diff --git a/README.md b/README.md
index b098fa1..0890633 100644
--- a/README.md
+++ b/README.md
@@ -55,7 +55,8 @@ in it is measured and what is not.
 **System view** — selecting a star flies the camera continuously into its system rather than
 cutting to a new scene. The Sun gets the real solar-system bodies, moving on JPL's mean orbital
 elements — Standish's for the planets, JPL SSD's satellite table for the moons, the Small-Body
-Database for Ceres, Eris, Haumea and Makemake — and turned by the IAU's rotational elements; other
+Database for Ceres, Eris, Haumea and Makemake — and turned by the IAU's rotational elements, Earth
+by the IERS Earth Rotation Angle; other
 stars get their confirmed exoplanets. Orbits are drawn as ellipses and bodies are propagated
 along them by a Kepler solver to the date on the map's clock. Under them, a dashed grid marks out
 round distances in AU — 5 AU rings for the solar system, 0.01 AU rings for TRAPPIST-1 — with a
@@ -329,7 +330,7 @@ plugin's own files are kept so it can be listed from a marketplace of its own la
 ## Data credits
 
 Star catalogue: [HYG database](https://github.com/astronexus/HYG-Database) (Hipparcos, Yale
-Bright Star, Gliese) — 68 388 stars within 250 pc. Solar-system orbits: JPL approximate planetary mean elements (Standish), JPL SSD satellite mean elements and the JPL Small-Body Database; rotation: the IAU WGCCRE 2015 report via NAIF's pck00011; physical data, and the positions the orbits are checked against: NASA/JPL Horizons. Exoplanets: NASA Exoplanet
+Bright Star, Gliese) — 68 388 stars within 250 pc. Solar-system orbits: JPL approximate planetary mean elements (Standish), JPL SSD satellite mean elements and the JPL Small-Body Database; rotation: the IAU WGCCRE 2015 report via NAIF's pck00011, and for Earth the IERS Conventions 2010; physical data, and the positions the orbits are checked against: NASA/JPL Horizons. Exoplanets: NASA Exoplanet
 Archive. Deep-sky objects: [OpenNGC](https://github.com/mattiaverga/OpenNGC). Body and skybox
 imagery: NASA/JPL/USGS public domain and Solar System Scope (CC BY 4.0) — per-file provenance
 is recorded in `src/assets/textures/README.md`.
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 f498fd1..bc176be 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, vi } from 'vitest';
 
-import { DEFAULT_EPOCH_JD, GM_SUN_AU3_PER_DAY2, TT_MINUS_UTC_DAYS, ttMinusUtSeconds } from '../../shared/astro/constants';
+import { DEFAULT_EPOCH_JD, GM_SUN_AU3_PER_DAY2, ttMinusUtSeconds } from '../../shared/astro/constants';
 import { keplerRates } from '../../shared/astro/kepler';
 import { eclipticToEquatorial, laplacePlaneToEquatorial, OBLIQUITY_J2000_DEG } from '../../shared/astro/coordinates';
 import { orientationAt } from '../../shared/astro/rotational-elements';
@@ -734,15 +734,17 @@ describe('solar-system bodies against Horizons', () => {
     expect((drawnPrimeMeridian('jupiter').angleTo(iauPrimeMeridian('jupiter', jdUt + 1574.1 / 86400)) * 180) / Math.PI).toBeLessThan(0.01);
   });
 
-  it('turns Earth by the UT the clock names, which is its turning: at AD 1000 its W is not moved on by ΔT', () => {
-    const jdUt = 2086307.5;
-    renderer.update(jdUt);
-    expect((drawnPrimeMeridian('earth').angleTo(iauPrimeMeridian('earth', jdUt + TT_MINUS_UTC_DAYS)) * 180) / Math.PI).toBeLessThan(0.01);
+  it('turns Earth by the UT the clock names, which is its turning: at AD 1000 the Sun stands over Horizons’ point', () => {
+    // Horizons' sub-solar longitude from the Sun (observer quantity 14, TIME_TYPE=UT) on JD 2086455,
+    // 1.0510 E, is Earth as it was 8.454 minutes before. Turned by the IAU's W at UT the drawn face
+    // was 2.3 degrees off; taken at TDB, 6.6.
+    renderer.update(2086455 - 8.45437443 / 1440);
+    expect(apart(facing('earth', new THREE.Vector3()).eastDeg, 1.05101)).toBeLessThan(0.15);
   });
 
   it('lights Earth where the Sun really stands: within 4 degrees of Greenwich at noon UTC', () => {
     // The equation of time is all that separates them: on 1 June 2025 it puts the Sun over 0.53 W,
-    // and the drawn sphere has it over 0.43 W.
+    // and the drawn sphere has it over 0.52 W.
     renderer.update(JUNE_1_2025_NOON_UTC);
     expect(Math.abs(facing('earth', new THREE.Vector3()).eastDeg)).toBeLessThan(4);
   });
diff --git a/src/app/shared/astro/constants.ts b/src/app/shared/astro/constants.ts
index 5146705..5c4cc9d 100644
--- a/src/app/shared/astro/constants.ts
+++ b/src/app/shared/astro/constants.ts
@@ -19,13 +19,6 @@ export const DEFAULT_EPOCH_JD = 2451545.0;
  */
 export const GM_SUN_AU3_PER_DAY2 = 0.01720209895 * 0.01720209895;
 
