Add moon orbit test suite and fix solar system config

- Created tests/test_moon_orbits.cpp with 4 test cases:
  * Moon orbital stability around Earth
  * Galilean moons orbital stability around Jupiter
  * Titan orbital stability around Saturn
  * Combined solar system parent stability

- Enhanced test_utilities.h/cpp:
  * Added min_time_days field to OrbitTracker
  * Added create_orbit_tracker_with_min_time() for short-period orbits

- Fixed parent indexes in tests/configs/solar_system.toml:
  * Moon: parent 3 (Mars) → 2 (Earth)
  * Io/Europa/Ganymede/Callisto: parent 5 (Saturn) → 4 (Jupiter)
  * Titan: parent 6 (Uranus) → 5 (Saturn)

- Updated semi_major_axis for moons to actual orbital distances

Tests reveal need for hierarchical orbit physics implementation.
Net change: +368 lines

Makefile

@@ -81,6 +81,8 @@ test-build:
 		-c $(TEST_DIR)/test_orbital_period.cpp -o build/test_orbital_period.o
 	$(CXX) $(CXXFLAGS) -I/usr/include/catch2 \
 		-c $(TEST_DIR)/test_comet_orbit.cpp -o build/test_comet_orbit.o
+	$(CXX) $(CXXFLAGS) -I/usr/include/catch2 \
+		-c $(TEST_DIR)/test_moon_orbits.cpp -o build/test_moon_orbits.o
 	$(CXX) $(CXXFLAGS) \
 		-c $(SRC_DIR)/test_utilities.cpp -o build/test_utilities.o
 	$(CXX) $(CXXFLAGS) \
@@ -91,7 +93,7 @@ test-build:
 		-c $(SRC_DIR)/config_loader.cpp -o build/test_config_loader.o
 	$(CC) -Wall -Wextra -g -ggdb3 -std=c99 -I./src -I./ext/tomlc17/src \
 		-c $(SRC_DIR)/../ext/tomlc17/src/tomlc17.c -o build/test_tomlc17.o
-	$(CXX) build/test_main.o build/test_integration.o build/test_energy.o build/test_orbital_period.o build/test_comet_orbit.o build/test_utilities.o build/test_physics.o build/test_simulation.o build/test_config_loader.o build/test_tomlc17.o \
+	$(CXX) build/test_main.o build/test_integration.o build/test_energy.o build/test_orbital_period.o build/test_comet_orbit.o build/test_moon_orbits.o build/test_utilities.o build/test_physics.o build/test_simulation.o build/test_config_loader.o build/test_tomlc17.o \
 		-o $(TEST_TARGET) -lCatch2Main -lCatch2 -lm
 
 # Run automated test suite

src/test_utilities.cpp

@@ -53,6 +53,19 @@ OrbitTracker* create_orbit_tracker(int body_index) {
     tracker->quadrant_transitions = 0;
     tracker->orbit_completed = false;
     tracker->time_at_completion = 0.0;
+    tracker->min_time_days = 100.0;
+    return tracker;
+}
+
+OrbitTracker* create_orbit_tracker_with_min_time(int body_index, double min_time_days) {
+    OrbitTracker* tracker = (OrbitTracker*)malloc(sizeof(OrbitTracker));
+    tracker->body_index = body_index;
+    tracker->initial_angle = 0.0;
+    tracker->previous_angle = 0.0;
+    tracker->quadrant_transitions = 0;
+    tracker->orbit_completed = false;
+    tracker->time_at_completion = 0.0;
+    tracker->min_time_days = min_time_days;
     return tracker;
 }
 
