diff --git a/Assets/Resources/Settings.asset b/Assets/Resources/Settings.asset
index df56f95ed..b007e4a33 100644
--- a/Assets/Resources/Settings.asset
+++ b/Assets/Resources/Settings.asset
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:9d641fe942dfd45990a0609b3017a63cdc27f502ab12aa74ffbf984ea3e8d346
-size 12644
+oid sha256:f5fda2fa736e02d879a317152738e0a4c44c10b356a6dbd41cf31d2d7e7e6fd8
+size 12571
diff --git a/Assets/Scripts/ServerShared/Settings.cs b/Assets/Scripts/ServerShared/Settings.cs
index 24078a658..f09a6b187 100644
--- a/Assets/Scripts/ServerShared/Settings.cs
+++ b/Assets/Scripts/ServerShared/Settings.cs
@@ -31,7 +31,6 @@ public class PlanetSettings
     public ExponentialLerp AsteroidRespawnTime;
     public float SecureAreaRadiusMultiplier = .45f;
 
-    public ExponentialCurve OrbitPeriod;
 
     // The global flow stardust and nebula follow and the sim knows (FlowField). Zero flow by default.
     public FlowSettings Flow = new FlowSettings();
diff --git a/Assets/Scripts/ServerShared/Targeting/BeltTargets.cs b/Assets/Scripts/ServerShared/Targeting/BeltTargets.cs
index cba7ea628..3e12536c3 100644
--- a/Assets/Scripts/ServerShared/Targeting/BeltTargets.cs
+++ b/Assets/Scripts/ServerShared/Targeting/BeltTargets.cs
@@ -59,8 +59,8 @@ public sealed class BeltTargets : ITargetProvider
         {
             order[i] = i;
             distances[i] = data.Asteroids[i].Distance;
-            // Pose divides by the float period the curve returns; the keys must use the same one.
-            _periods[i] = settings.OrbitPeriod.Evaluate(distances[i]);
+            // The belt's own periods (Zone.PeriodAround), the very floats Pose divides by.
+            _periods[i] = zone.AsteroidBelts[field].Periods[i];
         }
         Array.Sort(distances, order);
         _bands = new Band[(count + RocksPerBand - 1) / RocksPerBand];
diff --git a/Assets/Scripts/ServerShared/Zone.cs b/Assets/Scripts/ServerShared/Zone.cs
index db4bf6b3d..d6981f30d 100644
--- a/Assets/Scripts/ServerShared/Zone.cs
+++ b/Assets/Scripts/ServerShared/Zone.cs
@@ -158,15 +158,9 @@ public class Zone
 
         foreach (var orbit in pack.Orbits)
         {
-            Orbits.Add(orbit.Key, new Orbit(Settings, cache.Get(orbit)));
+            Orbits.Add(orbit.Key, new Orbit(cache.Get(orbit)));
         }
 
-        // Aetheria gravity (target invariant zone-valid-before-tick): every orbit is placed at the pack's time
-        // when the zone is built, so each well, light and body pose read before the first Update is where the
-        // body is. Velocity stays zero until the first Update differences two placed positions.
-        foreach (var orbit in Orbits)
-            GetOrbitPosition(orbit.Key);
-
         foreach (var body in pack.Planets)
         {
             var planet = cache.Get(body);
@@ -178,7 +172,6 @@ public class Zone
                     // the rule generation uses, and keeps it.
                     FieldKinds.Ensure(body.Key, belt, cache);
                     AsteroidBelts[body.Key] = new AsteroidBelt(belt);
-                    Targets.Add(new BeltTargets(this, body.Key, belt, settings, _time));
                     break;
                 case SunData sun:
                     var sunInstance = new Sun(settings, sun, Orbits[planet.Orbit.Key]);
@@ -194,6 +187,40 @@ public class Zone
             }
         }
 
+        // Orbits, bodies and belts exist: every rail's period is derived once, from the field it orbits in.
+        foreach (var orbit in Orbits)
+        {
+            var parent = orbit.Value.Data.Parent;
+            if (!parent.IsSet()) continue;
+            if (!_childOrbits.TryGetValue(parent.Key, out var siblings))
+                _childOrbits[parent.Key] = siblings = new List<CultRecordKey>();
+            siblings.Add(orbit.Key);
+        }
+        foreach (var body in PlanetInstances)
+            if (body.Value.WellStrength > 0)
+                _wellOn.TryAdd(Planets[body.Key].Orbit.Key, body.Value);
+        foreach (var orbit in Orbits)
+            orbit.Value.Period = orbit.Value.Data.Parent.IsSet()
+                ? PeriodAround(orbit.Value.Data.Parent.Key, orbit.Value.Data.Distance, orbit.Key)
+                : float.PositiveInfinity;
+        foreach (var belt in AsteroidBelts)
+        {
+            var beltOrbit = Planets[belt.Key].Orbit.Key;
+            var beltParent = Orbits[beltOrbit].Data.Parent.Key;
+            for (var i = 0; i < belt.Value.Periods.Length; i++)
+                belt.Value.Periods[i] = PeriodAround(beltParent, belt.Value.Data.Asteroids[i].Distance, beltOrbit);
+        }
+
+        // Aetheria gravity (target invariant zone-valid-before-tick): every orbit is placed at the pack's time
+        // when the zone is built, so each well, light and body pose read before the first Update is where the
+        // body is. Velocity stays zero until the first Update differences two placed positions.
+        foreach (var orbit in Orbits)
+            GetOrbitPosition(orbit.Key);
+
+        // BeltTargets copies the belts' periods, so it is built after them.
+        foreach (var belt in AsteroidBelts)
+            Targets.Add(new BeltTargets(this, belt.Key, belt.Value.Data, settings, _time));
+
         // Cut 2 (docs/mining-cut.md): read after belts are built so a later ChunkExists/ChunkRadius call has
         // Planets/AsteroidBelts populated. A pack from before key 6 existed has ChunkWear == null (nullable
         // persistence rule); that reads as no wear rather than throwing.
@@ -481,6 +508,8 @@ public class Zone
         {
             var orbit = Orbits[orbitID];
             float2 pos = float2.zero;
+            // An infinite period (a parentless orbit, or one with no field to circle in) leaves the orbit at its phase:
+            // frac(t / inf) is 0.
             if (orbit.Period > .01f)
             {
                 var phase = (float) frac(_time / orbit.Period);
@@ -503,6 +532,67 @@ public class Zone
         return Orbits[orbitID].Position;
     }
 
