Rossiter-McLaughlin Effect
As a planet transits, it blocks portions of the rotating stellar disk.
The star's approaching limb is blue-shifted, the receding limb red-shifted.
The RV anomaly depends on
which part of the disk is blocked,
revealing the
projected spin-orbit angle λ.
⚠ Ambiguities:
• We measure λ (projected), not true 3D obliquity ψ
• Stellar inclination i★ often unknown
• High-b transits have weaker constraints
• λ and (−λ) are degenerate for b=0