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# Encyclopedia

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). The light of an object with a substantial radial velocity will be subject to Doppler effect, so the frequency of the light decreases for receding objects (redshift) and increases for approaching objects (blueshift).

The radial velocity of a star or other luminous but distant objects can be measured accurately by taking a high-resolution spectrum and comparing the measured wavelengths of known spectral lines to wavelengths from laboratory measurements. By convention, a positive radial velocity indicates the object is receding; if the sign is negative, then the object is approaching.

For the figure on the right, the radial velocity of the moon is 0 because the distance between the moon and Earth does not change.

In many binary stars, the orbital motion usually causes radial velocity variations of several kilometers per second. As the spectra of these stars vary due to the Doppler effect, they are called spectroscopic binaries. Radial velocity studies can be used to estimate the masses of the stars, and some orbital elements, such as eccentricity and semimajor axis. The same method has also been used to detect planets around stars, in the way that the movement's measurement determines the planet's orbital period, while the resulting size of the displacement allows the calculation of the lower bound on a planet's mass. Radial velocity methods alone may only reveal a lower bound, since a large planet orbiting at a very high angle to the line of sight will perturb its star radially as much as a much smaller planet with an orbital plane on the line of sight. It has been suggested that planets with high eccentricities calculated by this method may be mimicking 2 planet systems of circular or near-circular resonant orbit. [1]