+    // orbit -> the well-bearing body on it, and orbit -> the orbits whose parent it is. Derived from the pack in the
+    // constructor, read only by PeriodAround and Strength.
+    private readonly Dictionary<CultRecordKey, Planet> _wellOn = new Dictionary<CultRecordKey, Planet>();
+    private readonly Dictionary<CultRecordKey, List<CultRecordKey>> _childOrbits = new Dictionary<CultRecordKey, List<CultRecordKey>>();
+    private readonly Dictionary<CultRecordKey, float> _strengths = new Dictionary<CultRecordKey, float>();
+
+    // The one owner of a rail's period (Orbit.Period, AsteroidBelt.Periods): 2 pi sqrt(d / a), a being the pull the
+    // field gives at distance d from the node being orbited. Round a body, that is the body's own well through
+    // GravityField. Round a bodyless node it is the ring law: the strength of the node's orbits inside d, plus the
+    // ring term for the orbits on d itself (within RingWindow of it), taken as a point at the node. A bodied self
+    // feels half the strength of its ring partners (as a binary does); a strengthless self (a station, a turret)
+    // takes the period its bodied ring partners have, the mean of what each of them feels. No pull gives an
+    // infinite period.
+    private const float RingWindow = 0.1f;
+
+    private float PeriodAround(CultRecordKey parentOrbit, float distance, CultRecordKey self)
+    {
+        float a;
+        if (_wellOn.TryGetValue(parentOrbit, out var body))
+            a = length(GravityField.Acceleration(body.WellStrength, body.RocheLimit, float2(distance, 0)));
+        else
+        {
+            var inside = 0f;
+            var ring = new List<(CultRecordKey Orbit, float Strength)>();
+            if (_childOrbits.TryGetValue(parentOrbit, out var children))
+                foreach (var child in children)
+                {
+                    var childDistance = Orbits[child].Data.Distance;
+                    var childStrength = Strength(child);
+                    if (childDistance < distance - RingWindow) inside += childStrength;
+                    else if (childDistance <= distance + RingWindow && childStrength > 0) ring.Add((child, childStrength));
+                }
+            // Whose pull self feels: its own if it has strength, else each bodied member of the ring.
+            var ringSum = 0f;
+            foreach (var member in ring) ringSum += member.Strength;
+            var selfBodied = Strength(self) > 0;
+            var feelings = 0f;
+            var feelers = 0;
+            foreach (var member in ring)
+                if (!selfBodied || member.Orbit.Equals(self))
+                {
+                    feelings += (ringSum - member.Strength) / 2;
+                    feelers++;
+                }
+            var strength = inside + (feelers > 0 ? feelings / feelers : 0f);
+            a = length(GravityField.Acceleration(strength, 0f, float2(distance, 0)));
+        }
+        return a > 0 && distance > 0 ? 2 * PI * sqrt(distance / a) : float.PositiveInfinity;
+    }
+
+    // The well strength an orbit carries as a node: its own body's plus every orbit round it, recursively.
+    private float Strength(CultRecordKey orbit)
+    {
+        if (_strengths.TryGetValue(orbit, out var known)) return known;
+        var strength = _wellOn.TryGetValue(orbit, out var body) ? body.WellStrength : 0f;
+        if (_childOrbits.TryGetValue(orbit, out var children))
+            foreach (var child in children)
+                strength += Strength(child);
+        return _strengths[orbit] = strength;
+    }
+
     public float2 GetOrbitVelocity(CultRecordKey orbit)
     {
         if (Orbits.ContainsKey(orbit))
@@ -571,7 +661,7 @@ public class Zone
         var beltData = Planets[belt] as AsteroidBeltData;
         var parentPosition = GetOrbitPosition(Orbits[beltData.Orbit.Key].Data.Parent.Key);
         var size = ChunkRadius(new ChunkId(belt, index));
-        return AsteroidBelts[belt].Pose(index, _time, parentPosition, Settings, size);
+        return AsteroidBelts[belt].Pose(index, _time, parentPosition, size);
     }
 
     // Mining Cut 3 (docs/mining-cut-refresh.md): how bright a chunk is -- Reflector's rule (cross-section × the light
@@ -592,7 +682,7 @@ public class Zone
         var parentPosition = GetOrbitPosition(Orbits[beltData.Orbit.Key].Data.Parent.Key);
         var beltInstance = AsteroidBelts[belt];
         for (var i = 0; i < beltData.Asteroids.Length; i++)
-            into[i] = beltInstance.Pose(i, _time, parentPosition, Settings, ChunkRadius(new ChunkId(belt, i)));
+            into[i] = beltInstance.Pose(i, _time, parentPosition, ChunkRadius(new ChunkId(belt, i)));
     }
 
     public SecurityLevel GetSecurityLevel(float2 pos)
@@ -748,9 +838,14 @@ public class AsteroidBelt
     public AsteroidBeltData Data;
     public float Radius { get; }
 
+    // Each asteroid's orbital period, in seconds: derived once at zone build by Zone.PeriodAround (the field at the
+    // asteroid's own distance from the belt's parent), the only period Pose and BeltTargets divide by.
+    public float[] Periods { get; }
+
     public AsteroidBelt(AsteroidBeltData data)
     {
         Data = data;
+        Periods = new float[data.Asteroids.Length];
         Radius = data.Asteroids.Max(a => a.Distance);
     }
 
@@ -758,12 +853,11 @@ public class AsteroidBelt
     // The only place a chunk pose is computed; no per-tick copy survives it. Cut 2: size is no longer read from
     // per-belt storage -- Zone owns wear now, so the caller (Zone.ChunkPose/EvaluateBelt) computes it through
     // Zone.ChunkRadius and passes it in.
-    public float4 Pose(int index, double time, float2 parentPosition, PlanetSettings settings, float size)
+    public float4 Pose(int index, double time, float2 parentPosition, float size)
     {
         var asteroid = Data.Asteroids[index];
         var rot = (float) (time * asteroid.RotationSpeed % (PI * 2));
-        var pos = OrbitData.Evaluate((float) frac(time / settings.OrbitPeriod.Evaluate(asteroid.Distance) +
-                                                  asteroid.Phase)) * asteroid.Distance + parentPosition;
+        var pos = OrbitData.Evaluate((float) frac(time / Periods[index] + asteroid.Phase)) * asteroid.Distance + parentPosition;
         return float4(pos.x, pos.y, rot, size);
     }
 
@@ -845,11 +939,13 @@ public class Orbit
     public float2 Velocity = float2.zero;
     public float2 Position = float2.zero;
     public float2 PreviousPosition = float2.zero;
-    public float Period;
 
-    public Orbit(PlanetSettings settings, OrbitData data)
+    // Seconds per turn, written only by the Zone constructor (Zone.PeriodAround); infinite when the orbit has no
+    // field to circle in.
+    public float Period { get; internal set; }
+
+    public Orbit(OrbitData data)
     {
         Data = data;
-        Period = settings.OrbitPeriod.Evaluate(data.Distance);
     }
 }
diff --git a/Assets/Scripts/ServerShared/ZoneGenerator.cs b/Assets/Scripts/ServerShared/ZoneGenerator.cs
index 156deb803..56778a619 100644
--- a/Assets/Scripts/ServerShared/ZoneGenerator.cs
+++ b/Assets/Scripts/ServerShared/ZoneGenerator.cs
@@ -531,7 +531,6 @@ public class GeneratorPlanet
 	public float Distance;
 	public float Phase;
 	public float Mass;
-	//public float Period;
 	public float ChildDistanceMinimum;
 	public float ChildDistanceMaximum;
 	public bool Empty = false;
@@ -586,7 +585,6 @@ public class GeneratorPlanet
 				ChildDistanceMaximum = (i % 2 == 0 ? p0ChildDist : p1ChildDist)
 			};
 			child.ChildDistanceMinimum = Settings.PlanetSafetyRadius.Evaluate(child.Mass) * 2;
-			//child.Period = Settings..Evaluate(child.Distance);
 			Children.Add(child);
 		}
 
