diff --git a/src/app/features/galaxy-system/system-orbits-renderer.spec.ts b/src/app/features/galaxy-system/system-orbits-renderer.spec.ts
index 2c228ec..db780fe 100644
--- a/src/app/features/galaxy-system/system-orbits-renderer.spec.ts
+++ b/src/app/features/galaxy-system/system-orbits-renderer.spec.ts
@@ -1,7 +1,7 @@
 import * as THREE from 'three/webgpu';
 import { describe, expect, it } from 'vitest';
 
-import { DEFAULT_EPOCH_JD, GM_SUN_AU3_PER_DAY2 } from '../../shared/astro/constants';
+import { DEFAULT_EPOCH_JD, GM_SUN_AU3_PER_DAY2, TT_MINUS_UTC_DAYS } from '../../shared/astro/constants';
 import { keplerRates } from '../../shared/astro/kepler';
 import { eclipticToEquatorial, OBLIQUITY_J2000_DEG } from '../../shared/astro/coordinates';
 import { BodyRecord, RotationalElements } from '../../shared/models/body.model';
@@ -203,7 +203,8 @@ describe('SystemOrbitsRenderer exoplanet propagation', () => {
       // A body at ecliptic longitude 0 sits on the +X axis in both frames, so it must not move.
       const atEquinox: BodyRecord = { ...EARTH, orbit: { ...EARTH.orbit, eccentricity: 0 } };
       const renderer = new SystemOrbitsRenderer([atEquinox], []);
-      renderer.update(DEFAULT_EPOCH_JD);
+      // The clock's UTC date whose TDB is the elements' epoch.
+      renderer.update(DEFAULT_EPOCH_JD - TT_MINUS_UTC_DAYS);
 
       const p = renderer.members[0].marker.position;
       expect(p.x).toBeCloseTo(1, 6);
@@ -489,7 +490,8 @@ describe('solar-system bodies against Horizons', () => {
   // Real records from bodies.json, and Horizons' own positions for them (ICRF, AU; heliocentric
   // for the planets, planet-centred for the moons) at dates across 1950-2100, so the whole path —
   // mean elements, their rates, the Laplace planes and the scene's frame — is checked against
-  // JPL's ephemeris rather than against itself.
+  // JPL's ephemeris rather than against itself. Horizons' dates are TDB and the renderer's are the
+  // clock's UTC, so each is handed over 69.184 s earlier.
   const RECORDS: Record<string, Pick<BodyRecord, 'kind' | 'orbit' | 'rates' | 'laplacePole' | 'parentBodyId' | 'massRatio'>> = {
     earth: {kind: 'planet', orbit: {semiMajorAxisAu: 1.00000018, eccentricity: 0.01673163, inclinationDeg: -0.00054346, longitudeOfAscendingNodeDeg: -5.11260389, argumentOfPeriapsisDeg: 108.04266274, meanAnomalyAtEpochDeg: -2.4631431299999917, epochJd: 2451545}, rates: {meanMotionDegPerDay: 0.9856091187759068, longitudeOfAscendingNodeDegPerDay: -0.000006604751813826146, argumentOfPeriapsisDegPerDay: 0.000015309819575633124, semiMajorAxisAuPerDay: -8.213552361396303e-13, eccentricityPerDay: -1.002327173169062e-9, inclinationDegPerDay: -3.6609938398357287e-7}},
     jupiter: {kind: 'planet', orbit: {semiMajorAxisAu: 5.20248019, eccentricity: 0.0485359, inclinationDeg: 1.29861416, longitudeOfAscendingNodeDeg: 100.29282654, argumentOfPeriapsisDeg: -86.0178741, meanAnomalyAtEpochDeg: 20.059839080000003, epochJd: 2451545}, rates: {meanMotionDegPerDay: 0.08309113532019165, longitudeOfAscendingNodeDegPerDay: 0.0000035659463381245725, argumentOfPeriapsisDegPerDay: 0.0000014167219712525667, semiMajorAxisAuPerDay: -7.841204654346339e-10, eccentricityPerDay: 4.935249828884326e-9, inclinationDegPerDay: -8.83501711156742e-8, meanAnomalyTerms: {b: -0.00012452, c: 0.0606406, s: -0.35635438, f: 38.35125}}},
@@ -517,6 +519,7 @@ describe('solar-system bodies against Horizons', () => {
     earth: {poleRaDeg: [0, -0.641, 0], poleDecDeg: [90, -0.557, 0], primeMeridianDeg: [190.147, 360.9856235, 0]},
     mars: {poleRaDeg: [317.269202, -0.10927547, 0], poleDecDeg: [54.432516, -0.05827105, 0], primeMeridianDeg: [176.049863, 350.891982443297, 0], terms: [{angleDeg: [79.398797, 0.5042615, 0], ra: 0.419057, dec: 0, pm: 0}, {angleDeg: [166.325722, 0.5042615, 0], ra: 0, dec: 1.591274, pm: 0}, {angleDeg: [95.391654, 0.5042615, 0], ra: 0, dec: 0, pm: 0.584542}]},
     jupiter: {poleRaDeg: [268.056595, -0.006499, 0], poleDecDeg: [64.495303, 0.002413, 0], primeMeridianDeg: [284.95, 870.536, 0]},
+    io: {poleRaDeg: [268.05, -0.009, 0], poleDecDeg: [64.5, 0.003, 0], primeMeridianDeg: [200.39, 203.4889538, 0], terms: [{angleDeg: [283.9, 4850.7], ra: 0.094, dec: 0.04, pm: -0.085}, {angleDeg: [355.8, 1191.3], ra: 0.024, dec: 0.011, pm: -0.022}]},
     saturn: {poleRaDeg: [40.589, -0.036, 0], poleDecDeg: [83.537, -0.004, 0], primeMeridianDeg: [38.9, 810.7939024, 0]},
     uranus: {poleRaDeg: [257.311, 0, 0], poleDecDeg: [-15.175, 0, 0], primeMeridianDeg: [203.81, -501.1600928, 0]},
     pluto: {poleRaDeg: [132.993, 0, 0], poleDecDeg: [-6.163, 0, 0], primeMeridianDeg: [302.695, 56.3625225, 0]},
@@ -552,7 +555,7 @@ describe('solar-system bodies against Horizons', () => {
 
