What motion component does the Doppler measurement technique primarily fail to register on its own?
Answer
Transverse velocity
The Doppler effect, whether causing redshift or blueshift, only measures motion directly toward or away from the observer—the radial velocity. It provides no information regarding movement across the line of sight, which is termed transverse velocity or proper motion. A star moving rapidly perpendicular to our view would have a radial velocity measurement of zero, even if its actual speed through space is substantial, necessitating a second observational method to find its total velocity.
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