@@ -647,7 +645,6 @@ public class GeneratorPlanet
 					Distance = distances[i],
 					Phase = random.NextFloat()
 				};
-				//child.Period = Context.GlobalData.OrbitalPeriod(child.Distance);
 				child.ChildDistanceMinimum = Settings.PlanetSafetyRadius.Evaluate(child.Mass) * 2;
 				// Maximum child distance of child is the smallest distance to either of its neighbors
 				child.ChildDistanceMaximum = min(i == 0 ? child.Distance - ChildDistanceMinimum : child.Distance - distances[i - 1],
diff --git a/tests/Aetheria.Shared.Tests/GravitySubtractTests.cs b/tests/Aetheria.Shared.Tests/GravitySubtractTests.cs
index d901b5a39..2b329ad5d 100644
--- a/tests/Aetheria.Shared.Tests/GravitySubtractTests.cs
+++ b/tests/Aetheria.Shared.Tests/GravitySubtractTests.cs
@@ -45,8 +45,7 @@ public sealed class GravitySubtractTests : IDisposable
         WellStrength = new ExponentialCurve { Exponent = .2f, Multiplier = 45.08f },
         RocheLimit = new ExponentialCurve { Exponent = .25f, Multiplier = 7.32f },
         BodyRadius = new ExponentialCurve { Exponent = .25f, Multiplier = 3f },
-        LightRadius = new ExponentialCurve { Exponent = .25f, Multiplier = 300f },
-        OrbitPeriod = new ExponentialCurve()
+        LightRadius = new ExponentialCurve { Exponent = .25f, Multiplier = 300f }
     };
 
     private Zone Bare(float packRadius) => new Zone(_items, Planets(), new ZonePack { Radius = packRadius },
diff --git a/tests/Aetheria.Shared.Tests/MiningCut1Tests.cs b/tests/Aetheria.Shared.Tests/MiningCut1Tests.cs
index 1467ae316..cea5467e1 100644
--- a/tests/Aetheria.Shared.Tests/MiningCut1Tests.cs
+++ b/tests/Aetheria.Shared.Tests/MiningCut1Tests.cs
@@ -33,11 +33,17 @@ public sealed class MiningCut1Tests : IDisposable
 
     // Authored-shape curves, not defaults: a default ExponentialLerp/ExponentialCurve evaluates to a flat
     // constant everywhere, which would hide a broken formula (every asteroid or every damage state would read
-    // alike). These give period, size and hitpoints real, distinct curvature over the fixture's distance and
-    // damage range.
+    // alike). These give size and hitpoints real, distinct curvature over the fixture's damage range. A body's
+    // well strength is its mass here (c = mass), with a small constant core, so the test spells every period from
+    // the masses it chose.
+    private const float SunStrength = 400f;
+    private const float PlanetStrength = 25f;
+
     private static PlanetSettings Settings() => new PlanetSettings
     {
-        OrbitPeriod = new ExponentialCurve { Multiplier = 0.6f, Exponent = 1.3f, Constant = 40f },
+        BodyRadius = new ExponentialCurve(),
+        WellStrength = new ExponentialCurve { Multiplier = 1f, Exponent = 1f },
+        RocheLimit = new ExponentialCurve { Constant = 2f },
         AsteroidSize = new ExponentialLerp { Minimum = 2f, Maximum = 9f, Exponent = 1.7f },
         AsteroidHitpoints = new ExponentialLerp { Minimum = 10f, Maximum = 300f, Exponent = 2f },
         AsteroidRespawnTime = new ExponentialLerp { Minimum = 5f, Maximum = 60f, Exponent = 1f }
@@ -48,7 +54,7 @@ public sealed class MiningCut1Tests : IDisposable
     private const float RootPhase = 0.17f;
 
     // A belt of AsteroidCount asteroids at distinct distances, phases, size parameters and rotation speeds,
-    // orbiting a parent (RootDistance/RootPhase) that is itself moving -- so a test that reads the parent's
+    // orbiting a planet (RootDistance/RootPhase round a fixed sun) that is itself moving -- so a test that reads the parent's
     // motion through ChunkPose is exercising real recursion, not a fixed point. Each call opens its own
     // catalog, so two zones built from this fixture never share cache state.
     private (Zone zone, CultRecordKey beltKey, Asteroid[] asteroids, PlanetSettings settings) BuildFixture()
@@ -63,8 +69,12 @@ public sealed class MiningCut1Tests : IDisposable
         var items = new ItemManager(cache, new ProvenanceLedger(), GameSettings(), _ => { });
         var settings = Settings();
 
-        var rootOrbit = new OrbitData { Distance = RootDistance, Phase = RootPhase };
+        var sunOrbitKey = cache.Upsert(new OrbitData()).Key;
+        var sunKey = cache.Upsert(new PlanetData { Name = "Sun", Orbit = new CultRecordRef<OrbitData>(sunOrbitKey), Mass = SunStrength }).Key;
+
+        var rootOrbit = new OrbitData { Parent = new CultRecordRef<OrbitData>(sunOrbitKey), Distance = RootDistance, Phase = RootPhase };
         var rootKey = cache.Upsert(rootOrbit).Key;
+        var planetKey = cache.Upsert(new PlanetData { Name = "Planet", Orbit = new CultRecordRef<OrbitData>(rootKey), Mass = PlanetStrength }).Key;
 
         var beltOrbit = new OrbitData { Parent = new CultRecordRef<OrbitData>(rootKey), Distance = 900f, Phase = 0.42f };
         var beltOrbitKey = cache.Upsert(beltOrbit).Key;
@@ -86,8 +96,8 @@ public sealed class MiningCut1Tests : IDisposable
 
         var pack = new ZonePack
         {
-            Orbits = { new CultRecordRef<OrbitData>(rootKey), new CultRecordRef<OrbitData>(beltOrbitKey) },
-            Planets = { new CultRecordRef<BodyData>(beltKey) },
+            Orbits = { new CultRecordRef<OrbitData>(sunOrbitKey), new CultRecordRef<OrbitData>(rootKey), new CultRecordRef<OrbitData>(beltOrbitKey) },
+            Planets = { new CultRecordRef<BodyData>(sunKey), new CultRecordRef<BodyData>(planetKey), new CultRecordRef<BodyData>(beltKey) },
             Radius = 5000f,
             Mass = 10000f,
             Time = 0
@@ -97,22 +107,22 @@ public sealed class MiningCut1Tests : IDisposable
         return (zone, beltKey, asteroids, settings);
     }
 