@@ -93,10 +106,10 @@ void update_orbit_tracker(OrbitTracker* tracker, CelestialBody* body, CelestialB
     if (total_rotation > M_PI) total_rotation -= 2.0 * M_PI;
 
     const double SECONDS_PER_DAY = 86400.0;
-    const double MIN_ORBIT_TIME = 100.0 * SECONDS_PER_DAY;
+    double min_time_seconds = tracker->min_time_days * SECONDS_PER_DAY;
 
     if (tracker->quadrant_transitions >= 2 &&
-        current_time > MIN_ORBIT_TIME &&
+        current_time > min_time_seconds &&
         fabs(total_rotation) < 0.05) {
         tracker->orbit_completed = true;
         tracker->time_at_completion = current_time;

src/test_utilities.h

@@ -20,6 +20,7 @@ struct OrbitTracker {
     bool orbit_completed;
     double time_at_completion;
     int body_index;
+    double min_time_days;
 };
 
 double calculate_kinetic_energy(CelestialBody* body);
@@ -28,6 +29,7 @@ double calculate_system_total_energy(SimulationState* sim);
 OrbitalMetrics calculate_orbital_metrics(CelestialBody* body, CelestialBody* parent);
 
 OrbitTracker* create_orbit_tracker(int body_index);
+OrbitTracker* create_orbit_tracker_with_min_time(int body_index, double min_time_days);
 void reset_orbit_tracker(OrbitTracker* tracker);
 void update_orbit_tracker(OrbitTracker* tracker, CelestialBody* body, CelestialBody* parent, double current_time);
 void destroy_orbit_tracker(OrbitTracker* tracker);

tests/configs/solar_system.toml

@@ -0,0 +1,141 @@
+# Solar System Configuration
+
+[[bodies]]
+name = "Sun"
+mass = 1.989e30        # kg
+radius = 6.96e8        # m
+position = { x = 0.0, y = 0.0, z = 0.0 }
+parent_index = -1
+color = { r = 1.0, g = 1.0, b = 0.0 }
+eccentricity = 0.0
+semi_major_axis = 0.0
+
+[[bodies]]
+name = "Venus"
+mass = 4.867e24
+radius = 6.0518e6
+position = { x = 1.082e11, y = 0.0, z = 0.0 }
+parent_index = 0
+color = { r = 0.9, g = 0.7, b = 0.3 }
+eccentricity = 0.0
+semi_major_axis = 1.082e11
+
+[[bodies]]
+name = "Earth"
+mass = 5.972e24
+radius = 6.371e6
+position = { x = 1.496e11, y = 0.0, z = 0.0 }
+parent_index = 0
+color = { r = 0.0, g = 0.5, b = 1.0 }
+eccentricity = 0.0
+semi_major_axis = 1.496e11
+
+[[bodies]]
+name = "Mars"
+mass = 6.39e23
+radius = 3.3895e6
+position = { x = 2.279e11, y = 0.0, z = 0.0 }
+parent_index = 0
+color = { r = 0.8, g = 0.3, b = 0.1 }
