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Radial velocity (RV) is the component of velocity of a body on a line between the body and the observer, i.e., the movement toward or away from the observer. (The line between observer and body is a radius of the sphere surrounding the observer at the distance of the body.) The radial velocity of an astronomical object can be determined by measuring the Doppler shift of known spectral lines (yielding the spectral radial velocity), a measurement that can be carried out at any distance as long as the source is sufficiently bright. This offers a relatively easy clue to the peculiar velocity of the object, and within the Milky Way offers clues to the orbits of stars around the galactic center, and to distance (kinematic distance).
Radial velocity of stars can also be used to identify the presence of unseen binary-star companions or extra-solar planets, the radial velocity method (aka RV method) of exoplanet detection: their orbit can produce a detectable pattern of variation in the radial-velocity component of the host star. The RV method is helpful for detecting planets with small (a day or few days) or medium (1-2 AU) orbits, but transits reveal more of the smaller orbits. Using the RV method requires high resolution spectrography (HRS), which requires a degree of brightness, an apparent magnitude of about +12 or brighter. Currently, such methods measure RV differences down to about a meter/second precision. (1/10 meter/second would be required to detect Earth from another star.) Observatories with instruments designed to do this are sometimes referred to as RV observatories (radial velocity observatories).
Radial velocity represents one component of a body's velocity relative to the observer, covering one of the three spatial dimensions. A two-dimensional velocity covering the other two dimensions (at right angles to the line of sight) is termed transverse velocity or tangential velocity. For nearby stars, proper motion offers a clue to the transverse velocity, which can be pinned down if the distance is known, e.g., using parallax. Transverse velocity does have a relativistic effect on Doppler shift (stemming from time dilation), which can significantly affect RV measurements if the magnitude of the transverse velocity is far greater than the radial velocity.
The term velocimetry is sometimes used for measurement of velocity, and radial velocimetry for the RV measurement.