-    // The formula of Zone.GetOrbitPosition (unchanged by this cut), reimplemented here so the expected value
-    // never calls production orbit code.
-    private static float2 OrbitPosition(double time, float distance, float phase, PlanetSettings settings, float2 fixedPosition = default)
+    // Zone.GetOrbitPosition's placement round a body of well strength `strength`, with the period spelled here from
+    // the field law outside the core (a = c/d, so T = 2 pi d / sqrt(c)), so the expected value never calls
+    // production orbit code.
+    private static float2 OrbitPosition(double time, float distance, float phase, float strength, float2 fixedPosition = default)
     {
-        var period = settings.OrbitPeriod.Evaluate(distance);
-        if (period <= .01f) return fixedPosition;
+        var period = 2 * PI * distance / sqrt(strength);
         var wrapped = (float) frac(time / period);
         return OrbitData.Evaluate(frac(wrapped + phase)) * distance + fixedPosition;
     }
 
-    // The formula of AsteroidBelt.Pose (Zone.cs:269-271 before this cut), reimplemented independently.
-    private static float4 ExpectedPose(double time, Asteroid asteroid, float2 parentPosition, PlanetSettings settings, float size)
+    // The pose of an asteroid circling a planet of well strength `strength`, spelled independently of AsteroidBelt.Pose.
+    private static float4 ExpectedPose(double time, Asteroid asteroid, float2 parentPosition, float strength, float size)
     {
         var rot = (float) (time * asteroid.RotationSpeed % (PI * 2));
-        var pos = OrbitData.Evaluate((float) frac(time / settings.OrbitPeriod.Evaluate(asteroid.Distance) + asteroid.Phase))
-                  * asteroid.Distance + parentPosition;
+        var period = 2 * PI * asteroid.Distance / sqrt(strength);
+        var pos = OrbitData.Evaluate((float) frac(time / period + asteroid.Phase)) * asteroid.Distance + parentPosition;
         return float4(pos.x, pos.y, rot, size);
     }
 
@@ -136,11 +146,11 @@ public sealed class MiningCut1Tests : IDisposable
             time += dt;
         }
 
-        var parentPosition = OrbitPosition(time, RootDistance, RootPhase, settings);
+        var parentPosition = OrbitPosition(time, RootDistance, RootPhase, SunStrength);
 
         for (var i = 0; i < asteroids.Length; i++)
         {
-            var expected = ExpectedPose(time, asteroids[i], parentPosition, settings, settings.AsteroidSize.Evaluate(asteroids[i].Size));
+            var expected = ExpectedPose(time, asteroids[i], parentPosition, PlanetStrength, settings.AsteroidSize.Evaluate(asteroids[i].Size));
             var actual = zone.ChunkPose(beltKey, i);
             AssertClose(expected.x, actual.x, 0.01f, $"asteroid {i} x");
             AssertClose(expected.y, actual.y, 0.01f, $"asteroid {i} y");
@@ -157,13 +167,13 @@ public sealed class MiningCut1Tests : IDisposable
     {
         var (zone, beltKey, asteroids, settings) = BuildFixture();
         zone.Update(40f);
-        var parentPosition = OrbitPosition(40.0, RootDistance, RootPhase, settings);
+        var parentPosition = OrbitPosition(40.0, RootDistance, RootPhase, SunStrength);
 
         var wrapped = 0;
         for (var i = 0; i < asteroids.Length; i++)
         {
             if (40.0 * asteroids[i].RotationSpeed > PI * 2) wrapped++;
-            var expected = ExpectedPose(40.0, asteroids[i], parentPosition, settings, settings.AsteroidSize.Evaluate(asteroids[i].Size));
+            var expected = ExpectedPose(40.0, asteroids[i], parentPosition, PlanetStrength, settings.AsteroidSize.Evaluate(asteroids[i].Size));
             var actual = zone.ChunkPose(beltKey, i);
             AssertClose(expected.z, actual.z, 0.001f, $"asteroid {i} rotation");
             Assert.InRange(actual.z, 0f, PI * 2);
diff --git a/tests/Aetheria.Shared.Tests/MiningCut2Tests.cs b/tests/Aetheria.Shared.Tests/MiningCut2Tests.cs
index 5ac5ecac8..c05fc1f32 100644
--- a/tests/Aetheria.Shared.Tests/MiningCut2Tests.cs
+++ b/tests/Aetheria.Shared.Tests/MiningCut2Tests.cs
@@ -40,7 +40,8 @@ public sealed class MiningCut2Tests : IDisposable
     // curve" is exercising real curvature, not a constant that any wrong formula would also satisfy.
     private static PlanetSettings Settings() => new PlanetSettings
     {
-        OrbitPeriod = new ExponentialCurve { Multiplier = 0.6f, Exponent = 1.3f, Constant = 40f },
+        WellStrength = new ExponentialCurve { Constant = 25f },
+        RocheLimit = new ExponentialCurve { Constant = 1f },
         AsteroidSize = new ExponentialLerp { Minimum = 2f, Maximum = 9f, Exponent = 1.7f },
         AsteroidHitpoints = new ExponentialLerp { Minimum = 20f, Maximum = 220f, Exponent = 1.6f },
         AsteroidRespawnTime = new ExponentialLerp { Minimum = 8f, Maximum = 96f, Exponent = 1.2f },
diff --git a/tests/Aetheria.Shared.Tests/MiningCut3Tests.cs b/tests/Aetheria.Shared.Tests/MiningCut3Tests.cs
index a9146613a..fdabaf6bd 100644
--- a/tests/Aetheria.Shared.Tests/MiningCut3Tests.cs
+++ b/tests/Aetheria.Shared.Tests/MiningCut3Tests.cs
@@ -47,11 +47,12 @@ public sealed partial class MiningCut3Tests : IDisposable
         FiringArc = 90f
     };
 