+eccentricity = 0.0
+semi_major_axis = 2.279e11
+
+[[bodies]]
+name = "Jupiter"
+mass = 1.898e27
+radius = 6.9911e7
+position = { x = 7.785e11, y = 0.0, z = 0.0 }
+parent_index = 0
+color = { r = 0.9, g = 0.7, b = 0.5 }
+eccentricity = 0.0
+semi_major_axis = 7.785e11
+
+[[bodies]]
+name = "Saturn"
+mass = 5.683e26
+radius = 5.8232e7
+position = { x = 1.434e12, y = 0.0, z = 0.0 }
+parent_index = 0
+color = { r = 0.9, g = 0.8, b = 0.6 }
+eccentricity = 0.0
+semi_major_axis = 1.434e12
+
+[[bodies]]
+name = "Uranus"
+mass = 8.681e25
+radius = 2.5362e7
+position = { x = 2.871e12, y = 0.0, z = 0.0 }
+parent_index = 0
+color = { r = 0.5, g = 0.8, b = 0.9 }
+eccentricity = 0.0
+semi_major_axis = 2.871e12
+
+[[bodies]]
+name = "Neptune"
+mass = 1.024e26
+radius = 2.4622e7
+position = { x = 4.495e12, y = 0.0, z = 0.0 }
+parent_index = 0
+color = { r = 0.2, g = 0.4, b = 0.9 }
+eccentricity = 0.0
+semi_major_axis = 4.495e12
+
+ [[bodies]]
+ name = "Moon"
+ mass = 7.342e22
+ radius = 1.737e6
+ position = { x = 1.500e11, y = 0.0, z = 0.0 }
+ parent_index = 2
+ color = { r = 0.7, g = 0.7, b = 0.7 }
+ eccentricity = 0.0
+ semi_major_axis = 3.844e8
+
+ [[bodies]]
+ name = "Io"
+ mass = 8.93e22
+ radius = 1.822e6
+ position = { x = 8.207e11, y = 0.0, z = 0.0 }
+ parent_index = 4
+ color = { r = 0.9, g = 0.9, b = 0.3 }
+ eccentricity = 0.0
+ semi_major_axis = 4.217e8
+
+ [[bodies]]
+ name = "Europa"
+ mass = 4.80e22
+ radius = 1.561e6
+ position = { x = 8.456e11, y = 0.0, z = 0.0 }
+ parent_index = 4
+ color = { r = 0.8, g = 0.8, b = 0.7 }
+ eccentricity = 0.0
+ semi_major_axis = 6.709e8
+
+ [[bodies]]
+ name = "Ganymede"
+ mass = 1.48e23
+ radius = 2.634e6
+ position = { x = 8.853e11, y = 0.0, z = 0.0 }
+ parent_index = 4
+ color = { r = 0.6, g = 0.6, b = 0.5 }
+ eccentricity = 0.0
+ semi_major_axis = 1.070e9
+
+ [[bodies]]
+ name = "Callisto"
+ mass = 1.08e23
+ radius = 2.410e6
+ position = { x = 9.670e11, y = 0.0, z = 0.0 }
+ parent_index = 4
+ color = { r = 0.5, g = 0.5, b = 0.4 }
+ eccentricity = 0.0
+ semi_major_axis = 1.883e9
+
+ [[bodies]]
+ name = "Titan"
+ mass = 1.345e23
+ radius = 2.575e6
+ position = { x = 1.556e12, y = 0.0, z = 0.0 }
+ parent_index = 5
+ color = { r = 0.9, g = 0.6, b = 0.3 }
+ eccentricity = 0.0
+ semi_major_axis = 1.222e9

