diff --git a/scripts/package-boundaries.json b/scripts/package-boundaries.json
index 3ce02469e..8c774b97b 100644
--- a/scripts/package-boundaries.json
+++ b/scripts/package-boundaries.json
@@ -2359,6 +2359,7 @@
       "src/geo/vector.js",
       "src/geo/ellipsoid.js",
       "src/geo/frames.js",
+      "src/geo/celestial.js",
       "src/layers/flights/icons.js",
       "src/data/aircraftIconLayer.js",
       "src/data/aircraftModelLayer.js",
@@ -5120,6 +5121,7 @@
       "src/geo/vector.js",
       "src/geo/ellipsoid.js",
       "src/geo/frames.js",
+      "src/geo/celestial.js",
       "src/agent/worldPacket.js",
       "src/data/layerSnapshot.js",
       "src/data/feedState.js",
@@ -5141,7 +5143,8 @@
       "src/geo/index.js",
       "src/geo/vector.js",
       "src/geo/ellipsoid.js",
-      "src/geo/frames.js"
+      "src/geo/frames.js",
+      "src/geo/celestial.js"
     ],
     "external": []
   },
@@ -5162,6 +5165,7 @@
       "src/geo/vector.js",
       "src/geo/ellipsoid.js",
       "src/geo/frames.js",
+      "src/geo/celestial.js",
       "src/render/stub/motion.js",
       "src/render/stub/camera.js",
       "src/render/stub/geodesic.js",
diff --git a/src/geo/celestial.js b/src/geo/celestial.js
new file mode 100644
index 000000000..127ba00c3
--- /dev/null
+++ b/src/geo/celestial.js
@@ -0,0 +1,814 @@
+/**
+ * @module geo/celestial
+ * @description The sun and the moon: a port of Cesium 1.138's
+ * `Simon1994PlanetaryPositions` (positions in Cesium's Earth inertial frame)
+ * and `Transforms.computeTemeToPseudoFixedMatrix` (inertial to Earth-fixed),
+ * with epoch milliseconds (UTC, as `Date.now()` gives) as the only time
+ * input. The time-scale handling (UTC to TAI through Cesium's leap-second
+ * table, TAI to TT to TDB) follows `JulianDate` step for step, so results
+ * are identical to Cesium's, bit for bit. Constants and operation order
+ * are Cesium's; do not "simplify" them. No engine.
+ *
+ * A rotation is returned as its axes `{x, y, z}` (the columns of Cesium's
+ * `Matrix3`), the form `geo/frames` uses. Times must be finite and inside
+ * what `Date` represents, and the model holds for a few thousand years
+ * either side of J2000; outside that its orbital elements leave their
+ * ranges and the functions throw a RangeError, where Cesium's debug build
+ * throws.
+ */
+import { frameVector } from './frames.js';
+import { RADIANS_PER_DEGREE, TWO_PI, vec, zeroToTwoPi } from './vector.js';
+
+// ---------------------------------------------------------------------------
+// Constants (Core/Math.js, Core/TimeConstants.js)
+
+const PI = Math.PI;
+const RADIANS_PER_ARCSECOND = RADIANS_PER_DEGREE / 3600.0;
+const SECONDS_PER_MILLISECOND = 0.001;
+const SECONDS_PER_MINUTE = 60.0;
+const SECONDS_PER_HOUR = 3600.0;
+const SECONDS_PER_DAY = 86400.0;
+const DAYS_PER_JULIAN_CENTURY = 36525.0;
+
+// ---------------------------------------------------------------------------
+// Julian dates (Core/JulianDate.js). A date is `{dayNumber, secondsOfDay}`,
+// in TAI unless a comment says otherwise, as Cesium keeps them.
+
+function setComponents(wholeDays, secondsOfDay, julianDate) {
+  const extraDays = (secondsOfDay / SECONDS_PER_DAY) | 0;
+  wholeDays += extraDays;
+  secondsOfDay -= SECONDS_PER_DAY * extraDays;
+
+  if (secondsOfDay < 0) {
+    wholeDays--;
+    secondsOfDay += SECONDS_PER_DAY;
+  }
+
+  julianDate.dayNumber = wholeDays;
+  julianDate.secondsOfDay = secondsOfDay;
+  return julianDate;
+}
+
+const taiDate = (dayNumber, secondsOfDay) =>
+  setComponents(dayNumber, secondsOfDay, {});
+
+const addSeconds = (julianDate, seconds, result) =>
+  setComponents(
+    julianDate.dayNumber,
+    julianDate.secondsOfDay + seconds,
+    result,
+  );
+
+const totalDays = (julianDate) =>
+  julianDate.dayNumber + julianDate.secondsOfDay / SECONDS_PER_DAY;
+
+function secondsDifference(left, right) {
+  const dayDifference = (left.dayNumber - right.dayNumber) * SECONDS_PER_DAY;
+  return dayDifference + (left.secondsOfDay - right.secondsOfDay);
+}
+
+function compare(left, right) {
+  const julianDayNumberDifference = left.dayNumber - right.dayNumber;
+  if (julianDayNumberDifference !== 0) return julianDayNumberDifference;
+  return left.secondsOfDay - right.secondsOfDay;
+}
+
+/** `JulianDate.leapSeconds`: the TAI instant of each leap second, and TAI−UTC from it. */
+const LEAP_SECONDS = [
+  [2441317, 43210.0, 10], // January 1, 1972 00:00:00 UTC
+  [2441499, 43211.0, 11], // July 1, 1972 00:00:00 UTC
+  [2441683, 43212.0, 12], // January 1, 1973 00:00:00 UTC
+  [2442048, 43213.0, 13], // January 1, 1974 00:00:00 UTC
+  [2442413, 43214.0, 14], // January 1, 1975 00:00:00 UTC
+  [2442778, 43215.0, 15], // January 1, 1976 00:00:00 UTC
+  [2443144, 43216.0, 16], // January 1, 1977 00:00:00 UTC
+  [2443509, 43217.0, 17], // January 1, 1978 00:00:00 UTC
