What name is given to velocity along an observer's line of sight?

What name is given to velocity along an observer’s line of sight? It is called radial velocity. More precisely, radial velocity is the component of an object’s relative velocity directed toward or away from the observer. It is the projection of the full velocity vector onto the line connecting the object and observer.

An object can move quickly while having little radial velocity if most of its motion is sideways. That sideways component is tangential velocity. A plane passing across a radar station, for example, has both radial and tangential components, and only the radial part directly changes its distance from the station.

Radial velocity is central to the Doppler effect. An approaching object produces a higher observed frequency, while a receding object produces a lower one. Astronomers measure stellar radial velocity by tracking shifts in known spectral lines: motion toward Earth causes blueshift, and motion away causes redshift.

The term also underpins exoplanet searches. An orbiting planet makes its star wobble around a shared center of mass, producing a repeating radial-velocity signal. The method can reveal an orbital period and usually a minimum planetary mass, because the system’s tilt is not always known.