   for (const [id, jd, x, y, z, maxDeg] of HORIZONS) {
     it(`puts ${id} within ${maxDeg} degrees of Horizons on JD ${jd}`, () => {
-      renderer.update(jd);
+      renderer.update(jd - TT_MINUS_UTC_DAYS);
       const drawn = renderer.members.find((member) => member.id === id)!.marker.position;
       const angleDeg = (drawn.angleTo(new THREE.Vector3(x, y, z)) * 180) / Math.PI;
       expect(angleDeg).toBeLessThan(maxDeg);
@@ -560,9 +563,9 @@ describe('solar-system bodies against Horizons', () => {
   }
 
   it('puts Pluto where Horizons has it round its barycentre with Charon, 2 131 km out and opposite Charon', () => {
-    // Horizons, Pluto (999) from the Pluto-system barycentre (9), on JD 2488069.5 (2100).
+    // Horizons, Pluto (999) from the Pluto-system barycentre (9), on JD 2488069.5 TDB (2100).
     const horizons = new THREE.Vector3(0.000003313612032581019, 0.000001023040948538272, -0.00001381793390079716);
-    renderer.update(2488069.5);
+    renderer.update(2488069.5 - TT_MINUS_UTC_DAYS);
     const charon = renderer.members.find((member) => member.id === 'charon')!.marker;
     const barycentre = charon.parent!.position;
     const pluto = renderer.members.find((member) => member.id === 'pluto')!.marker.position.clone().sub(barycentre);
@@ -712,12 +715,14 @@ describe('solar-system bodies against Horizons', () => {
   //
   // Measured: every longitude within 0.09 degrees and every latitude within 0.03, but for the
   // Moon's face towards Earth, 0.70 and 0.09 out because its mean orbit is (its evection alone is
-  // 1.27 degrees); its face towards the Sun is within 0.002.
+  // 1.27 degrees); its face towards the Sun is within 0.002. Io's face towards Jupiter is 0.012 out:
+  // with its orbit taken at the clock's UTC and its spin at TDB it was 0.175, the 69 s between them.
   const SUB_POINTS: Array<[id: string, observer: string | undefined, lightMinutes: number, west: boolean, flattening: number, observerLon: number, observerLat: number, sunLon: number, sunLat: number, maxObserverDeg: number]> = [
     ['earth', undefined, 8.43351424, false, 1 / 298.257, 1.5855, 22.261204, 1.579501, 22.260426, 0.1],
     ['mars', 'earth', 14.13295841, true, 1 - 3376.2 / 3396.19, 307.365389, 21.27653, 269.287887, 25.451264, 0.1],
     ['moon', 'earth', 0.02150549, false, 0, 7.256763, -3.462104, 116.285934, 1.503004, 0.8],
-    ['jupiter', 'earth', 50.70337676, true, 1 - 66854 / 71492, 251.139846, 2.58787, 247.855871, 2.572658, 0.1]
+    ['jupiter', 'earth', 50.70337676, true, 1 - 66854 / 71492, 251.139846, 2.58787, 247.855871, 2.572658, 0.1],
+    ['io', 'jupiter', 0.02340584, true, 0, 359.964094, -0.002537, 355.673108, 2.26528, 0.05]
   ];
 