+  [2443874, 43218.0, 18], // January 1, 1979 00:00:00 UTC
+  [2444239, 43219.0, 19], // January 1, 1980 00:00:00 UTC
+  [2444786, 43220.0, 20], // July 1, 1981 00:00:00 UTC
+  [2445151, 43221.0, 21], // July 1, 1982 00:00:00 UTC
+  [2445516, 43222.0, 22], // July 1, 1983 00:00:00 UTC
+  [2446247, 43223.0, 23], // July 1, 1985 00:00:00 UTC
+  [2447161, 43224.0, 24], // January 1, 1988 00:00:00 UTC
+  [2447892, 43225.0, 25], // January 1, 1990 00:00:00 UTC
+  [2448257, 43226.0, 26], // January 1, 1991 00:00:00 UTC
+  [2448804, 43227.0, 27], // July 1, 1992 00:00:00 UTC
+  [2449169, 43228.0, 28], // July 1, 1993 00:00:00 UTC
+  [2449534, 43229.0, 29], // July 1, 1994 00:00:00 UTC
+  [2450083, 43230.0, 30], // January 1, 1996 00:00:00 UTC
+  [2450630, 43231.0, 31], // July 1, 1997 00:00:00 UTC
+  [2451179, 43232.0, 32], // January 1, 1999 00:00:00 UTC
+  [2453736, 43233.0, 33], // January 1, 2006 00:00:00 UTC
+  [2454832, 43234.0, 34], // January 1, 2009 00:00:00 UTC
+  [2456109, 43235.0, 35], // July 1, 2012 00:00:00 UTC
+  [2457204, 43236.0, 36], // July 1, 2015 00:00:00 UTC
+  [2457754, 43237.0, 37], // January 1, 2017 00:00:00 UTC
+].map(([dayNumber, secondsOfDay, offset]) =>
+  Object.freeze({ julianDate: taiDate(dayNumber, secondsOfDay), offset }),
+);
+
+/** Core/binarySearch.js over the leap-second table. */
+function searchLeapSeconds(julianDate) {
+  let low = 0;
+  let high = LEAP_SECONDS.length - 1;
+  while (low <= high) {
+    const i = ~~((low + high) / 2);
+    const comparison = compare(LEAP_SECONDS[i].julianDate, julianDate);
+    if (comparison < 0) {
+      low = i + 1;
+      continue;
+    }
+    if (comparison > 0) {
+      high = i - 1;
+      continue;
+    }
+    return i;
+  }
+  return ~(high + 1);
+}
+
+/** `convertUtcToTai`: shifts a UTC date to TAI in place. */
+function convertUtcToTai(julianDate) {
+  // Even though julianDate is in UTC, Cesium treats it as TAI to search.
+  let index = searchLeapSeconds(julianDate);
+  if (index < 0) index = ~index;
+  if (index >= LEAP_SECONDS.length) index = LEAP_SECONDS.length - 1;
+
+  let offset = LEAP_SECONDS[index].offset;
+  if (index > 0) {
+    const difference = secondsDifference(
+      LEAP_SECONDS[index].julianDate,
+      julianDate,
+    );
+    if (difference > offset) {
+      index--;
+      offset = LEAP_SECONDS[index].offset;
+    }
+  }
+
+  addSeconds(julianDate, offset, julianDate);
+}
+
+/** `JulianDate.computeTaiMinusUtc`. */
+function computeTaiMinusUtc(julianDate) {
+  let index = searchLeapSeconds(julianDate);
+  if (index < 0) {
+    index = ~index;
+    --index;
+    if (index < 0) index = 0;
+  }
+  return LEAP_SECONDS[index].offset;
+}
+
+/**
+ * `JulianDate.fromDate(new Date(ms))`: the TAI Julian date of a UTC epoch
+ * millisecond (Seidelmann 1992, Explanatory Supplement p. 604).
+ */
+function julianDateFromEpochMs(ms) {
+  const date = new Date(ms);
+  if (!Number.isFinite(date.getTime()))
+    throw new RangeError(`not a representable time: ${ms}`);
+  const year = date.getUTCFullYear();
+  const month = date.getUTCMonth() + 1;
+  const day = date.getUTCDate();
+  let hour = date.getUTCHours();
+  const minute = date.getUTCMinutes();
+  const second = date.getUTCSeconds();
+  const millisecond = date.getUTCMilliseconds();
+
+  const a = ((month - 14) / 12) | 0;
+  const b = year + 4800 + a;
+  let dayNumber =
+    (((1461 * b) / 4) | 0) +
+    (((367 * (month - 2 - 12 * a)) / 12) | 0) -
+    (((3 * (((b + 100) / 100) | 0)) / 4) | 0) +
+    day -
+    32075;
+
+  // Julian dates are noon-based.
+  hour = hour - 12;
+  if (hour < 0) hour += 24;
+
+  let secondsOfDay =
+    second +
+    (hour * SECONDS_PER_HOUR +
+      minute * SECONDS_PER_MINUTE +
+      millisecond * SECONDS_PER_MILLISECOND);
+
+  if (secondsOfDay >= 43200.0) dayNumber -= 1;
+
+  // The JulianDate constructor: split a fractional day number (none here).
+  const wholeDays = dayNumber | 0;
+  secondsOfDay = secondsOfDay + (dayNumber - wholeDays) * SECONDS_PER_DAY;
+  const julianDate = setComponents(wholeDays, secondsOfDay, {});
+  convertUtcToTai(julianDate);
+  return julianDate;
+}
+
+// ---------------------------------------------------------------------------
+// Matrices
+
+/** Cesium's `Matrix3.multiplyByVector`, column-major. */
+function multiplyByVector(matrix, v) {
+  return {
+    x: matrix[0] * v.x + matrix[3] * v.y + matrix[6] * v.z,
+    y: matrix[1] * v.x + matrix[4] * v.y + matrix[7] * v.z,
+    z: matrix[2] * v.x + matrix[5] * v.y + matrix[8] * v.z,
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Simon 1994 (Core/Simon1994PlanetaryPositions.js)
+
+function computeTdbMinusTtSpice(daysSinceJ2000InTerrestrialTime) {
+  // SPICE N0051 (unitim.c) constants, with TT taken as the independent
+  // variable instead of TDB, as Cesium does.
