A cos²(Ecliptic Longitude) Anomaly in Gaia DR3 Astrometry

We report a 20σ systematic modulation of the astrometric reduced chi-squared in Gaia DR3 as a function of ecliptic longitude. For 500,000 well-measured stars (parallax/error > 20, RUWE < 1.4, G < 15, |β| < 30°), the reduced chi-squared follows χ²ν = A + B·cos²(λ), with fractional amplitude B/A = 7.8%. The signal survives controlling for observation count (t = 20.0 with N_obs covariate), is phase-locked to the ecliptic (zero crossing at 60° shift), and strengthens toward the ecliptic plane (9.3% at |β| < 5°, 5.9% at |β| ≥ 20°). Random-phase controls (N = 100) produce |t| < 2.9. The amplitude was predicted a priori from a geometric projection model calibrated on the Moon's orbital inclination oscillation (δ/φ = 1.815°/30° = 6.05%); the high-latitude subset agrees to 3%. This deposit includes the complete reproducibility package: LaTeX source, the 500,000-star dataset (CSV), the Gaia ADQL query, the Python analysis script (requires only NumPy), and the full analysis output. To reproduce all results: download Star_Data.csv and run_gaia_test.py, then run python run_gaia_test.py