tests/test_moon_orbits.cpp

@@ -0,0 +1,264 @@
+#include <catch2/catch_test_macros.hpp>
+#include "../src/physics.h"
+#include "../src/simulation.h"
+#include "../src/config_loader.h"
+#include "../src/test_utilities.h"
+#include <cmath>
+#include <vector>
+
+TEST_CASE("Moon orbital stability around Earth", "[moon][earth]") {
+    const double TIME_STEP = 60.0;
+    const double EXPECTED_PERIOD_DAYS = 27.3;
+    const double SECONDS_PER_DAY = 86400.0;
+    const double MAX_SIMULATION_DAYS = 35.0;
+    const double MOON_DISTANCE_FROM_EARTH = 384400000.0;
+
+    SimulationState* sim = create_simulation(20, TIME_STEP);
+
+    REQUIRE(load_system_config(sim, "tests/configs/solar_system.toml"));
+
+    const int EARTH_INDEX = 2;
+    const int MOON_INDEX = 8;
+
+    double max_time = MAX_SIMULATION_DAYS * SECONDS_PER_DAY;
+    OrbitTracker* tracker = create_orbit_tracker_with_min_time(MOON_INDEX, 5.0);
+
+    int initial_parent = sim->bodies[MOON_INDEX].parent_index;
+    Vec3 initial_pos_relative_to_earth = vec3_sub(
+        sim->bodies[MOON_INDEX].position,
+        sim->bodies[EARTH_INDEX].position
+    );
+    double initial_distance = vec3_magnitude(initial_pos_relative_to_earth);
+
+    INFO("Moon initial distance from Earth: " << initial_distance << " m");
+    INFO("Expected distance: " << MOON_DISTANCE_FROM_EARTH << " m");
+
+    while (sim->time < max_time && !tracker->orbit_completed) {
+        update_simulation(sim);
+
+        int current_parent = sim->bodies[MOON_INDEX].parent_index;
+
+        REQUIRE(current_parent == initial_parent);
+
+        if (current_parent != EARTH_INDEX) {
+            INFO("Moon parent changed from " << initial_parent << " to " << current_parent);
+            REQUIRE(current_parent == EARTH_INDEX);
+        }
+
+        Vec3 current_pos_relative_to_earth = vec3_sub(
+            sim->bodies[MOON_INDEX].position,
+            sim->bodies[EARTH_INDEX].position
+        );
+        double current_distance = vec3_magnitude(current_pos_relative_to_earth);
+
+        double distance_drift = fabs(current_distance - initial_distance);
+        double drift_percentage = (distance_drift / initial_distance) * 100.0;
+
+        REQUIRE(drift_percentage < 20.0);
+
+        update_orbit_tracker(tracker, &sim->bodies[MOON_INDEX], &sim->bodies[EARTH_INDEX], sim->time);
+    }
+
+    REQUIRE(tracker->orbit_completed);
+
+    double measured_period_days = tracker->time_at_completion / SECONDS_PER_DAY;
+    double period_error_days = fabs(measured_period_days - EXPECTED_PERIOD_DAYS);
+
+    INFO("Expected Moon period: " << EXPECTED_PERIOD_DAYS << " days");
+    INFO("Measured Moon period: " << measured_period_days << " days");
+    INFO("Period error: " << period_error_days << " days");
+
+    REQUIRE(period_error_days < 3.0);
+
+    Vec3 final_pos_relative_to_earth = vec3_sub(
+        sim->bodies[MOON_INDEX].position,
+        sim->bodies[EARTH_INDEX].position
+    );
+    double final_distance = vec3_magnitude(final_pos_relative_to_earth);
+
+    double final_drift_percentage = (fabs(final_distance - initial_distance) / initial_distance) * 100.0;
+
+    INFO("Final distance from Earth: " << final_distance << " m");
+    INFO("Initial distance from Earth: " << initial_distance << " m");
+    INFO("Final drift: " << final_drift_percentage << "%");
+
+    REQUIRE(final_drift_percentage < 10.0);
+
+    destroy_orbit_tracker(tracker);
+    destroy_simulation(sim);
+}
+
+TEST_CASE("Galilean moons orbital stability around Jupiter", "[moon][jupiter]") {
+    const double TIME_STEP = 60.0;
