Radial velocity (RV) measurements of a star with an orbiting companion are only sensitive to the star's relative line-of-sight motion. As a result, there is a direct relationship between the RV semi-amplitude K of a body with mass m and the apparent inclination i of its orbit that gives rise to the RV method's inability to determine a companion's true mass, but rather only its minimum mass msini.
As a planet transits, it blocks a portion of its host's starlight. Most stars rotate in a measurable way, meaning that at any time, half of the star's projected surface area is Doppler shifted (blue) towards Earth, while the other half is Doppler shifted away (red) from Earth. As a result, a transiting planet may block out portions of the star's rotation-induced Doppler shift in its spectrum as observed from Earth. Known as the Rossiter-McLaughlin (RM) effect, this produces a relative RV shift that depends on a variety of factors, including the relative spin-orbit angle of the planet with respect to the star. This is a simple animation illustrating the RM effect for a transiting exoplanet that is well-aligned to the rotation of its host star.
A time lapse of a lunar eclipse as seen from the light-polluted skies of Philadelphia.
