Radial Velocity

Radial velocity is the velocity of an object in the direction of the line of sight (i.e., its speed straight towards or away from an observer). In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity. The spectroscopic radial velocity is the radial component of the velocity of the source at emission and the observer at observation, as determined by spectroscopy. Astrometric radial velocity is the radial velocity as determined by astrometric observations (for example, a secular change in the annual parallax).