An Etymological Dictionary of Astronomy and Astrophysics

A shift in the lines of an object’s spectrum toward longer wavelengths. Redshift indicates that an object is moving away from the observer. The larger the redshift, the faster the object is moving. Redshift is expressed by z = Δλ/λ = v/c, where λ is the wavelength, Δλ the wavelength shift, v the velocity of the source relative to the observer, and c the → speed of light. When v approaches c, redshift is expressed by the → relativistic formula

z = ((1 +v/c)/(1 - v/c))^&frac12 - 1.

See also: → red; → shift.