+  const g = 6.239996 + 0.0172019696544 * daysSinceJ2000InTerrestrialTime;
+  return 1.657e-3 * Math.sin(g + 1.671e-2 * Math.sin(g));
+}
+
+const TdtMinusTai = 32.184;
+const J2000d = 2451545;
+function taiToTdb(date) {
+  // TAI to TT.
+  const result = addSeconds(date, TdtMinusTai, {});
+  // TT to TDB.
+  const days = totalDays(result) - J2000d;
+  return addSeconds(result, computeTdbMinusTtSpice(days), result);
+}
+
+const epoch = taiDate(2451545, 0); // Actually TDB (not TAI), as in Cesium.
+const MetersPerKilometer = 1000.0;
+const MetersPerAstronomicalUnit = 1.4959787e11; // IAU 1976 value
+
+/** Days since J2000 TDB at a TAI date. */
+function daysSinceEpochTdb(date) {
+  const tdb = taiToTdb(date);
+  return (
+    tdb.dayNumber -
+    epoch.dayNumber +
+    (tdb.secondsOfDay - epoch.secondsOfDay) / SECONDS_PER_DAY
+  );
+}
+
+function elementsToCartesian(
+  semimajorAxis,
+  eccentricity,
+  inclination,
+  longitudeOfPerigee,
+  longitudeOfNode,
+  meanLongitude,
+) {
+  if (inclination < 0.0) {
+    inclination = -inclination;
+    longitudeOfNode += PI;
+  }
+  // Reached in the far past (Date's earliest times), as Cesium's debug
+  // check is.
+  if (inclination > PI)
+    throw new RangeError(
+      'inclination out of range: the time is too far from J2000',
+    );
+
+  const radiusOfPeriapsis = semimajorAxis * (1.0 - eccentricity);
+  const argumentOfPeriapsis = longitudeOfPerigee - longitudeOfNode;
+  const rightAscensionOfAscendingNode = longitudeOfNode;
+  const trueAnomaly = meanAnomalyToTrueAnomaly(
+    meanLongitude - longitudeOfPerigee,
+    eccentricity,
+  );
+
+  const perifocalToEquatorial = perifocalToCartesianMatrix(
+    argumentOfPeriapsis,
+    inclination,
+    rightAscensionOfAscendingNode,
+  );
+  const semilatus = radiusOfPeriapsis * (1.0 + eccentricity);
+  const costheta = Math.cos(trueAnomaly);
+  const sintheta = Math.sin(trueAnomaly);
+
+  const denom = 1.0 + eccentricity * costheta;
+  const radius = semilatus / denom;
+  return multiplyByVector(perifocalToEquatorial, {
+    x: radius * costheta,
+    y: radius * sintheta,
+    z: 0.0,
+  });
+}
+
+function meanAnomalyToTrueAnomaly(meanAnomaly, eccentricity) {
+  if (!(eccentricity >= 0.0 && eccentricity < 1.0))
+    throw new RangeError(
+      'eccentricity out of range: the time is too far from J2000',
+    );
+  const eccentricAnomaly = meanAnomalyToEccentricAnomaly(
+    meanAnomaly,
+    eccentricity,
+  );
+  return eccentricAnomalyToTrueAnomaly(eccentricAnomaly, eccentricity);
+}
+
+const maxIterationCount = 50;
+const keplerEqConvergence = 0.00000001; // CesiumMath.EPSILON8
+function meanAnomalyToEccentricAnomaly(meanAnomaly, eccentricity) {
+  const revs = Math.floor(meanAnomaly / TWO_PI);
+
+  // Angle in the current revolution.
+  meanAnomaly -= revs * TWO_PI;
+
+  // Starting value for the iteration.
+  let iterationValue =
+    meanAnomaly +
+    (eccentricity * Math.sin(meanAnomaly)) /
+      (1.0 - Math.sin(meanAnomaly + eccentricity) + Math.sin(meanAnomaly));
+
+  // Newton-Raphson on Kepler's equation.
+  let eccentricAnomaly = Number.MAX_VALUE;
+  let count = 0;
+  for (
+    ;
+    count < maxIterationCount &&
+    Math.abs(eccentricAnomaly - iterationValue) > keplerEqConvergence;
+    ++count
+  ) {
+    eccentricAnomaly = iterationValue;
+    const NRfunction =
+      eccentricAnomaly -
+      eccentricity * Math.sin(eccentricAnomaly) -
+      meanAnomaly;
+    const dNRfunction = 1 - eccentricity * Math.cos(eccentricAnomaly);
+    iterationValue = eccentricAnomaly - NRfunction / dNRfunction;
+  }
+
+  // Not reached for an elliptic orbit; kept as Cesium's debug check.
+  if (count >= maxIterationCount)
+    throw new RangeError('Kepler equation did not converge');
+  eccentricAnomaly = iterationValue + revs * TWO_PI;
+  return eccentricAnomaly;
+}
+
+function eccentricAnomalyToTrueAnomaly(eccentricAnomaly, eccentricity) {
+  const revs = Math.floor(eccentricAnomaly / TWO_PI);
+
+  eccentricAnomaly -= revs * TWO_PI;
+
+  const trueAnomalyX = Math.cos(eccentricAnomaly) - eccentricity;
+  const trueAnomalyY =
+    Math.sin(eccentricAnomaly) * Math.sqrt(1 - eccentricity * eccentricity);
+
+  let trueAnomaly = Math.atan2(trueAnomalyY, trueAnomalyX);
+
+  // Correct quadrant.
+  trueAnomaly = zeroToTwoPi(trueAnomaly);
+  if (eccentricAnomaly < 0) trueAnomaly -= TWO_PI;
+
+  // Previous revolutions.
