Christian Doppler described in 1842 the change in perceived wave frequency when source and observer move relative to each other. Initially formulated for the light of binary stars, the principle was experimentally confirmed for sound in 1845 and generalized to any wave type. In astronomy, the Doppler shift of spectral lines allows measuring the radial velocity of celestial objects, a technique Mayor and Queloz used in 1995 to detect 51 Pegasi b, the first confirmed exoplanet around a solar-type star.