Christian Doppler was the Austrian physicist who discovered the effect that bears his name. Doppler's 'principle', which he first mentions in a paper in 1842, explains the change in frequency observed as a source of waves approaches or recedes from an observer (for example, a stationary person on a station platform hears the pitch of the sound coming from a train rise as it approaches and then fall as the train moves away). The 'principle' was verified experimentally by using an open wagon full of playing trumpeters. These were pulled to and fro in front of a number of other musicians who were able to detect the changes in pitch, confirming Doppler's equations. Doppler also suggested that his 'principle' could be applied to any type of wave motion, such as light, and in 1848 Armand Fizeau showed that shifts in the spectral lines of stars could be observed and ascribed to the Doppler effect. This effect was first used in 1868, when William Huggins found that Sirius (the dog star) was moving away from our solar system. More recently, the Doppler effect has become particularly important in astronomy, because it has enabled scientists to provide evidence that the Universe is expanding the spectra of distant galaxies are generally red-shifted because they are receding from the Milky Way.
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