The Rossiter–McLaughlin effect is an astrophysical phenomenon observed during transits of exoplanets, specifically when a planet passes in front of its host star from our viewpoint. This effect occurs because the planet partially blocks the star's light and causes a distortion in the Doppler shift of the star's light as seen from Earth. When a planet transits, it blocks different parts of the star's surface as it moves across it.
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A transiting body hides different velocity regions of a rotating stellar disc and causes an anomalous measured Doppler shift. Its time dependence constrains the sky-projected spin-orbit angle, which is not generally the full stellar obliquity.