Though the pitch of a train whistle is constant, the whistle seems higher when the train steams toward the station and lower when it steams away. A 38-year-old Austrian physicist named Christian Doppler explained this phenomenon, now known as the Doppler Effect, in 1842.

According to Doppler, the sound waves in front of an approaching source bunch closer together than those in front of a stationary source, thus causing the listener to receive a larger number of waves (and a higher pitch). The waves in front of a source that is moving away spread further apart, which causes the listener to receive a smaller number of waves (and a lower pitch). The theory also applies to light, radio and water waves.

In its greatest application, the Doppler Effect has enabled scientists to determine the motion and velocity of a star by studying the light that the star emits; the light is closer toward the red end of the spectrum (lower frequency) if the star is moving away from Earth, and closer toward the violet end (higher frequency) if it is moving toward Earth. The American astronomer Edwin Hubble thereby discovered in 1929 that galaxies are moving away from Earth at speeds that increase with their distance from Earth, giving rise to the popular theory that the universe is expanding.

PostScript: Closer to home, the Doppler Effect has been used to determine the speed of passing cars, the location of airplanes and missiles, and the rhythms of the human heart.