The increase in wavelength, towards the red end of the visible-light spectrum, of electromagnetic radiation emitted from a body that is receding from an observer, the extent of the shift being proportional to the speed of recession. Radiation from an approaching body is shifted in the opposite direction, towards the shorter, blue end of the spectrum. The phenomenon was first proposed in 1842 by the Austrian physicist Christian Doppler, and verified in 1845 by the Dutch meteorologist C. H. D. Buys Ballot, who observed the changing pitch of the sound made by musicians playing on an open car on a train as it approached, passed, and receded from a stationary trackside observer. It occurs because each wave crest in the radiation from the moving body commences farther from or closer to the observer and so takes a slightly longer or shorter time to arrive. Thus, successive crests arrive farther apart or closer together, causing the wavelength to increase or decrease, and the pitch of a sound to fall or rise or the wavelength of light to shift towards red or blue.