+  trueAnomaly += revs * TWO_PI;
+
+  return trueAnomaly;
+}
+
+/** Perifocal (PQW) to inertial axes, column-major. */
+function perifocalToCartesianMatrix(
+  argumentOfPeriapsis,
+  inclination,
+  rightAscension,
+) {
+  const cosap = Math.cos(argumentOfPeriapsis);
+  const sinap = Math.sin(argumentOfPeriapsis);
+
+  const cosi = Math.cos(inclination);
+  const sini = Math.sin(inclination);
+
+  const cosraan = Math.cos(rightAscension);
+  const sinraan = Math.sin(rightAscension);
+
+  const result = new Float64Array(9);
+  result[0] = cosraan * cosap - sinraan * sinap * cosi;
+  result[1] = sinraan * cosap + cosraan * sinap * cosi;
+  result[2] = sinap * sini;
+  result[3] = -cosraan * sinap - sinraan * cosap * cosi;
+  result[4] = -sinraan * sinap + cosraan * cosap * cosi;
+  result[5] = cosap * sini;
+  result[6] = sinraan * sini;
+  result[7] = -cosraan * sini;
+  result[8] = cosi;
+  return result;
+}
+
+// From section 5.8
+const semiMajorAxis0 = 1.0000010178 * MetersPerAstronomicalUnit;
+const meanLongitude0 = 100.46645683 * RADIANS_PER_DEGREE;
+const meanLongitude1 = 1295977422.83429 * RADIANS_PER_ARCSECOND;
+
+// From table 6
+const p1u = 16002;
+const p2u = 21863;
+const p3u = 32004;
+const p4u = 10931;
+const p5u = 14529;
+const p6u = 16368;
+const p7u = 15318;
+const p8u = 32794;
+
+const Ca1 = 64 * 1e-7 * MetersPerAstronomicalUnit;
+const Ca2 = -152 * 1e-7 * MetersPerAstronomicalUnit;
+const Ca3 = 62 * 1e-7 * MetersPerAstronomicalUnit;
+const Ca4 = -8 * 1e-7 * MetersPerAstronomicalUnit;
+const Ca5 = 32 * 1e-7 * MetersPerAstronomicalUnit;
+const Ca6 = -41 * 1e-7 * MetersPerAstronomicalUnit;
+const Ca7 = 19 * 1e-7 * MetersPerAstronomicalUnit;
+const Ca8 = -11 * 1e-7 * MetersPerAstronomicalUnit;
+
+const Sa1 = -150 * 1e-7 * MetersPerAstronomicalUnit;
+const Sa2 = -46 * 1e-7 * MetersPerAstronomicalUnit;
+const Sa3 = 68 * 1e-7 * MetersPerAstronomicalUnit;
+const Sa4 = 54 * 1e-7 * MetersPerAstronomicalUnit;
+const Sa5 = 14 * 1e-7 * MetersPerAstronomicalUnit;
+const Sa6 = 24 * 1e-7 * MetersPerAstronomicalUnit;
+const Sa7 = -28 * 1e-7 * MetersPerAstronomicalUnit;
+const Sa8 = 22 * 1e-7 * MetersPerAstronomicalUnit;
+
+const q1u = 10;
+const q2u = 16002;
+const q3u = 21863;
+const q4u = 10931;
+const q5u = 1473;
+const q6u = 32004;
+const q7u = 4387;
+const q8u = 73;
+
+const Cl1 = -325 * 1e-7;
+const Cl2 = -322 * 1e-7;
+const Cl3 = -79 * 1e-7;
+const Cl4 = 232 * 1e-7;
+const Cl5 = -52 * 1e-7;
+const Cl6 = 97 * 1e-7;
+const Cl7 = 55 * 1e-7;
+const Cl8 = -41 * 1e-7;
+
+const Sl1 = -105 * 1e-7;
+const Sl2 = -137 * 1e-7;
+const Sl3 = 258 * 1e-7;
+const Sl4 = 35 * 1e-7;
+const Sl5 = -116 * 1e-7;
+const Sl6 = -88 * 1e-7;
+const Sl7 = -112 * 1e-7;
+const Sl8 = -80 * 1e-7;
+
+/** The Earth-Moon barycentre (section 6), at a TAI date. */
+function computeSimonEarthMoonBarycenter(date) {
+  // t is thousands of years from J2000 TDB.
+  const x = daysSinceEpochTdb(date);
+  const t = x / (DAYS_PER_JULIAN_CENTURY * 10.0);
+
+  const u = 0.3595362 * t;
+  const semimajorAxis =
+    semiMajorAxis0 +
+    Ca1 * Math.cos(p1u * u) +
+    Sa1 * Math.sin(p1u * u) +
+    Ca2 * Math.cos(p2u * u) +
+    Sa2 * Math.sin(p2u * u) +
+    Ca3 * Math.cos(p3u * u) +
+    Sa3 * Math.sin(p3u * u) +
+    Ca4 * Math.cos(p4u * u) +
+    Sa4 * Math.sin(p4u * u) +
+    Ca5 * Math.cos(p5u * u) +
+    Sa5 * Math.sin(p5u * u) +
+    Ca6 * Math.cos(p6u * u) +
+    Sa6 * Math.sin(p6u * u) +
+    Ca7 * Math.cos(p7u * u) +
+    Sa7 * Math.sin(p7u * u) +
+    Ca8 * Math.cos(p8u * u) +
+    Sa8 * Math.sin(p8u * u);
+  const meanLongitude =
+    meanLongitude0 +
+    meanLongitude1 * t +
+    Cl1 * Math.cos(q1u * u) +
+    Sl1 * Math.sin(q1u * u) +
+    Cl2 * Math.cos(q2u * u) +
+    Sl2 * Math.sin(q2u * u) +
+    Cl3 * Math.cos(q3u * u) +
+    Sl3 * Math.sin(q3u * u) +
+    Cl4 * Math.cos(q4u * u) +
+    Sl4 * Math.sin(q4u * u) +
+    Cl5 * Math.cos(q5u * u) +
+    Sl5 * Math.sin(q5u * u) +
+    Cl6 * Math.cos(q6u * u) +
+    Sl6 * Math.sin(q6u * u) +
+    Cl7 * Math.cos(q7u * u) +
+    Sl7 * Math.sin(q7u * u) +
+    Cl8 * Math.cos(q8u * u) +
+    Sl8 * Math.sin(q8u * u);
+
+  // All constants in this part are from section 5.8.
