Doppler Effect is the apparent variation is frequency of any emitted wave as the source of the wave approaches or moves away, relative to an observer. The principle explains why, if a source of sound is moving toward an observer, the sound seems higher in pitch. The lines in the spectrum of a star are similarly shifted toward the violet or red if the distances between the star and the earth is changing.

Red Shift is the shift toward longer wavelengths observed in the special lines of astronomical objects. American astronomer Edwin Powell Hubble theorized that the red shift observed in the spectra of galaxies is caused by the Doppler effect and indicates the speed of recession of, and the distances of, the galaxies. A second mechanism for the red shift, predicted by the general theory of relativity, is the gravitational red shift, in which periodic processes are slowed down in a gravitational field.