-    // Flat terrain (no zone depth, no gravity wells), one sun whose light reaches 1000 units, and the chunk
+    // Every sun carries a well (the belts circle in it, c = 9), one sun whose light reaches 1000 units, and the chunk
     // curves MiningCut2Tests uses, so sizes and wear vary over the fixture's range.
     private static PlanetSettings PlanetSettings(float lightRadius = 1000f) => new PlanetSettings
     {
-        OrbitPeriod = new ExponentialCurve { Multiplier = 0.6f, Exponent = 1.3f, Constant = 40f },
+        WellStrength = new ExponentialCurve { Constant = 9f },
+        RocheLimit = new ExponentialCurve { Constant = 1f },
         AsteroidSize = new ExponentialLerp { Minimum = 2f, Maximum = 9f, Exponent = 1.7f },
         AsteroidHitpoints = new ExponentialLerp { Minimum = 20f, Maximum = 220f, Exponent = 1.6f },
         AsteroidRespawnTime = new ExponentialLerp { Minimum = 8f, Maximum = 96f, Exponent = 1.2f },
diff --git a/tests/Aetheria.Shared.Tests/MiningIndexTests.cs b/tests/Aetheria.Shared.Tests/MiningIndexTests.cs
index e398700df..e47f9a4a1 100644
--- a/tests/Aetheria.Shared.Tests/MiningIndexTests.cs
+++ b/tests/Aetheria.Shared.Tests/MiningIndexTests.cs
@@ -458,7 +458,8 @@ public sealed partial class MiningCut3Tests
         var spread = Enumerable.Range(0, 700).Select(_ => Rock(50f + (float) rng.NextDouble() * 5950f, (float) rng.NextDouble(), (float) rng.NextDouble())).ToArray();
         var s = BuildSceneAt(centre, 20000f, 777.25, null, new[] { kind }, Belt(kind, spread), Belt(kind, Annulus(rng, 3000, 999f, 1001f)));
         var game = PlanetSettings(20000f);
-        game.OrbitPeriod = new ExponentialCurve { Multiplier = 1f, Exponent = 1f, Constant = 0f };
+        // The game's periods: T = 2 pi d / sqrt(c) with c = 4 pi^2 is T = d.
+        game.WellStrength = new ExponentialCurve { Constant = 4f * PI * PI };
         var searches = 0;
         foreach (var zone in new[] { s.Zone, new Zone(s.Items, game, s.Pack, new GalaxyZone { Name = "Game periods", Owner = null }, null) })
         foreach (var step in new[] { 0f, 13f, 1e4f, 3.3e7f })
diff --git a/tests/Aetheria.Shared.Tests/OrbitsFromFieldTests.cs b/tests/Aetheria.Shared.Tests/OrbitsFromFieldTests.cs
new file mode 100644
index 000000000..10de2c62e
--- /dev/null
+++ b/tests/Aetheria.Shared.Tests/OrbitsFromFieldTests.cs
@@ -0,0 +1,362 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using CultMath;
+using GameCult.Caching;
+using Xunit;
+using static CultMath.math;
+using float2 = CultMath.float2;
+using float3 = CultMath.float3;
+
+// Aetheria gravity, orbits-from-field: every rail's period is derived from the field it orbits in. Round a body it is
+// that body's own well; round a bodyless node it is the ring law (the strength inside the rail, plus half of what shares
+// it). The zones here are hand-built packs read by the production constructor, with the shipped well curves
+// (c = 45.08 m^.2, R = 7.32 m^.25). Each expected period is spelled here from the field law outside the core, T = 2 pi d / sqrt(c),
+// with c and R read from the zone's own bodies and every distance outside the core.
+public sealed class OrbitsFromFieldTests : IDisposable
+{
+    private readonly string _root = Path.Combine(Path.GetTempPath(), "aetheria-orbitsfield-" + Guid.NewGuid().ToString("N"));
+    private readonly List<CultCache> _caches = new List<CultCache>();
+
+    public OrbitsFromFieldTests() => Directory.CreateDirectory(_root);
+
+    public void Dispose()
+    {
+        foreach (var cache in _caches) cache.Dispose();
+        Directory.Delete(_root, true);
+    }
+
+    private static GameplaySettings GameSettings() => new GameplaySettings
+    {
+        DefaultEntitySettings = new EntitySettings(),
+        Tiers = new[] { new RarityTier { Name = "Common", Quality = .5f, Rarity = 0, Color = new float3(1, 1, 1) } },
+        QualityPriceModifier = new ExponentialLerp()
+    };
+
+    private static PlanetSettings Curves() => new PlanetSettings
+    {
+        WellStrength = new ExponentialCurve { Exponent = .2f, Multiplier = 45.08f },
+        RocheLimit = new ExponentialCurve { Exponent = .25f, Multiplier = 7.32f },
+        BodyRadius = new ExponentialCurve { Exponent = .25f, Multiplier = 3f },
+        LightRadius = new ExponentialCurve()
+    };
+
+    private sealed class World
+    {
+        public CultCache Cache;
+        public ItemManager Items;
+        public ZonePack Pack = new ZonePack { Radius = 20000f, Mass = 10000f };
+
+        public CultRecordKey Orbit(CultRecordKey parent, float distance, float phase = 0f)
+        {
+            var data = new OrbitData { Distance = distance, Phase = phase };
+            if (parent.IsSet()) data.Parent = new CultRecordRef<OrbitData>(parent);
+            var key = Cache.Upsert(data).Key;
+            Pack.Orbits.Add(new CultRecordRef<OrbitData>(key));
+            return key;
+        }
+
+        public CultRecordKey Body(CultRecordKey orbit, float mass)
+        {
+            var key = Cache.Upsert(new PlanetData { Name = "Body", Orbit = new CultRecordRef<OrbitData>(orbit), Mass = mass }).Key;
+            Pack.Planets.Add(new CultRecordRef<BodyData>(key));
+            return key;
+        }
+
+        public Zone Build() => new Zone(Items, Curves(), Pack, new GalaxyZone { Name = "OrbitsFromField", Owner = null }, null);
+    }
+
+    private World NewWorld()
+    {
+        var dir = Path.Combine(_root, $"world{_caches.Count}");
+        Directory.CreateDirectory(dir);
+        var cache = AetheriaStores.Open(Path.Combine(dir, "Aetheria.cc"), Path.Combine(dir, "run.cc"), catalogWritable: true);
+        _caches.Add(cache);
+        return new World { Cache = cache, Items = new ItemManager(cache, new ProvenanceLedger(), GameSettings(), _ => { }) };
+    }
+
+    private static float C(Zone zone, CultRecordKey body) => zone.PlanetInstances[body].WellStrength;
+    private static float Core(Zone zone, CultRecordKey body) => zone.PlanetInstances[body].RocheLimit;
+    private static float Outside(float distance, float c) => 2 * PI * distance / sqrt(c);
+
+    private static void AssertPeriod(float expected, float actual, string what) =>
+        Assert.True(Math.Abs(actual / expected - 1f) <= 1e-4f, $"{what}: expected {expected}, got {actual}");
+
+    [Fact]
+    public void AMoonsPeriodIsItsPlanetsFieldAlone()
+    {
+        var w = NewWorld();
+        var sunOrbit = w.Orbit(default, 0f);
+        var sun = w.Body(sunOrbit, 5000f);
+        var planetOrbit = w.Orbit(sunOrbit, 3000f);
+        var planet = w.Body(planetOrbit, 100f);
+        var moonOrbit = w.Orbit(planetOrbit, 150f);
+        w.Body(moonOrbit, 1f);
+        var zone = w.Build();
+
+        Assert.True(Core(zone, planet) < 150f && Core(zone, sun) < 3000f, "fixture: both rails run outside the core");
+        AssertPeriod(Outside(150f, C(zone, planet)), zone.Orbits[moonOrbit].Period, "the moon, in its planet's field");
+        AssertPeriod(Outside(3000f, C(zone, sun)), zone.Orbits[planetOrbit].Period, "the planet, in its sun's field");
+    }
+
+    [Fact]
+    public void AChildInsideItsParentsCoreOrbitsAsASolidBody()
+    {
+        var w = NewWorld();
+        var sunOrbit = w.Orbit(default, 0f);
+        w.Body(sunOrbit, 5000f);
+        var planetOrbit = w.Orbit(sunOrbit, 3000f);
+        var planet = w.Body(planetOrbit, 100f);
+        var coreRadius = Curves().RocheLimit.Evaluate(100f);
+        var near = w.Orbit(planetOrbit, coreRadius * .3f);
+        var far = w.Orbit(planetOrbit, coreRadius * .7f);
+        var zone = w.Build();
+
+        var solid = 2 * PI * coreRadius / sqrt(C(zone, planet)); // a = c d / R^2, so T = 2 pi R / sqrt(c) at every d inside
+        AssertPeriod(solid, zone.Orbits[near].Period, "the inner child");
+        AssertPeriod(solid, zone.Orbits[far].Period, "the outer child");
+    }
+
+    [Fact]
+    public void AnInnerSiblingDoesNotSpeedABodysRail()
+    {
+        var w = NewWorld();
+        var sunOrbit = w.Orbit(default, 0f);
+        var sun = w.Body(sunOrbit, 5000f);
+        var innerOrbit = w.Orbit(sunOrbit, 800f);
+        w.Body(innerOrbit, 20000f);
+        var outerOrbit = w.Orbit(sunOrbit, 3000f);
+        w.Body(outerOrbit, 100f);
+        var zone = w.Build();
+
+        AssertPeriod(Outside(3000f, C(zone, sun)), zone.Orbits[outerOrbit].Period, "the outer planet, with a heavy planet inside it");
+    }
+
+    [Fact]
+    public void ABinaryOrbitsUnderHalfItsPartnersPull()
+    {
+        var w = NewWorld();
+        var node = w.Orbit(default, 0f);
+        var a = w.Orbit(node, 400f, 0f);
+        var heavy = w.Orbit(node, 400f, .5f);
+        var bodyA = w.Body(a, 100f);
+        var bodyHeavy = w.Body(heavy, 2000f);
+        var zone = w.Build();
+
+        Assert.True(C(zone, bodyHeavy) > 1.5f * C(zone, bodyA), "fixture: unequal members");
+        AssertPeriod(Outside(400f, C(zone, bodyHeavy) / 2), zone.Orbits[a].Period, "the light member");
+        AssertPeriod(Outside(400f, C(zone, bodyA) / 2), zone.Orbits[heavy].Period, "the heavy member");
+    }
+
+    [Fact]
+    public void AnOuterBodyFeelsTheWholePairAndItsMoons()
+    {
+        var w = NewWorld();
+        var node = w.Orbit(default, 0f);
+        var m1 = w.Orbit(node, 400f, 0f);
+        var m2 = w.Orbit(node, 400f, .5f);
+        var moon = w.Orbit(m1, 40f);
+        var body1 = w.Body(m1, 100f);
+        var body2 = w.Body(m2, 300f);
+        var moonBody = w.Body(moon, 5f);
+        var outer = w.Orbit(node, 3000f);
+        w.Body(outer, 50f);
+        var zone = w.Build();
+
+        var whole = C(zone, body1) + C(zone, moonBody) + C(zone, body2);
+        AssertPeriod(Outside(3000f, whole), zone.Orbits[outer].Period, "the outer body, round the pair and the first one's moon");
+    }
+
+    [Fact]
+    public void ACompanionKeepsItsBodysPeriod()
+    {
+        // Under a body: a bodyless orbit at the planet's distance round the same sun.
+        var w = NewWorld();
+        var sunOrbit = w.Orbit(default, 0f);
+        w.Body(sunOrbit, 5000f);
+        var planetOrbit = w.Orbit(sunOrbit, 2000f, 0f);
+        w.Body(planetOrbit, 100f);
+        var companion = w.Orbit(sunOrbit, 2000f, 1f / 6f);