+  const eccentricity = 0.0167086342 - 0.0004203654 * t;
+  const longitudeOfPerigee =
+    102.93734808 * RADIANS_PER_DEGREE + 11612.3529 * RADIANS_PER_ARCSECOND * t;
+  const inclination = 469.97289 * RADIANS_PER_ARCSECOND * t;
+  const longitudeOfNode =
+    174.87317577 * RADIANS_PER_DEGREE - 8679.27034 * RADIANS_PER_ARCSECOND * t;
+
+  return elementsToCartesian(
+    semimajorAxis,
+    eccentricity,
+    inclination,
+    longitudeOfPerigee,
+    longitudeOfNode,
+    meanLongitude,
+  );
+}
+
+/** The Moon relative to the Earth (section 4), at a TAI date. */
+function computeSimonMoon(date) {
+  const x = daysSinceEpochTdb(date);
+  const t = x / DAYS_PER_JULIAN_CENTURY;
+  const t2 = t * t;
+  const t3 = t2 * t;
+  const t4 = t3 * t;
+
+  // Terms from section 3.4 (b.1)
+  let semimajorAxis = 383397.7725 + 0.004 * t;
+  let eccentricity = 0.055545526 - 0.000000016 * t;
+  const inclinationConstant = 5.15668983 * RADIANS_PER_DEGREE;
+  let inclinationSecPart =
+    -0.00008 * t + 0.02966 * t2 - 0.000042 * t3 - 0.00000013 * t4;
+  const longitudeOfPerigeeConstant = 83.35324312 * RADIANS_PER_DEGREE;
+  let longitudeOfPerigeeSecPart =
+    14643420.2669 * t - 38.2702 * t2 - 0.045047 * t3 + 0.00021301 * t4;
+  const longitudeOfNodeConstant = 125.04455501 * RADIANS_PER_DEGREE;
+  let longitudeOfNodeSecPart =
+    -6967919.3631 * t + 6.3602 * t2 + 0.007625 * t3 - 0.00003586 * t4;
+  const meanLongitudeConstant = 218.31664563 * RADIANS_PER_DEGREE;
+  let meanLongitudeSecPart =
+    1732559343.4847 * t - 6.391 * t2 + 0.006588 * t3 - 0.00003169 * t4;
+
+  // Delaunay arguments from section 3.5 b
+  const D =
+    297.85019547 * RADIANS_PER_DEGREE +
+    RADIANS_PER_ARCSECOND *
+      (1602961601.209 * t - 6.3706 * t2 + 0.006593 * t3 - 0.00003169 * t4);
+  const F =
+    93.27209062 * RADIANS_PER_DEGREE +
+    RADIANS_PER_ARCSECOND *
+      (1739527262.8478 * t - 12.7512 * t2 - 0.001037 * t3 + 0.00000417 * t4);
+  const l =
+    134.96340251 * RADIANS_PER_DEGREE +
+    RADIANS_PER_ARCSECOND *
+      (1717915923.2178 * t + 31.8792 * t2 + 0.051635 * t3 - 0.0002447 * t4);
+  const lprime =
+    357.52910918 * RADIANS_PER_DEGREE +
+    RADIANS_PER_ARCSECOND *
+      (129596581.0481 * t - 0.5532 * t2 + 0.000136 * t3 - 0.00001149 * t4);
+  const psi =
+    310.17137918 * RADIANS_PER_DEGREE -
+    RADIANS_PER_ARCSECOND *
+      (6967051.436 * t + 6.2068 * t2 + 0.007618 * t3 - 0.00003219 * t4);
+
+  // Add terms from Table 4
+  const twoD = 2.0 * D;
+  const fourD = 4.0 * D;
+  const sixD = 6.0 * D;
+  const twol = 2.0 * l;
+  const threel = 3.0 * l;
+  const fourl = 4.0 * l;
+  const twoF = 2.0 * F;
+  semimajorAxis +=
+    3400.4 * Math.cos(twoD) -
+    635.6 * Math.cos(twoD - l) -
+    235.6 * Math.cos(l) +
+    218.1 * Math.cos(twoD - lprime) +
+    181.0 * Math.cos(twoD + l);
+  eccentricity +=
+    0.014216 * Math.cos(twoD - l) +
+    0.008551 * Math.cos(twoD - twol) -
+    0.001383 * Math.cos(l) +
+    0.001356 * Math.cos(twoD + l) -
+    0.001147 * Math.cos(fourD - threel) -
+    0.000914 * Math.cos(fourD - twol) +
+    0.000869 * Math.cos(twoD - lprime - l) -
+    0.000627 * Math.cos(twoD) -
+    0.000394 * Math.cos(fourD - fourl) +
+    0.000282 * Math.cos(twoD - lprime - twol) -
+    0.000279 * Math.cos(D - l) -
+    0.000236 * Math.cos(twol) +
+    0.000231 * Math.cos(fourD) +
+    0.000229 * Math.cos(sixD - fourl) -
+    0.000201 * Math.cos(twol - twoF);
+  inclinationSecPart +=
+    486.26 * Math.cos(twoD - twoF) -
+    40.13 * Math.cos(twoD) +
+    37.51 * Math.cos(twoF) +
+    25.73 * Math.cos(twol - twoF) +
+    19.97 * Math.cos(twoD - lprime - twoF);
+  longitudeOfPerigeeSecPart +=
+    -55609 * Math.sin(twoD - l) -
+    34711 * Math.sin(twoD - twol) -
+    9792 * Math.sin(l) +
+    9385 * Math.sin(fourD - threel) +
+    7505 * Math.sin(fourD - twol) +
+    5318 * Math.sin(twoD + l) +
+    3484 * Math.sin(fourD - fourl) -
+    3417 * Math.sin(twoD - lprime - l) -
+    2530 * Math.sin(sixD - fourl) -
+    2376 * Math.sin(twoD) -
+    2075 * Math.sin(twoD - threel) -
+    1883 * Math.sin(twol) -
+    1736 * Math.sin(sixD - 5.0 * l) +
+    1626 * Math.sin(lprime) -
+    1370 * Math.sin(sixD - threel);
+  longitudeOfNodeSecPart +=
+    -5392 * Math.sin(twoD - twoF) -
+    540 * Math.sin(lprime) -
+    441 * Math.sin(twoD) +
+    423 * Math.sin(twoF) -
+    288 * Math.sin(twol - twoF);
+  meanLongitudeSecPart +=
