The Doppler effect is the apparent difference between the frequency at which sound or light waves leave a source and that at which they reach an observer, caused by relative motion of the observer and the wave source. This phenomenon is used in astronomical measurements and in radar and modern navigation. It was first described (1842) by Austrian physicist Christian Doppler.
The following is an example of the Doppler effect: as one approaches a blowing horn, the perceived pitch is higher until the horn is reached and then becomes lower as the horn is passed. Similarly, the light from a star, observed from the Earth, shifts toward the red end of the spectrum (lower frequency or longer wavelength) if the Earth and star are receding from each other and toward the violet (higher frequency or shorter wavelength) if they are approaching each other. The Doppler effect is used in studying the motion of stars and to search for double stars and is an integral part of modern theories of the universe.
Excerpt from the Encyclopedia Britannica without permission.