+        var zone = w.Build();
+        Assert.True(float.IsFinite(zone.Orbits[planetOrbit].Period));
+        Assert.Equal(zone.Orbits[planetOrbit].Period, zone.Orbits[companion].Period);
+
+        // Under a bodyless node: the same, round a node whose only field is an inner member's.
+        w = NewWorld();
+        var node = w.Orbit(default, 0f);
+        var innerOrbit = w.Orbit(node, 100f);
+        var inner = w.Body(innerOrbit, 3000f);
+        var planet2 = w.Orbit(node, 500f, 0f);
+        w.Body(planet2, 100f);
+        var companion2 = w.Orbit(node, 500f, 1f / 6f);
+        zone = w.Build();
+        AssertPeriod(Outside(500f, C(zone, inner)), zone.Orbits[planet2].Period, "the planet");
+        Assert.Equal(zone.Orbits[planet2].Period, zone.Orbits[companion2].Period);
+    }
+
+    // A station on a binary's ring, with nothing inside, feels what the pair's members feel (the mean of what each does),
+    // and so does one a hair off the ring's distance but within its window.
+    [Theory]
+    [InlineData(0f)]
+    [InlineData(.05f)]
+    [InlineData(-.05f)]
+    public void ACompanionOnABinarysRingTakesTheMembersPeriod(float offset)
+    {
+        var w = NewWorld();
+        var node = w.Orbit(default, 0f);
+        var light = w.Orbit(node, 400f, 0f);
+        var heavy = w.Orbit(node, 400f, .5f);
+        var lightBody = w.Body(light, 100f);
+        var heavyBody = w.Body(heavy, 2000f);
+        var companion = w.Orbit(node, 400f + offset, .25f);
+        var zone = w.Build();
+
+        Assert.True(C(zone, heavyBody) > 1.5f * C(zone, lightBody), "fixture: unequal members");
+        AssertPeriod(Outside(400f + offset, (C(zone, lightBody) + C(zone, heavyBody)) / 4), zone.Orbits[companion].Period,
+            "the companion, between its two members");
+
+        // An equal pair: the companion has exactly the members' period.
+        w = NewWorld();
+        node = w.Orbit(default, 0f);
+        var a = w.Orbit(node, 400f, 0f);
+        var b = w.Orbit(node, 400f, .5f);
+        w.Body(a, 300f);
+        w.Body(b, 300f);
+        companion = w.Orbit(node, 400f, .25f);
+        zone = w.Build();
+        Assert.True(float.IsFinite(zone.Orbits[a].Period));
+        Assert.Equal(zone.Orbits[a].Period, zone.Orbits[companion].Period);
+        Assert.Equal(zone.Orbits[b].Period, zone.Orbits[companion].Period);
+    }
+
+    // The ring is the 0.1 u window about a rail's distance: a member just inside it is a ring partner (pulls half), one just
+    // outside is inside or beyond the rail (pulls whole, or not at all).
+    [Theory]
+    [InlineData(-.09f, true)]
+    [InlineData(.09f, true)]
+    [InlineData(-.11f, false)]
+    public void TheRingWindowIsATenthOfAUnit(float partnerOffset, bool onRing)
+    {
+        var w = NewWorld();
+        var node = w.Orbit(default, 0f);
+        var rail = w.Orbit(node, 400f, 0f);
+        var partner = w.Orbit(node, 400f + partnerOffset, .5f);
+        w.Body(rail, 100f);
+        var partnerBody = w.Body(partner, 2000f);
+        var zone = w.Build();
+
+        var c = C(zone, partnerBody);
+        AssertPeriod(Outside(400f, onRing ? c / 2 : c), zone.Orbits[rail].Period, "the rail, with a partner at " + partnerOffset);
+    }
+
+    [Fact]
+    public void AMemberJustBeyondTheWindowIsOutsideTheRail()
+    {
+        var w = NewWorld();
+        var node = w.Orbit(default, 0f);
+        var rail = w.Orbit(node, 400f, 0f);
+        var beyond = w.Orbit(node, 400.11f, .5f);
+        w.Body(rail, 100f);
+        w.Body(beyond, 2000f);
+        var zone = w.Build();
+
+        Assert.True(float.IsPositiveInfinity(zone.Orbits[rail].Period), "a body beyond the rail does not pull it");
+    }
+
+    // A bodyless node's children read only that node's ring: the well of the body the node itself orbits does not enter.
+    [Fact]
+    public void ABodylessNodesChildrenNeverReadItsParentsWell()
+    {
+        var w = NewWorld();
+        var sunOrbit = w.Orbit(default, 0f);
+        w.Body(sunOrbit, 5000f);
+        var node = w.Orbit(sunOrbit, 3000f);
+        var alone = w.Orbit(node, 200f, 0f);
+        w.Body(alone, 100f);
+        var zone = w.Build();
+        Assert.True(float.IsPositiveInfinity(zone.Orbits[alone].Period), "a lone body on a bodyless node has no ring partner and no field");
+
+        w = NewWorld();
+        sunOrbit = w.Orbit(default, 0f);
+        w.Body(sunOrbit, 5000f);
+        node = w.Orbit(sunOrbit, 3000f);
+        var a = w.Orbit(node, 200f, 0f);
+        var b = w.Orbit(node, 200f, .5f);
+        var bodyA = w.Body(a, 100f);
+        var bodyB = w.Body(b, 300f);
+        zone = w.Build();
+        AssertPeriod(Outside(200f, C(zone, bodyB) / 2), zone.Orbits[a].Period, "the first member, under its partner alone");
+        AssertPeriod(Outside(200f, C(zone, bodyA) / 2), zone.Orbits[b].Period, "the second member, under its partner alone");
+    }
+
+    [Fact]
+    public void AnOrbitWithNoFieldStandsStill()
+    {
+        var w = NewWorld();
+        var node = w.Orbit(default, 0f);
+        var lonely = w.Orbit(node, 300f, .25f);
+        var fixedOne = w.Orbit(default, 50f, .25f);
+        var zone = w.Build();
+
+        Assert.True(float.IsPositiveInfinity(zone.Orbits[lonely].Period), "no field to circle in");
+        Assert.True(float.IsPositiveInfinity(zone.Orbits[fixedOne].Period), "a parentless orbit has nothing to circle");
+        var before = (zone.Orbits[lonely].Position, zone.Orbits[fixedOne].Position);
+        foreach (var dt in new[] { .5f, 3f, 70f, 5000f })
+            zone.Update(dt);
+        Assert.Equal(before, (zone.Orbits[lonely].Position, zone.Orbits[fixedOne].Position));
+        Assert.True(length(zone.Orbits[lonely].Velocity) == 0f && length(zone.Orbits[fixedOne].Velocity) == 0f);
+    }
+
+    // Generated zones, by the production generator: every orbit with a parent has a finite period and moves against its
+    // parent across an Update, and the seeds held a bodyless node with two members (a root rosette) and a pair on one ring.
+    [Fact]
+    public void EveryGeneratedOrbitMoves()
+    {
+        var rosettes = 0;
+        var binaries = 0;
+        for (uint seed = 1; seed <= 12; seed++)
+        {
+            var dir = Path.Combine(_root, $"galaxy{seed}");
+            Directory.CreateDirectory(dir);
+            var catalog = Path.Combine(dir, "Aetheria.cc");
+            File.Copy(TestCatalog.Repo, catalog);
+            var cache = new CultCache(TestCatalog.Registry());
+            _caches.Add(cache);
+            cache.AddBackingStore(new GameCult.Caching.MessagePack.SingleFileMessagePackBackingStore(catalog), AetheriaStores.CatalogTypes);
+            cache.AddBackingStore(new GameCult.Caching.MessagePack.SingleFileMessagePackBackingStore(Path.Combine(dir, "run.cc")), AetheriaStores.RunTypes);
+            var authored = AuthoredSettings.Load(RunStartTests.FindRepoRoot());
+            var tutorial = authored.Read<TutorialGenerationSettings>("TutorialGenerationSettings");
+            var galaxy = new Galaxy(tutorial, authored.Read<SectorBackgroundSettings>("TutorialBackgroundSettings"),
+                authored.Read<NameGeneratorSettings>("NameGeneratorSettings"), cache, new PlayerSettings(),
+                Directory.CreateDirectory(Path.Combine(dir, "Narrative")), _ => { }, null, seed);
+            var items = new ItemManager(cache, new ProvenanceLedger(), authored.Read<GameplaySettings>("GameplaySettings"), _ => { });
+            var pack = ZoneGenerator.GenerateZone(items, authored.Read<ZoneGenerationSettings>("ZoneSettings"), galaxy, galaxy.Entrance,
+                galaxy.IsPrelude, false);
+            var zone = new Zone(items, authored.Read<PlanetSettings>("PlanetSettings"), pack, galaxy.Entrance, galaxy);
+
+            var hosting = zone.Planets.Values.Select(body => body.Orbit.Key).ToHashSet();
+            foreach (var key in zone.Orbits.Keys.Where(k => !hosting.Contains(k)))
+            {
+                var members = zone.Orbits.Where(o => o.Value.Data.Parent.IsSet() && o.Value.Data.Parent.Key.Equals(key) && hosting.Contains(o.Key)).ToList();
+                if (members.Count >= 2) rosettes++;
+                if (members.Any(a => members.Any(b => !a.Key.Equals(b.Key) && Math.Abs(a.Value.Data.Distance - b.Value.Data.Distance) < .1f))) binaries++;
+            }
+
+            zone.Update(.02f);
+            var before = zone.Orbits.ToDictionary(o => o.Key, o => o.Value.Position - ParentAt(zone, o.Value));
+            zone.Update(.5f);
+            foreach (var orbit in zone.Orbits.Where(o => o.Value.Data.Parent.IsSet()))
+            {
+                Assert.True(float.IsFinite(orbit.Value.Period), $"seed {seed}: an orbit with a parent has no finite period");
+                Assert.True(lengthsq(orbit.Value.Position - ParentAt(zone, orbit.Value) - before[orbit.Key]) > 0f,
+                    $"seed {seed}: an orbit did not move against its parent (period {orbit.Value.Period}, distance {orbit.Value.Data.Distance})");
+            }
+        }
+        Assert.True(rosettes > 0, "the seeds hold a bodyless node with two bodied members");
+        Assert.True(binaries > 0, "the seeds hold two bodied orbits on one ring");
+    }
+
+    private static float2 ParentAt(Zone zone, Orbit orbit) =>
+        orbit.Data.Parent.IsSet() ? zone.Orbits[orbit.Data.Parent.Key].Position : orbit.Data.FixedPosition;
+}
diff --git a/tests/Aetheria.Shared.Tests/PotentialOwnerTests.cs b/tests/Aetheria.Shared.Tests/PotentialOwnerTests.cs
index bcb5310b5..3cd03747e 100644
--- a/tests/Aetheria.Shared.Tests/PotentialOwnerTests.cs
+++ b/tests/Aetheria.Shared.Tests/PotentialOwnerTests.cs
@@ -46,7 +46,7 @@ public sealed class PotentialOwnerTests : IDisposable
         WellStrength = new ExponentialCurve { Exponent = .2f, Multiplier = 45.08f },
         RocheLimit = new ExponentialCurve { Exponent = .25f, Multiplier = 7.32f },
         BodyRadius = new ExponentialCurve { Exponent = .25f, Multiplier = 2f },
-        LightRadius = new ExponentialCurve(), OrbitPeriod = new ExponentialCurve(),
+        LightRadius = new ExponentialCurve(),
         HeightExaggeration = exaggeration
     };
 