   for (const [id, observer, lightMinutes, west, flattening, observerLon, observerLat, sunLon, sunLat, maxObserverDeg] of SUB_POINTS) {
diff --git a/src/app/features/galaxy-system/system-orbits-renderer.ts b/src/app/features/galaxy-system/system-orbits-renderer.ts
index eee8041..5950d2d 100644
--- a/src/app/features/galaxy-system/system-orbits-renderer.ts
+++ b/src/app/features/galaxy-system/system-orbits-renderer.ts
@@ -6,6 +6,7 @@ import { planetTexture } from '../../shared/rendering/procedural-planet-texture'
 import { bodyTexturePath, loadCachedTexture, saturnRing } from '../../shared/rendering/texture-catalog';
 import { isPropagatableOrbit, keplerRates, meanElementsAt, orbitEllipsePoints, positionAtEpoch, resolveGravitationalParameter, resolveOrbitalElements } from '../../shared/astro/kepler';
 import { CartesianCoordinates, OBLIQUITY_J2000_DEG } from '../../shared/astro/coordinates';
+import { tdbFromUtc } from '../../shared/astro/constants';
 import { BodyRecord, MeanElementRates, OrbitalElements, RotationalElements } from '../../shared/models/body.model';
 import { bodyOrientation, poleFrame } from '../../shared/rendering/body-orientation';
 import { bodyMarkerRadiusAu, systemGridRingsAu } from './system-framing';
@@ -466,10 +467,14 @@ export class SystemOrbitsRenderer {
     this.object.add(starLight());
   }
 
-  /** Recomputes every marker's position for the given Julian date. Call once per tick. */
+  /**
+   * Recomputes every marker's position for the given Julian date, UTC as the map's clock gives it:
+   * the orbits are taken at its TDB, as the spins are. Call once per tick.
+   */
   update(epochJd: number): void {
+    const jdTdb = tdbFromUtc(epochJd);
     for (const body of this.topLevelBodies) {
-      const current = meanElementsAt(body.elements, body.rates, epochJd);
+      const current = meanElementsAt(body.elements, body.rates, jdTdb);
       const orbital = positionAtEpoch(current);
       body.position.set(orbital.x, orbital.y, orbital.z).applyQuaternion(body.frame);
       body.marker.position.copy(body.position);
@@ -477,7 +482,7 @@ export class SystemOrbitsRenderer {
       if (body.rotationalElements) {
         bodyOrientation(body.rotationalElements, epochJd, body.marker.quaternion);
       } else if (body.rotationPeriodHours) {
-        body.marker.quaternion.copy(spinFor(current, body.frame, body.rotationPeriodHours, epochJd - body.elements.epochJd));
+        body.marker.quaternion.copy(spinFor(current, body.frame, body.rotationPeriodHours, jdTdb - body.elements.epochJd));
       }
     }
 
