Update documentation for eccentric orbit support

Update README with new 12-field config format and examples showing both
circular and eccentric orbits. Update config_assumptions.md to reflect
fixed issues and new capabilities.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

README.md

@@ -5,6 +5,7 @@ A 3D orbital mechanics simulation using a 2-body gravitational model with sphere
 ## Features
 
 - **2-body gravitational physics** with Euler integration
+- **Eccentric orbit support** - circular and elliptical orbits using vis-viva equation
 - **Sphere of influence (SOI)** transitions between gravitational parents
 - **3D real-time visualization** using raylib
 - **Configurable star systems** via simple text files
@@ -99,22 +100,29 @@ Configuration files define celestial bodies in a simple text format:
 
 ```
 # Comments start with #
-# Format: name mass(kg) radius(m) x(m) y(m) z(m) parent_index r g b
+# Format: name mass(kg) radius(m) x(m) y(m) z(m) parent_index r g b eccentricity semi_major_axis(m)
 
-Sun 1.989e30 6.96e8 0 0 0 -1 1.0 1.0 0.0
-Earth 5.972e24 6.371e6 1.496e11 0 0 0 0.0 0.5 1.0
-Moon 7.342e22 1.737e6 1.500e11 0 0 1 0.7 0.7 0.7
+# Sun at origin
+Sun 1.989e30 6.96e8 0 0 0 -1 1.0 1.0 0.0 0 0
+
+# Earth - circular orbit (e=0)
+Earth 5.972e24 6.371e6 1.496e11 0 0 0 0.0 0.5 1.0 0 1.496e11
+
+# Comet - eccentric orbit (e=0.7)
+Comet 1e14 5e3 1.122e11 0 0 0 0.5 0.8 1.0 0.7 3.74e11
 ```
 
 Fields:
 - **name**: Body name (string, no spaces)
 - **mass**: Mass in kilograms
 - **radius**: Radius in meters
-- **x, y, z**: Initial position in meters
+- **x, y, z**: Initial position in meters (absolute coordinates)
 - **parent_index**: Index of gravitational parent (-1 for root bodies like stars)
 - **r, g, b**: RGB color values (0.0 to 1.0)
+- **eccentricity**: Orbital eccentricity (0 = circular, >0 = elliptical)
+- **semi_major_axis**: Semi-major axis in meters (for e=0, use orbital radius)
 
-Velocities are calculated automatically for circular orbits.
+Velocities are calculated automatically using the vis-viva equation based on position and orbital parameters.
 
 ## Technical Details
 

docs/config_assumptions.md

@@ -1,11 +1,16 @@
 # Configuration File Assumptions and Issues
 
+## Recent Updates:
+- ✅ **FIXED**: Eccentric orbit support added via eccentricity and semi_major_axis parameters
+- ✅ **FIXED**: Binary star barycenter calculation
+- ⚠️ **TODO**: Moon and Jupiter's moons positions still incorrect in solar_system.txt
+
 ## Key Assumptions Found:
 
 ### **1. Format Assumptions (from config_loader.cpp)**
-- Line buffer limited to 256 characters (config_loader.cpp:30)
-- Name limited to 64 characters (config_loader.cpp:31)
-- All 10 fields must be present: `name mass radius x y z parent_index r g b` (config_loader.cpp:19)
+- Line buffer limited to 256 characters (config_loader.cpp:39)
+- Name limited to 64 characters (config_loader.cpp:40)
+- All 12 fields must be present: `name mass radius x y z parent_index r g b eccentricity semi_major_axis` (config_loader.cpp:16-20)
 - File format is space-delimited
 
 ### **2. Solar System Configuration Issues:**
@@ -36,12 +41,15 @@ The moon positions appear to use **absolute coordinates** matching their orbital
 **Problem 2: Planet positions**
 - PlanetA1 is at `(3.5e11, 0, 0)` while StarA is at `(3.0e11, 0, 0)` - only 50 million km apart
 - PlanetB1 is at `(-4.2e11, 0, 0)` while StarB is at `(-3.7e11, 0, 0)` - only 50 million km apart
-- These seem reasonable as absolute positions, but the velocity calculation assumes circular orbits
-
-### **4. Velocity Calculation Assumptions (config_loader.cpp:63-118):**
-- All orbits are assumed to be **circular**
-- Orbits are calculated in the XY plane perpendicular to Z-axis (config_loader.cpp:92)
-- Root bodies (parent_index = -1) get zero velocity
+- These seem reasonable as absolute positions (now supports both circular and eccentric orbits)
+
+### **4. Velocity Calculation Assumptions (config_loader.cpp:78-245):**
+- ~~All orbits are assumed to be **circular**~~ - **FIXED**: Now supports both circular (e=0) and eccentric (e>0) orbits
+- Velocities calculated using **vis-viva equation**: `v = sqrt(G*M*(2/r - 1/a))`
+  - For circular orbits: e=0, a=orbital_radius
+  - For eccentric orbits: e>0, a=semi_major_axis
+- Orbits are calculated in the XY plane perpendicular to Z-axis (config_loader.cpp:219-226)
+- Root bodies (parent_index = -1) get zero velocity (or orbit barycenter if multiple roots)
 - Velocities are calculated relative to parent, then parent's velocity is added
 
 ### **5. Critical Assumption:**