The change in frequency or wavelength of light from a celestial object due to its motion relative to the observer, appearing as a shift toward blue (approaching) or red (receding) wavelengths. This effect enables measurement of stellar velocities and cosmic expansion.
Named after Austrian physicist Christian Doppler, who first described the effect in 1842. Initially proposed for sound waves, the principle was later applied to light waves and became fundamental to astronomical observations and cosmology.
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