
     "edu:check": "node tools/snippets.js --check",

test/v4/horizons-ref.mjs

@@ -0,0 +1,77 @@
+// JPL Horizons reference comparison. Run with: node test/v4/horizons-ref.mjs
+//
+// Reference data supplied by the project owner from the Horizons API
+// (source DE441), observer site Tokyo E139.741 N35.658 h=25m,
+// airless apparent quantities 2 (RA/Dec of date, topocentric),
+// 4 (azimuth/elevation) and 20 (range), 2026-07-18 00:00..24:00 UTC.
+//
+// Tolerances state each body's SERIES accuracy class, not the pipeline's:
+// the residual must look like the documented truncation error of the
+// underlying theory (a smooth offset shared by the equatorial and
+// horizontal coordinates), and anything beyond it fails the build.
+//   moon: Meeus 60-term series, ~10" in longitude, tens of km in range.
+//         Measured on capture: RA -8", Dec +1", Az/El within 8",
+//         range -30 km — consistent with the series class.
+import assert from 'assert';
+
+import { Instant } from '../../src/time/instant.js';
+import { DEG } from '../../src/math/angles.js';
+import { observer } from '../../src/observer/observer.js';
+import { moon } from '../../src/bodies/moon.js';
+
+const AU_KM = 149597870.7;
+
+let failed = 0;
+const test = (name, fn) => {
+  try {
+    fn();
+    console.log('ok - ' + name);
+  } catch (e) {
+    failed++;
+    console.error('NG - ' + name + ': ' + e.message);
+  }
+};
+
+const TOKYO = observer({ latitude: 35.658, longitude: 139.741, height: 25 });
+
+// rows: [iso, RA deg, Dec deg, Az deg, El deg, delta au]
+const CASES = [
+  {
+    body: moon, name: 'moon',
+    skyTolArcsec: 15, rangeTolKm: 50,
+    rows: [
+      ['2026-07-18T00:00:00Z', 165.423160268, 4.665362159, 86.287282626, 2.830527623, 0.00251170278392],
+      ['2026-07-18T06:00:00Z', 167.688729940, 3.118631144, 176.512941294, 57.414144138, 0.00248826202850],
+      ['2026-07-18T12:00:00Z', 169.876940753, 1.492171722, 268.984296789, 3.974817572, 0.00253126073634],
+      ['2026-07-18T18:00:00Z', 173.527125098, -0.085041911, 347.705201508, -53.796385069, 0.00257917480388],
+      ['2026-07-19T00:00:00Z', 177.328839749, -1.646624904, 85.015043564, -9.710551650, 0.00256196526041]
+    ]
+  }
+  // mars / sun blocks are appended as their Horizons captures arrive.
+];
+
+for (const c of CASES) {
+  test(`horizons: ${c.name} topocentric apparent place (Tokyo)`, () => {
+    for (const [iso, ra, dec, az, el, delta] of c.rows) {
+      const o = TOKYO.observe(c.body, Instant.fromISO(iso));
+      const dRa = (o.ra - ra) * Math.cos(dec * DEG) * 3600;
+      const dDec = (o.dec - dec) * 3600;
+      const dAz = (o.azimuth - az) * Math.cos(el * DEG) * 3600;
+      const dEl = (o.elevation - el) * 3600;
+      const skyEq = Math.hypot(dRa, dDec);
+      const skyHor = Math.hypot(dAz, dEl);
+      assert.ok(skyEq < c.skyTolArcsec, `${iso} radec off ${skyEq.toFixed(1)}"`);
+      assert.ok(skyHor < c.skyTolArcsec, `${iso} azel off ${skyHor.toFixed(1)}"`);
+      // the equatorial and horizontal offsets must be the SAME sky error,
+      // i.e. the topocentric geometry itself adds nothing
+      assert.ok(Math.abs(skyEq - skyHor) < 3, `${iso} geometry residual`);
+      assert.ok(Math.abs(o.range - delta * AU_KM) < c.rangeTolKm, `${iso} range`);
+    }
+  });
+}
+
+if (failed > 0) {
+  console.error(failed + ' test(s) failed');
+  process.exit(1);
+}
+console.log('all horizons reference tests passed');