+    const double SECONDS_PER_DAY = 86400.0;
+    const double MAX_SIMULATION_DAYS = 20.0;
+
+    const double IO_PERIOD_DAYS = 1.77;
+    const double EUROPA_PERIOD_DAYS = 3.55;
+    const double GANYMEDE_PERIOD_DAYS = 7.15;
+    const double CALLISTO_PERIOD_DAYS = 16.69;
+
+    SimulationState* sim = create_simulation(20, TIME_STEP);
+
+    REQUIRE(load_system_config(sim, "tests/configs/solar_system.toml"));
+
+    const int JUPITER_INDEX = 4;
+    const int IO_INDEX = 9;
+    const int EUROPA_INDEX = 10;
+    const int GANYMEDE_INDEX = 11;
+    const int CALLISTO_INDEX = 12;
+
+    OrbitTracker* io_tracker = create_orbit_tracker_with_min_time(IO_INDEX, 1.0);
+    OrbitTracker* europa_tracker = create_orbit_tracker_with_min_time(EUROPA_INDEX, 2.0);
+    OrbitTracker* ganymede_tracker = create_orbit_tracker_with_min_time(GANYMEDE_INDEX, 5.0);
+    OrbitTracker* callisto_tracker = create_orbit_tracker_with_min_time(CALLISTO_INDEX, 10.0);
+
+    double max_time = MAX_SIMULATION_DAYS * SECONDS_PER_DAY;
+
+    while (sim->time < max_time) {
+        update_simulation(sim);
+
+        REQUIRE(sim->bodies[IO_INDEX].parent_index == JUPITER_INDEX);
+        REQUIRE(sim->bodies[EUROPA_INDEX].parent_index == JUPITER_INDEX);
+        REQUIRE(sim->bodies[GANYMEDE_INDEX].parent_index == JUPITER_INDEX);
+        REQUIRE(sim->bodies[CALLISTO_INDEX].parent_index == JUPITER_INDEX);
+
+        update_orbit_tracker(io_tracker, &sim->bodies[IO_INDEX], &sim->bodies[JUPITER_INDEX], sim->time);
+        update_orbit_tracker(europa_tracker, &sim->bodies[EUROPA_INDEX], &sim->bodies[JUPITER_INDEX], sim->time);
+        update_orbit_tracker(ganymede_tracker, &sim->bodies[GANYMEDE_INDEX], &sim->bodies[JUPITER_INDEX], sim->time);
+        update_orbit_tracker(callisto_tracker, &sim->bodies[CALLISTO_INDEX], &sim->bodies[JUPITER_INDEX], sim->time);
+    }
+
+    REQUIRE(io_tracker->orbit_completed);
+    REQUIRE(europa_tracker->orbit_completed);
+    REQUIRE(ganymede_tracker->orbit_completed);
+    REQUIRE(callisto_tracker->orbit_completed);
+
+    double io_period_days = io_tracker->time_at_completion / SECONDS_PER_DAY;
+    double europa_period_days = europa_tracker->time_at_completion / SECONDS_PER_DAY;
+    double ganymede_period_days = ganymede_tracker->time_at_completion / SECONDS_PER_DAY;
+    double callisto_period_days = callisto_tracker->time_at_completion / SECONDS_PER_DAY;
+
+    INFO("Io period: " << io_period_days << " days (expected: " << IO_PERIOD_DAYS << ")");
+    INFO("Europa period: " << europa_period_days << " days (expected: " << EUROPA_PERIOD_DAYS << ")");
+    INFO("Ganymede period: " << ganymede_period_days << " days (expected: " << GANYMEDE_PERIOD_DAYS << ")");
+    INFO("Callisto period: " << callisto_period_days << " days (expected: " << CALLISTO_PERIOD_DAYS << ")");
+
+    REQUIRE(fabs(io_period_days - IO_PERIOD_DAYS) < 0.5);
+    REQUIRE(fabs(europa_period_days - EUROPA_PERIOD_DAYS) < 1.0);
+    REQUIRE(fabs(ganymede_period_days - GANYMEDE_PERIOD_DAYS) < 2.0);
+    REQUIRE(fabs(callisto_period_days - CALLISTO_PERIOD_DAYS) < 4.0);
+
+    destroy_orbit_tracker(io_tracker);
+    destroy_orbit_tracker(europa_tracker);
+    destroy_orbit_tracker(ganymede_tracker);
+    destroy_orbit_tracker(callisto_tracker);
+    destroy_simulation(sim);
+}
+
+TEST_CASE("Titan orbital stability around Saturn", "[moon][saturn]") {
+    const double TIME_STEP = 60.0;
+    const double EXPECTED_PERIOD_DAYS = 15.95;
+    const double SECONDS_PER_DAY = 86400.0;
+    const double MAX_SIMULATION_DAYS = 25.0;
+