@@ -487,7 +492,7 @@ export class SystemOrbitsRenderer {
         continue;
       }
       moon.pivot.position.copy(parent.position);
-      const current = meanElementsAt(moon.elements, moon.rates, epochJd);
+      const current = meanElementsAt(moon.elements, moon.rates, jdTdb);
       const orbital = positionAtEpoch(current);
       moon.marker.position.set(orbital.x, orbital.y, orbital.z).applyQuaternion(moon.frame);
       orientOrbit(moon.orbitLine.quaternion, current, moon.frame);
@@ -502,7 +507,7 @@ export class SystemOrbitsRenderer {
       if (moon.rotationalElements) {
         bodyOrientation(moon.rotationalElements, epochJd, moon.marker.quaternion);
       } else if (moon.rotationPeriodHours) {
-        moon.marker.quaternion.copy(spinFor(current, moon.frame, moon.rotationPeriodHours, epochJd - moon.elements.epochJd));
+        moon.marker.quaternion.copy(spinFor(current, moon.frame, moon.rotationPeriodHours, jdTdb - moon.elements.epochJd));
       }
     }
 
diff --git a/src/app/shared/astro/constants.ts b/src/app/shared/astro/constants.ts
index 41e2569..b54dae3 100644
--- a/src/app/shared/astro/constants.ts
+++ b/src/app/shared/astro/constants.ts
@@ -27,6 +27,17 @@ export const GM_SUN_AU3_PER_DAY2 = 0.01720209895 * 0.01720209895;
  */
 export const TT_MINUS_UTC_DAYS = 69.184 / 86400;
 
+/**
+ * The TDB date every element set here is evaluated at, for a date on the map's clock, which is
+ * UTC: Standish's T_eph, the SSD satellite and SBDB epochs and the IAU's d and T all run on TDB.
+ * Positions and spins both go through this, so a locked moon's face and the orbit it is drawn on
+ * are taken at the same instant; taken at the clock's date, the orbits ran 69 s behind the spins,
+ * which is 0.9 degrees of Phobos's orbit and 0.16 of Io's.
+ */
+export function tdbFromUtc(jdUtc: number): number {
+  return jdUtc + TT_MINUS_UTC_DAYS;
+}
+
 /** Converts a JS `Date` into a Julian date (days), for driving the Kepler propagator "now". */
 export function dateToJulianDate(date: Date = new Date()): number {
   return date.getTime() / 86400000 + 2440587.5;
diff --git a/src/app/shared/rendering/body-orientation.spec.ts b/src/app/shared/rendering/body-orientation.spec.ts
index 6421380..82bef8a 100644
--- a/src/app/shared/rendering/body-orientation.spec.ts
+++ b/src/app/shared/rendering/body-orientation.spec.ts
@@ -1,8 +1,11 @@
 import * as THREE from 'three/webgpu';
 import { describe, expect, it } from 'vitest';
 
+import { TT_MINUS_UTC_DAYS } from '../astro/constants';
+import { eclipticToEquatorial } from '../astro/coordinates';
+import { meanElementsAt, positionAtEpoch } from '../astro/kepler';
 import { BodyRecord } from '../models/body.model';
-import { bodyPageView } from './body-orientation';
+import { bodyOrientation, bodyPageView } from './body-orientation';
 
 // Earth (the Earth-Moon barycentre's mean elements) and the Moon as bodies.json carries them.
 const EARTH: BodyRecord = {
@@ -51,6 +54,17 @@ describe('bodyPageView', () => {
     expect(Math.abs(moon.latDeg - 1.503004)).toBeLessThan(0.05);
   });
 
+  it('takes the Sun where it stands at the same TDB instant the body is turned for', () => {
+    // Earth's own sphere, turned as the system view turns it, and the Sun seen from Earth's mean
+    // place at the clock's date taken to TDB: the page must light that same point of its map.
+    const planet = new THREE.Quaternion();
+    const sun = new THREE.Vector3();
+    bodyPageView(EARTH, BODIES, JUNE_1_2025_NOON_UTC, SUN_AZIMUTH, planet, sun);
+    const place = eclipticToEquatorial(positionAtEpoch(meanElementsAt(EARTH.orbit, EARTH.rates, JUNE_1_2025_NOON_UTC + TT_MINUS_UTC_DAYS)));
+    const expected = new THREE.Vector3(-place.x, -place.y, -place.z).normalize().applyQuaternion(bodyOrientation(EARTH.rotationalElements!, JUNE_1_2025_NOON_UTC).invert());
+    expect(sun.clone().applyQuaternion(planet.clone().invert()).angleTo(expected)).toBeLessThan(1e-9);
+  });
+
   it('keeps the pole up and the Sun where the page’s light stands, turning the body under it', () => {
     const planet = new THREE.Quaternion();
     const sun = new THREE.Vector3();
diff --git a/src/app/shared/rendering/body-orientation.ts b/src/app/shared/rendering/body-orientation.ts
index a0ea9d9..3755e84 100644
--- a/src/app/shared/rendering/body-orientation.ts
+++ b/src/app/shared/rendering/body-orientation.ts
@@ -1,6 +1,6 @@
 import * as THREE from 'three/webgpu';
 
