Editing Radial velocity (section)

{{Short description|Velocity of an object as the rate of distance change between the object and a point}}
{{distinguish-redirect|Radial speed|Circular motion{{!}}radial motion}}
[[File:Radialgeschwindigkeit.gif|thumb|A plane flying past a radar station: the plane's velocity vector (red) is the sum of the radial velocity (green) and the tangential velocity (blue).]]

The '''radial velocity''' or '''line-of-sight velocity''' of a target with respect to an observer is the [[temporal rate of change|rate of change]] of the [[vector quantity|vector]] [[displacement (geometry)|displacement]] between the two points. It is formulated as the [[vector projection]] of the target-observer [[relative velocity]] onto the [[relative direction (geometry)|relative direction]] or [[Line of sight|line-of-sight]] (LOS) connecting the two points. 

The '''radial speed''' or '''range rate''' is the [[temporal rate]] of the [[Euclidean distance|distance]] or [[Slant range|range]] between the two points. It is a [[Sign (mathematics)|signed]] [[Scalar (mathematics)|scalar quantity]], formulated as the [[scalar projection]] of the relative velocity vector onto the LOS direction. Equivalently, radial speed equals the [[vector norm|norm]] of the radial velocity, [[Modulo (mathematics)|modulo]] the sign.{{efn|The norm, a nonnegative number, is multiplied by -1 if velocity (red arrow in the figure) and relative position form an [[obtuse angle]] or if relative velocity (green arrow) and relative position are antiparallel.}}

In astronomy, the point is usually taken to be the observer on Earth, so the radial velocity then denotes the speed with which the object moves away from the Earth (or approaches it, for a negative radial velocity).