Radial Velocity

Synonyms

Doppler shift; Line-of-sight velocity

Definition

In astronomy, the radial velocity is the velocity of an object along the line of sight (the line connecting the observer to the object), i.e., along the radius vector to the object.

The radial velocity is usually measured using the observed Doppler shift of spectral lines, given by the formula Δλ/λ = v/c, where Δλ is the shift in wavelength observed for the object compared to the rest wavelength, λ, and vis the velocity of the object along the line of sight. The quantity c is the speed of light (299,792 km/s). From this formula, a positive velocity away from the observer results in a shift of the wavelength to larger values. This phenomenon is often called a redshift. Similarly, a negative velocity toward the observer causes the wavelength to shift toward smaller values, resulting in a blue shift. When the velocity is significant compared to the speed of light, a relativistic version of the formula should be used. Radial...