+    -3332.9 * Math.sin(twoD) +
+    1197.4 * Math.sin(twoD - l) -
+    662.5 * Math.sin(lprime) +
+    396.3 * Math.sin(l) -
+    218.0 * Math.sin(twoD - lprime);
+
+  // Add terms from Table 5
+  const twoPsi = 2.0 * psi;
+  const threePsi = 3.0 * psi;
+  inclinationSecPart +=
+    46.997 * Math.cos(psi) * t -
+    0.614 * Math.cos(twoD - twoF + psi) * t +
+    0.614 * Math.cos(twoD - twoF - psi) * t -
+    0.0297 * Math.cos(twoPsi) * t2 -
+    0.0335 * Math.cos(psi) * t2 +
+    0.0012 * Math.cos(twoD - twoF + twoPsi) * t2 -
+    0.00016 * Math.cos(psi) * t3 +
+    0.00004 * Math.cos(threePsi) * t3 +
+    0.00004 * Math.cos(twoPsi) * t3;
+  const perigeeAndMean =
+    2.116 * Math.sin(psi) * t -
+    0.111 * Math.sin(twoD - twoF - psi) * t -
+    0.0015 * Math.sin(psi) * t2;
+  longitudeOfPerigeeSecPart += perigeeAndMean;
+  meanLongitudeSecPart += perigeeAndMean;
+  longitudeOfNodeSecPart +=
+    -520.77 * Math.sin(psi) * t +
+    13.66 * Math.sin(twoD - twoF + psi) * t +
+    1.12 * Math.sin(twoD - psi) * t -
+    1.06 * Math.sin(twoF - psi) * t +
+    0.66 * Math.sin(twoPsi) * t2 +
+    0.371 * Math.sin(psi) * t2 -
+    0.035 * Math.sin(twoD - twoF + twoPsi) * t2 -
+    0.015 * Math.sin(twoD - twoF + psi) * t2 +
+    0.0014 * Math.sin(psi) * t3 -
+    0.0011 * Math.sin(threePsi) * t3 -
+    0.0009 * Math.sin(twoPsi) * t3;
+
+  // Add constants and convert units
+  semimajorAxis *= MetersPerKilometer;
+  const inclination =
+    inclinationConstant + inclinationSecPart * RADIANS_PER_ARCSECOND;
+  const longitudeOfPerigee =
+    longitudeOfPerigeeConstant +
+    longitudeOfPerigeeSecPart * RADIANS_PER_ARCSECOND;
+  const meanLongitude =
+    meanLongitudeConstant + meanLongitudeSecPart * RADIANS_PER_ARCSECOND;
+  const longitudeOfNode =
+    longitudeOfNodeConstant + longitudeOfNodeSecPart * RADIANS_PER_ARCSECOND;
+
+  return elementsToCartesian(
+    semimajorAxis,
+    eccentricity,
+    inclination,
+    longitudeOfPerigee,
+    longitudeOfNode,
+    meanLongitude,
+  );
+}
+
+// The Earth relative to the Earth-Moon barycentre, from the Moon and the
+// 1992 mu value (Moon/Earth mass ratio) in Table 2.
+const moonEarthMassRatio = 0.012300034;
+const factor = (moonEarthMassRatio / (moonEarthMassRatio + 1.0)) * -1;
+function computeSimonEarth(date) {
+  const moon = computeSimonMoon(date);
+  return { x: moon.x * factor, y: moon.y * factor, z: moon.z * factor };
+}
+
+// Ecliptic to Cesium's Earth inertial axes (STK Components'
+// GeographicTransformer), column-major.
+const axesTransformation = Float64Array.of(
+  1.0000000000000002,
+  -5.154129427414611e-16,
+  -2.23970096136568e-16,
+  5.619723173785822e-16,
+  0.9174820620691819,
+  0.39777715593191376,
+  4.690511510146299e-19,
+  -0.39777715593191376,
+  0.9174820620691819,
+);
+
+/**
+ * The Sun in Cesium's Earth-centred inertial frame, in metres
+ * (`Simon1994PlanetaryPositions.computeSunPositionInEarthInertialFrame`).
+ * @param {number} ms UTC epoch milliseconds.
+ * @returns {{x: number, y: number, z: number}}
+ */
+export function sunPositionInertial(ms) {
+  const date = julianDateFromEpochMs(ms);
+  const translation = computeSimonEarthMoonBarycenter(date);
+  const earth = computeSimonEarth(date);
+  return multiplyByVector(axesTransformation, {
+    x: -translation.x - earth.x,
+    y: -translation.y - earth.y,
+    z: -translation.z - earth.z,
+  });
+}
+
+/**
+ * The Moon in Cesium's Earth-centred inertial frame, in metres
+ * (`Simon1994PlanetaryPositions.computeMoonPositionInEarthInertialFrame`).
+ * @param {number} ms UTC epoch milliseconds.
+ * @returns {{x: number, y: number, z: number}}
+ */
+export function moonPositionInertial(ms) {
+  return multiplyByVector(
+    axesTransformation,
+    computeSimonMoon(julianDateFromEpochMs(ms)),
+  );
+}
+
+// ---------------------------------------------------------------------------
+// TEME to pseudo-fixed (Core/Transforms.js)
+
+const gmstConstant0 = 6 * 3600 + 41 * 60 + 50.54841;
+const gmstConstant1 = 8640184.812866;
+const gmstConstant2 = 0.093104;
+const gmstConstant3 = -6.2e-6;
+const rateCoef = 1.1772758384668e-19;
+const wgs84WRPrecessing = 7.2921158553e-5;
+const twoPiOverSecondsInDay = TWO_PI / 86400.0;
+
+/**
+ * The rotation from True Equator Mean Equinox axes to Cesium's pseudo-fixed
+ * axes (`Transforms.computeTemeToPseudoFixedMatrix`), treating UT1 as UTC.