@@ -83,7 +83,7 @@ public sealed class PotentialOwnerTests : IDisposable
         var planets = new PlanetSettings
         {
             BodyRadius = new ExponentialCurve(),
-            LightRadius = new ExponentialCurve(), OrbitPeriod = new ExponentialCurve()
+            LightRadius = new ExponentialCurve()
         };
         var zone = ZoneOf(planets, (100f, float2.zero), (5000f, new float2(200f, 0f)));
         foreach (var p in new[] { float2.zero, float2(200f, 0f), float2(37f, -911f), float2(5000f, 5000f) })
@@ -138,7 +138,7 @@ public sealed class PotentialOwnerTests : IDisposable
             WellStrength = new ExponentialCurve { Constant = 100f },
             RocheLimit = new ExponentialCurve { Constant = roche },
             BodyRadius = new ExponentialCurve { Constant = 10f },
-            LightRadius = new ExponentialCurve(), OrbitPeriod = new ExponentialCurve()
+            LightRadius = new ExponentialCurve()
         };
         Assert.Throws<InvalidOperationException>(() => ZoneOf(Constants(0f), (100f, float2.zero)));
         Assert.Throws<InvalidOperationException>(() => ZoneOf(Constants(-5f), (100f, float2.zero)));
diff --git a/tests/Aetheria.Shared.Tests/WellFixture.cs b/tests/Aetheria.Shared.Tests/WellFixture.cs
index 54237862d..08bed1ad4 100644
--- a/tests/Aetheria.Shared.Tests/WellFixture.cs
+++ b/tests/Aetheria.Shared.Tests/WellFixture.cs
@@ -10,7 +10,6 @@ internal static class WellFixture
     public static void Dig(CultCache cache, ZonePack pack, PlanetSettings settings, float strength, float coreRadius, float2 at = default)
     {
         settings.BodyRadius = new ExponentialCurve();
-        settings.OrbitPeriod = new ExponentialCurve();
         settings.WellStrength = new ExponentialCurve { Constant = strength };
         settings.RocheLimit = new ExponentialCurve { Constant = coreRadius };
         var orbit = cache.Upsert(new OrbitData { FixedPosition = at }).Key;
diff --git a/tests/Aetheria.Shared.Tests/ZonePlacedAtBuildTests.cs b/tests/Aetheria.Shared.Tests/ZonePlacedAtBuildTests.cs
index e9673c3f5..f02992ff5 100644
--- a/tests/Aetheria.Shared.Tests/ZonePlacedAtBuildTests.cs
+++ b/tests/Aetheria.Shared.Tests/ZonePlacedAtBuildTests.cs
@@ -16,7 +16,7 @@ public sealed class ZonePlacedAtBuildTests : IDisposable
 {
     private static readonly float2 RootAt = new float2(300f, -200f);
     private const float Distance = 400f;
-    private const float Period = 40f;
+    private const float SunStrength = 100f; // the root's body: the planet circles in its field
 
