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Radial Velocity ○◂｜Definition｜1st｜20251119205401-00-⌔
Radial velocity
The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity onto the relative direction or line-of-sight (LOS) connecting the two points.
The radial speed or range rate is the temporal rate of the distance or range between the two points. It is a signed scalar quantity, formulated as the scalar projection of the relative velocity vector onto the LOS direction. Equivalently, radial speed equals the norm of the radial velocity, modulo the sign.1
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).
Printed 2026-06-28.
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Link to original Footnotes
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. ↩
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