+ * Epoch milliseconds cannot name an instant inside a leap second
+ * (23:59:60), so none reaches it; the instants either side are Cesium's.
+ * @param {number} ms UTC epoch milliseconds.
+ * @returns {{x: object, y: object, z: object, origin: object}} A frame.
+ */
+export function temeToPseudoFixed(ms) {
+  const date = julianDateFromEpochMs(ms);
+  // GMST is computed from UT1; UTC stands in for it.
+  const dateInUtc = addSeconds(date, -computeTaiMinusUtc(date), {});
+  const utcDayNumber = dateInUtc.dayNumber;
+  const utcSecondsIntoDay = dateInUtc.secondsOfDay;
+
+  let t;
+  const diffDays = utcDayNumber - 2451545;
+  if (utcSecondsIntoDay >= 43200.0) {
+    t = (diffDays + 0.5) / DAYS_PER_JULIAN_CENTURY;
+  } else {
+    t = (diffDays - 0.5) / DAYS_PER_JULIAN_CENTURY;
+  }
+
+  const gmst0 =
+    gmstConstant0 +
+    t * (gmstConstant1 + t * (gmstConstant2 + t * gmstConstant3));
+  const angle = (gmst0 * twoPiOverSecondsInDay) % TWO_PI;
+  const ratio = wgs84WRPrecessing + rateCoef * (utcDayNumber - 2451545.5);
+  const secondsSinceMidnight =
+    (utcSecondsIntoDay + SECONDS_PER_DAY * 0.5) % SECONDS_PER_DAY;
+  const gha = angle + ratio * secondsSinceMidnight;
+  const cosGha = Math.cos(gha);
+  const sinGha = Math.sin(gha);
+
+  return {
+    x: vec(cosGha, -sinGha, 0.0),
+    y: vec(sinGha, cosGha, 0.0),
+    z: vec(0.0, 0.0, 1.0),
+    origin: vec(0.0, 0.0, 0.0),
+  };
+}
+
+/**
+ * The Sun in Earth-fixed (ECEF) metres: `sunPositionInertial` rotated by
+ * `temeToPseudoFixed`. This is the pseudo-fixed frame Core falls back to
+ * while EOP data is missing, not the EOP-corrected ICRF-to-fixed one
+ * (`Transforms.computeIcrfToFixedMatrix`); the two differ by precession,
+ * nutation and polar motion.
+ * @param {number} ms UTC epoch milliseconds.
+ * @returns {{x: number, y: number, z: number}}
+ */
+export function sunPositionFixed(ms) {
+  return frameVector(temeToPseudoFixed(ms), sunPositionInertial(ms));
+}
+
+/**
+ * The Moon in Earth-fixed (ECEF) metres, in the same pseudo-fixed frame as
+ * `sunPositionFixed`.
+ * @param {number} ms UTC epoch milliseconds.
+ * @returns {{x: number, y: number, z: number}}
+ */
+export function moonPositionFixed(ms) {
+  return frameVector(temeToPseudoFixed(ms), moonPositionInertial(ms));
+}
diff --git a/src/geo/celestial.test.mjs b/src/geo/celestial.test.mjs
new file mode 100644
index 000000000..b81b0ed8c
--- /dev/null
+++ b/src/geo/celestial.test.mjs
@@ -0,0 +1,200 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import * as Cesium from 'cesium';
+import {
+  moonPositionFixed,
+  moonPositionInertial,
+  sunPositionFixed,
+  sunPositionInertial,
+  temeToPseudoFixed,
+} from './celestial.js';
+
+/*
+ * The port runs Cesium's operations in the same order with the same Math
+ * functions, so every component must be identical (Object.is, so signed
+ * zeros count). Any operation-order slip shows up as a difference.
+ */
+
+/** A seeded generator, so a failure names a reproducible point. */
+function random(seed) {
+  let s = seed >>> 0;
+  return () => {
+    s = (s * 1664525 + 1013904223) >>> 0;
+    return s / 2 ** 32;
+  };
+}
+
+const START = Date.UTC(1990, 0, 1);
+const END = Date.UTC(2050, 0, 1);
+const year = (y) => new Date(0).setUTCFullYear(y, 0, 1);
+
+/** Seeded times in 1990-2050 and -500 to 3000, the epoch, and steps across every leap second. */
+function sampleTimes() {
+  const next = random(0x5eed);
+  const times = [];
+  for (let i = 0; i < 4000; i++) {
+    times.push(Math.round(START + next() * (END - START)));
+  }
+  // Far from J2000, where Kepler's revolutions count, and before 1972, where
+  // UTC to TAI takes the table's first offset.
+  for (let i = 0; i < 400; i++)
+    times.push(Math.round(year(-500) + next() * (year(3000) - year(-500))));
+  times.push(
+    year(1900),
+    year(1960),
+    -1,
+    0,
+    Date.UTC(1971, 11, 31, 23, 59, 59, 999),
+    Date.UTC(1972, 0, 1),
+    Date.UTC(1972, 6, 1) - 1,
+    year(2100),
+    year(3000),
+  );
+  // Each leap second Cesium knows, 1.5 s either side.
+  const boundaries = [
+    [1972, 6],
+    [1973, 0],
+    [1974, 0],
+    [1975, 0],
+    [1976, 0],
+    [1977, 0],
+    [1978, 0],
+    [1979, 0],
+    [1980, 0],
+    [1981, 6],
+    [1982, 6],
+    [1983, 6],
+    [1985, 6],
+    [1988, 0],
+    [1990, 0],
+    [1991, 0],
+    [1992, 6],
+    [1993, 6],
+    [1994, 6],
+    [1996, 0],
+    [1997, 6],
+    [1999, 0],
+    [2006, 0],
+    [2009, 0],
+    [2012, 6],
+    [2015, 6],
+    [2017, 0],
+  ];
+  for (const [year, month] of boundaries) {
+    const at = Date.UTC(year, month, 1);
+    for (let d = -1500; d <= 1500; d += 125) times.push(at + d);
+  }
+  // The span the task names, millisecond by millisecond at the edges.