     private readonly string _root = Path.Combine(Path.GetTempPath(), "aetheria-zoneplaced-" + Guid.NewGuid().ToString("N"));
     private readonly List<CultCache> _caches = new List<CultCache>();
@@ -38,13 +38,13 @@ public sealed class ZonePlacedAtBuildTests : IDisposable
 
     private static ExponentialCurve Flat(float v) => new ExponentialCurve { Constant = v };
 
-    // One planet with a well (c 100, Roche limit and core 20 u), a fixed root orbit at RootAt and the
-    // planet orbiting it at Distance from `phase`. Every orbit period is Period seconds.
+    // A sun on the fixed root orbit at RootAt, a planet orbiting it at Distance from `phase`, and a light moon body on
+    // the moon orbit so the moon's moon has a field to circle in. A body's well is 100 per unit of mass; every core
+    // (Roche limit) is 20 u.
     private static PlanetSettings Settings() => new PlanetSettings
     {
-        OrbitPeriod = Flat(Period),
         BodyRadius = Flat(20f),
-        WellStrength = Flat(100f),
+        WellStrength = new ExponentialCurve { Multiplier = 100f, Exponent = 1f },
         RocheLimit = Flat(20f),
         LightRadius = Flat(0f)
     };
@@ -86,10 +86,12 @@ public sealed class ZonePlacedAtBuildTests : IDisposable
         {
             Parent = new CultRecordRef<OrbitData>(moonOrbit), Distance = 40f
         }).Key;
+        var sun = cache.Upsert(new PlanetData { Orbit = new CultRecordRef<OrbitData>(rootOrbit), Mass = 1f }).Key;
         var planet = cache.Upsert(new PlanetData { Orbit = new CultRecordRef<OrbitData>(planetOrbit), Mass = 1f }).Key;
+        var moon = cache.Upsert(new PlanetData { Orbit = new CultRecordRef<OrbitData>(moonOrbit), Mass = .01f }).Key;
         var pack = new ZonePack
         {
-            Planets = { new CultRecordRef<BodyData>(planet) },
+            Planets = { new CultRecordRef<BodyData>(sun), new CultRecordRef<BodyData>(planet), new CultRecordRef<BodyData>(moon) },
             Radius = 5000f,
             Mass = 10000f,
             Time = time
@@ -154,7 +156,7 @@ public sealed class ZonePlacedAtBuildTests : IDisposable
     public void TheFirstTickVelocityIsTheOrbitsSpeed(bool moonFirst)
     {
         var s = Build(.35f, 0.0, moonFirst);
-        var expected = 2f * PI * Distance / Period;
+        var expected = sqrt(SunStrength); // outside the core T = 2 pi d / sqrt(c): the speed is sqrt(c), whatever d
         foreach (var dt in new[] { 1f / 60f, 1f / 30f, 1f / 60f })
         {
             s.Zone.Update(dt);