-/**
- * TT - UTC today, in days: 32.184 s plus the 37 leap seconds UTC has taken since 1972, the last at
- * the end of 2016. TDB, which ephemerides run on, stays within 2 ms of TT. See {@link ttMinusUtSeconds}
- * for other dates.
- */
-export const TT_MINUS_UTC_DAYS = 69.184 / 86400;
-
 /** The first day of each month UTC took a leap second at the start of, from its 10 s of 1972. */
 const LEAP_SECONDS_FROM = [
   [1972, 7], [1973, 1], [1974, 1], [1975, 1], [1976, 1], [1977, 1], [1978, 1], [1979, 1], [1980, 1], [1981, 7],
diff --git a/src/app/shared/rendering/body-orientation.spec.ts b/src/app/shared/rendering/body-orientation.spec.ts
index 5519052..c978d81 100644
--- a/src/app/shared/rendering/body-orientation.spec.ts
+++ b/src/app/shared/rendering/body-orientation.spec.ts
@@ -47,6 +47,15 @@ describe('bodyPageView', () => {
     expect(Math.abs(geodetic - 22.260426)).toBeLessThan(0.05);
   });
 
+  it('lights Earth’s face where Horizons does at the far end of the clock too: AD 1000 and AD 1', () => {
+    // Horizons' sub-solar longitude from the Sun (observer quantity 14, TIME_TYPE=UT), Earth taken
+    // one light-time back: 1.0510 E on JD 2086455 and 1.5606 E on JD 1721600. The IAU's W, taken at
+    // UT, drew them 2.3 and 4.5 degrees west of that.
+    for (const [jdUt, lightMinutes, eastDeg] of [[2086455, 8.45437443, 1.05101], [1721600, 8.45020842, 1.560644]]) {
+      expect(Math.abs(subSolarPoint(EARTH, jdUt - lightMinutes / 1440).eastDeg - eastDeg)).toBeLessThan(0.15);
+    }
+  });
+
   it('takes a moon’s Sun from where it and its planet are: the Moon’s sub-solar point is Horizons’', () => {
     const moon = subSolarPoint(MOON, JUNE_1_2025_NOON_UTC);
     // 116.2859 E and 1.5030 N, seen from Earth's centre.
diff --git a/src/app/shared/rendering/body-orientation.ts b/src/app/shared/rendering/body-orientation.ts
index 42dd4c8..9c03139 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 { tdbFromUtc, 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';
@@ -54,14 +54,18 @@ export function poleFrame(pole: { raDeg: number; decDeg: number }, target = new
  *
  * The clock is UT and the IAU's elements run on TDB, 69.184 s ahead today and 1 574 s at AD 1000;
  * in 69 s Earth turns 0.29 degrees, Jupiter 0.70 and Phobos 0.90, so the date is taken to TDB here
- * (see `tdbFromUtc`). Earth, `followsUt`, is the one exception: its turning is what UT counts,
- * so the clock's date already says how far it has turned, and its W, fitted to today, is taken at
- * that date plus today's TT - UTC. Taken at TDB, it would turn ΔT further: 44 degrees at AD 1.
+ * (see `tdbFromUtc`). Earth, `followsUt`, is the one exception: its turning is what UT counts, so
+ * it is turned by the IERS Earth Rotation Angle at the clock's date (IERS Conventions 2010, eq.
+ * 5.15), counted from the node its W starts at, 90 degrees past its pole's right ascension. The
+ * IAU's W for Earth, fitted to today, runs 6.3e-6 degrees a day slow of that once its pole's drift
+ * is counted: taken at UT, it left Earth's lit face 2.3 degrees off Horizons at AD 1000 and 4.5 at
+ * AD 1. Taken at TDB, it would have turned ΔT further, 44 degrees at AD 1.
  */
 export function bodyOrientation(elements: RotationalElements, jdUtc: number, target = new THREE.Quaternion(), followsUt = false): THREE.Quaternion {
-  const { poleRaDeg, poleDecDeg, primeMeridianDeg } = orientationAt(elements, followsUt ? jdUtc + TT_MINUS_UTC_DAYS : tdbFromUtc(jdUtc));
+  const { poleRaDeg, poleDecDeg, primeMeridianDeg } = orientationAt(elements, tdbFromUtc(jdUtc));
+  const turnDeg = followsUt ? 360 * (0.779057273264 + 1.00273781191135448 * (jdUtc - 2451545)) - 90 - poleRaDeg : primeMeridianDeg;
   return poleFrame({ raDeg: poleRaDeg, decDeg: poleDecDeg }, target)
-    .multiply(scratchTurn.setFromAxisAngle(Z_AXIS, primeMeridianDeg * DEG_TO_RAD))
+    .multiply(scratchTurn.setFromAxisAngle(Z_AXIS, turnDeg * DEG_TO_RAD))
     .multiply(MAP_TO_BODY);
 }