+  const leap = Date.UTC(2017, 0, 1);
+  for (let d = -1000; d <= 1000; d += 1) {
+    if (Math.abs(d) <= 5 || Math.abs(d) >= 995 || d % 50 === 0) {
+      times.push(leap + d);
+    }
+  }
+  // Noon/midnight edges (secondsOfDay wrap), J2000, and before the epoch.
+  times.push(
+    Date.UTC(2000, 0, 1, 12),
+    Date.UTC(2000, 0, 1, 11, 58, 55, 816),
+    Date.UTC(2000, 0, 1, 11, 59, 59, 999),
+    Date.UTC(2024, 5, 30, 12, 0, 0, 0),
+    Date.UTC(2024, 5, 30, 11, 59, 59, 999),
+    Date.UTC(2024, 5, 30, 23, 59, 59, 999),
+    START,
+    END,
+  );
+  return times;
+}
+
+const TIMES = sampleTimes();
+const julian = (ms) => Cesium.JulianDate.fromDate(new Date(ms));
+const iso = (ms) => new Date(ms).toISOString();
+
+function sameOver(label, compute, expected) {
+  for (const ms of TIMES) {
+    const ours = compute(ms);
+    const theirs = expected(ms);
+    for (const k of ['x', 'y', 'z'])
+      assert.ok(
+        Object.is(ours[k], theirs[k]),
+        `${label} ${k} at ${iso(ms)} (${ms}): ${ours[k]} vs ${theirs[k]}`,
+      );
+  }
+}
+
+function cesiumFixed(ms, compute) {
+  const time = julian(ms);
+  const inertial = compute(time, new Cesium.Cartesian3());
+  const matrix = Cesium.Transforms.computeTemeToPseudoFixedMatrix(time);
+  return Cesium.Matrix3.multiplyByVector(
+    matrix,
+    inertial,
+    new Cesium.Cartesian3(),
+  );
+}
+
+const { Simon1994PlanetaryPositions: Simon } = Cesium;
+
+test('sunPositionInertial matches Simon1994PlanetaryPositions', () => {
+  sameOver('sun inertial', sunPositionInertial, (ms) =>
+    Simon.computeSunPositionInEarthInertialFrame(julian(ms)),
+  );
+});
+
+test('moonPositionInertial matches Simon1994PlanetaryPositions', () => {
+  sameOver('moon inertial', moonPositionInertial, (ms) =>
+    Simon.computeMoonPositionInEarthInertialFrame(julian(ms)),
+  );
+});
+
+test('temeToPseudoFixed matches Transforms.computeTemeToPseudoFixedMatrix', () => {
+  for (const ms of TIMES) {
+    const ours = temeToPseudoFixed(ms);
+    const theirs = Cesium.Transforms.computeTemeToPseudoFixedMatrix(julian(ms));
+    ['x', 'y', 'z'].forEach((axis, c) =>
+      ['x', 'y', 'z'].forEach((k, r) =>
+        assert.ok(
+          Object.is(ours[axis][k], theirs[c * 3 + r]),
+          `${axis}.${k} at ${iso(ms)} (${ms}): ${ours[axis][k]} vs ${theirs[c * 3 + r]}`,
+        ),
+      ),
+    );
+  }
+});
+
+test('sunPositionFixed matches the pseudo-fixed rotation of the sun', () => {
+  sameOver('sun fixed', sunPositionFixed, (ms) =>
+    cesiumFixed(ms, Simon.computeSunPositionInEarthInertialFrame),
+  );
+});
+
+test('moonPositionFixed matches the pseudo-fixed rotation of the moon', () => {
+  sameOver('moon fixed', moonPositionFixed, (ms) =>
+    cesiumFixed(ms, Simon.computeMoonPositionInEarthInertialFrame),
+  );
+});
+
+test('positions are plausible: 1 AU sun, ~384,000 km moon', () => {
+  const ms = Date.UTC(2026, 10, 5);
+  const sun = sunPositionInertial(ms);
+  const moon = moonPositionInertial(ms);
+  const au = Math.hypot(sun.x, sun.y, sun.z) / 1.4959787e11;
+  const moonKm = Math.hypot(moon.x, moon.y, moon.z) / 1000;
+  assert.ok(au > 0.98 && au < 1.02, `sun at ${au} AU`);
+  assert.ok(moonKm > 356000 && moonKm < 407000, `moon at ${moonKm} km`);
+});
+
+test('times outside the model throw', () => {
+  for (const fn of [
+    sunPositionInertial,
+    moonPositionInertial,
+    temeToPseudoFixed,
+    sunPositionFixed,
+    moonPositionFixed,
+  ])
+    for (const ms of [NaN, Infinity, -Infinity, 8.64e15 + 1])
+      assert.throws(() => fn(ms), RangeError, `${fn.name}(${ms})`);
+  const farFuture = Date.parse('+050000-01-01T00:00:00Z');
+  assert.throws(() => sunPositionInertial(farFuture), /eccentricity/);
+  for (const fn of [sunPositionInertial, moonPositionInertial])
+    assert.throws(() => fn(-8.64e15), /inclination/);
+});
diff --git a/src/geo/index.js b/src/geo/index.js
index 6eec330aa..f80ae09ac 100644
--- a/src/geo/index.js
+++ b/src/geo/index.js
@@ -1,10 +1,11 @@
 /**
  * @module geo
  * @description Renderer-neutral geodesy and vector maths: WGS84, ECEF and
- * geodetic conversions, east-north-up frames, quaternions, and the sun and
+ * geodetic conversions, east-north-up frames, and the sun and
  * moon. Plain numbers and `{x, y, z}` objects only, so code that needs maths
  * but not a renderer does not import one.
  */
 export * from './vector.js';
 export * from './ellipsoid.js';
 export * from './frames.js';
+export * from './celestial.js';