-import { TT_MINUS_UTC_DAYS } from '../astro/constants';
+import { tdbFromUtc } from '../astro/constants';
 import { CartesianCoordinates, eclipticToEquatorial, laplacePlaneToEquatorial } from '../astro/coordinates';
 import { meanElementsAt, positionAtEpoch } from '../astro/kepler';
 import { orientationAt } from '../astro/rotational-elements';
@@ -51,10 +51,10 @@ export function poleFrame(pole: { raDeg: number; decDeg: number }, target = new
  * and on to where the pole points.
  *
  * The clock is UTC and the IAU's elements run on TDB, 69.184 s ahead; in that time Earth turns
- * 0.29 degrees, Jupiter 0.70 and Phobos 0.90, so the difference is added here.
+ * 0.29 degrees, Jupiter 0.70 and Phobos 0.90, so the date is taken to TDB here (see `tdbFromUtc`).
  */
 export function bodyOrientation(elements: RotationalElements, jdUtc: number, target = new THREE.Quaternion()): THREE.Quaternion {
-  const { poleRaDeg, poleDecDeg, primeMeridianDeg } = orientationAt(elements, jdUtc + TT_MINUS_UTC_DAYS);
+  const { poleRaDeg, poleDecDeg, primeMeridianDeg } = orientationAt(elements, tdbFromUtc(jdUtc));
   return poleFrame({ raDeg: poleRaDeg, decDeg: poleDecDeg }, target)
     .multiply(scratchTurn.setFromAxisAngle(Z_AXIS, primeMeridianDeg * DEG_TO_RAD))
     .multiply(MAP_TO_BODY);
@@ -62,13 +62,14 @@ export function bodyOrientation(elements: RotationalElements, jdUtc: number, tar
 
 /** Where a body is from the Sun at a date, in the ICRF, AU: a moon's planet's place plus its own. */
 function heliocentricPosition(body: BodyRecord, bodies: readonly BodyRecord[], jdUtc: number): CartesianCoordinates {
-  const own = positionAtEpoch(meanElementsAt(body.orbit, body.rates, jdUtc));
+  const jdTdb = tdbFromUtc(jdUtc);
+  const own = positionAtEpoch(meanElementsAt(body.orbit, body.rates, jdTdb));
   const parent = body.parentBodyId ? bodies.find((candidate) => candidate.id === body.parentBodyId) : undefined;
   if (!parent) {
     return eclipticToEquatorial(own);
   }
   const offset = body.laplacePole ? laplacePlaneToEquatorial(own, body.laplacePole) : eclipticToEquatorial(own);
-  const centre = eclipticToEquatorial(positionAtEpoch(meanElementsAt(parent.orbit, parent.rates, jdUtc)));
+  const centre = eclipticToEquatorial(positionAtEpoch(meanElementsAt(parent.orbit, parent.rates, jdTdb)));
   return { x: centre.x + offset.x, y: centre.y + offset.y, z: centre.z + offset.z };
 }
 
@@ -92,7 +93,7 @@ export function bodyPageView(body: BodyRecord, bodies: readonly BodyRecord[], jd
   if (!elements) {
     return false;
   }
-  const { poleRaDeg, poleDecDeg } = orientationAt(elements, jdUtc + TT_MINUS_UTC_DAYS);
+  const { poleRaDeg, poleDecDeg } = orientationAt(elements, tdbFromUtc(jdUtc));
   // From the ICRF into the body's frame with its pole on +Y, before the turn about that pole.
   const toPage = poleFrame({ raDeg: poleRaDeg, decDeg: poleDecDeg }, scratchPage).multiply(MAP_TO_BODY).invert();
   const position = heliocentricPosition(body, bodies, jdUtc);