When an object that is emitting waves, such as sound waves, is moving, the wavelength ahead of the object is shortened, relative to its actual frequency. You might say that the forward motion crushes the waves together ; at the same time, relatively, the waves behind the moving source are stretched out. This change in wave pattern is what is meant by the Doppler effect, or the Doppler shift, and is familiar from the altering sound of a race car or police siren as it passes.
Austrian physicist Christian Doppler(1803-53) first described the effect in relation to astronomy in On the Colored Light of the Binary Stars and Some Other Stars of the Heavens(1842). Doppler was determined, after enduring an at times humiliating journey through academia, to demonstrate his ability to apply his genius to natural phenomena. He named the effect while seeking to explain differences in the colors of double stars. his principle was swiftly demonstrated in relation to any type of wave -- sound, light, water -- emanating from a moving source. It also applies to an observer moving relative to the medium through which the wave is transmitted, and to situations in which the medium itself is the method of transmission.
The Doppler effect now helps with predictions in such diverse fields as meteorology, navigation, and medical diagnosis. As Doppler himself predicted, as soon as instruments developed sufficiency to take measurements, scientists could determine the directional motion of stars. This alerted them to "the red shift," whereby light from a star, as observed from the Earth, shifts toward the red end of the spectrum (with a lower frequency or longer wavelength) if the Earth and star are receding from each other. In other words, they discovered that we are in an expanding universe. It follows that things were once very much closer. The Doppler effect thus contributed to a seismic shift in universal understanding.
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