Add Body.EARTH with Luna moon and switch Body indexing to 1-based

CHANGELOG.md

@@ -5,6 +5,18 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.3.1] - 2026-04-22
+
+### Added
+
+- `Body.EARTH` enum value with `rotation_hours = 23.934`, `orbital_hours = 8766.0`, discovery epoch 1543-01-01 (Copernicus), contact epoch 1969-07-20 (Apollo 11)
+- `Luna` moon (Earth's Moon): tidally locked, `rotation_hours = orbital_hours = 655.72`, discovery epoch 1609-11-01 (Galileo), contact epoch 1969-07-20 (Apollo 11)
+- `LUNA` constant exported from `moon.py`
+
+### Changed
+
+- `Body.__getitem__` now uses 1-based indexing — `Body.MARS[1]` returns Phobos, `Body.MARS[2]` returns Deimos
+
 ## [1.3.0] - 2026-04-21
 
 ### Added