Doppler shift

A Doppler shift is the change in position of spectral lines due to the Doppler effect; named after Christian Doppler. If an object is approaching, its light is compressed and any lines in its spectrum appear at shorter wavelengths than if the object were at rest (see blue shift). If the object is receding, the opposite effect occurs and the spectral lines are shifted toward the longer wavelength or red end of the spectrum (see red shift).