Encyclopedia of Astrobiology

2015 Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James (Jim) CleavesII, Daniele L. Pinti, José Cernicharo Quintanilla, Daniel Rouan, Tilman Spohn, Stéphane Tirard, Michel Viso

Radial Velocity

  • David W. LathamEmail author
Reference work entry
DOI: https://doi.org/10.1007/978-3-662-44185-5_1331



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...

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© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.Harvard-Smithsonian Center for AstrophysicsCambridgeUSA