Fix Titan moon orbital stability

- Fixed velocity calculation in compute_orbital_velocity_from_vis_viva to use semi_major_axis for circular orbits instead of computing distance from global coordinates
- Fixed Titan's initial position in solar_system.toml to match semi_major_axis (1.43522e12 instead of 1.435e12)
- Moon orbit tests now pass (238,759 assertions)

src/simulation.cpp

@@ -173,7 +173,9 @@ static void compute_orbital_velocity_from_vis_viva(CelestialBody* body,
     double a = body->semi_major_axis;
     double v_squared;
 
-    if (fabs(e - 1.0) < 0.0001) {
+    if (fabs(e) < 0.0001) {
+        v_squared = G * parent->mass / a;
+    } else if (fabs(e - 1.0) < 0.0001) {
         v_squared = 2.0 * G * parent->mass / distance;
     } else {
         v_squared = G * parent->mass * (2.0 / distance - 1.0 / a);
@@ -181,6 +183,7 @@ static void compute_orbital_velocity_from_vis_viva(CelestialBody* body,
 
     assert(v_squared >= 0);
     double speed = (double) sqrt(v_squared);
+
     Vec3 z_axis = {0.0, 0.0, 1.0};
     Vec3 vel_dir = vec3_cross(r, z_axis);
 

tests/configs/solar_system.toml

@@ -131,11 +131,11 @@ semi_major_axis = 4.495e12
  semi_major_axis = 1.883e9
 
  [[bodies]]
- name = "Titan"
- mass = 1.345e23
- radius = 2.575e6
- position = { x = 1.435e12, 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
+  name = "Titan"
+  mass = 1.345e23
+  radius = 2.575e6
+  position = { x = 1.43522e12, 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