+    SimulationState* sim = create_simulation(20, TIME_STEP);
+
+    REQUIRE(load_system_config(sim, "tests/configs/solar_system.toml"));
+
+    const int SATURN_INDEX = 5;
+    const int TITAN_INDEX = 13;
+
+    OrbitTracker* tracker = create_orbit_tracker_with_min_time(TITAN_INDEX, 10.0);
+
+    Vec3 initial_pos_relative_to_saturn = vec3_sub(
+        sim->bodies[TITAN_INDEX].position,
+        sim->bodies[SATURN_INDEX].position
+    );
+    double initial_distance = vec3_magnitude(initial_pos_relative_to_saturn);
+
+    double max_time = MAX_SIMULATION_DAYS * SECONDS_PER_DAY;
+
+    while (sim->time < max_time && !tracker->orbit_completed) {
+        update_simulation(sim);
+
+        REQUIRE(sim->bodies[TITAN_INDEX].parent_index == SATURN_INDEX);
+
+        Vec3 current_pos_relative_to_saturn = vec3_sub(
+            sim->bodies[TITAN_INDEX].position,
+            sim->bodies[SATURN_INDEX].position
+        );
+        double current_distance = vec3_magnitude(current_pos_relative_to_saturn);
+
+        double drift_percentage = (fabs(current_distance - initial_distance) / initial_distance) * 100.0;
+        REQUIRE(drift_percentage < 10.0);
+
+        update_orbit_tracker(tracker, &sim->bodies[TITAN_INDEX], &sim->bodies[SATURN_INDEX], sim->time);
+    }
+
+    REQUIRE(tracker->orbit_completed);
+
+    double measured_period_days = tracker->time_at_completion / SECONDS_PER_DAY;
+    double period_error_days = fabs(measured_period_days - EXPECTED_PERIOD_DAYS);
+
+    INFO("Expected Titan period: " << EXPECTED_PERIOD_DAYS << " days");
+    INFO("Measured Titan period: " << measured_period_days << " days");
+    INFO("Period error: " << period_error_days << " days");
+
+    REQUIRE(period_error_days < 3.0);
+
+    destroy_orbit_tracker(tracker);
+    destroy_simulation(sim);
+}
+
+TEST_CASE("Combined solar system with all moons - parent stability", "[moon][integration]") {
+    const double TIME_STEP = 60.0;
+    const double SECONDS_PER_DAY = 86400.0;
+    const double MAX_SIMULATION_DAYS = 60.0;
+
+    SimulationState* sim = create_simulation(20, TIME_STEP);
+
+    REQUIRE(load_system_config(sim, "tests/configs/solar_system.toml"));
+
+    struct ParentChange {
+        double time_days;
+        int body_index;
+        int old_parent;
+        int new_parent;
+    };
+
+    std::vector<ParentChange> parent_changes;
+
+    int initial_parents[14];
+    for (int i = 0; i < 14; i++) {
+        initial_parents[i] = sim->bodies[i].parent_index;
+    }
+
+    double max_time = MAX_SIMULATION_DAYS * SECONDS_PER_DAY;
+
+    while (sim->time < max_time) {
+        update_simulation(sim);
+
+        for (int i = 0; i < sim->body_count; i++) {
+            if (sim->bodies[i].parent_index != initial_parents[i]) {
+                ParentChange change;
+                change.time_days = sim->time / SECONDS_PER_DAY;
+                change.body_index = i;
+                change.old_parent = initial_parents[i];
+                change.new_parent = sim->bodies[i].parent_index;
+                parent_changes.push_back(change);
+                initial_parents[i] = sim->bodies[i].parent_index;
+            }
+        }
+    }
+
+    INFO("Total parent changes detected: " << parent_changes.size());
+    for (const auto& change : parent_changes) {
+        INFO("Body " << sim->bodies[change.body_index].name
+             << " (index " << change.body_index << ") changed parent "
+             << "from " << change.old_parent << " to " << change.new_parent
+             << " at day " << change.time_days);
+    }
+
+    destroy_simulation(sim);
+}