The Doppler Effect was proposed by Austrian physicist Christian Doppler in 1842. The theory was tested in 1845 by Dutch meteorologist C.H.D. Buys Ballot. Ballot's findings were supported by Scottish scientist John Scott Russell in 1848.
The Doppler Effect describes the difference in frequency between when light or sound leaves its source and when it reaches the observer. An example of this occurs when a train horn approaches. The sound of the horn changes pitch as the train gets closer. Similarly, the color of stars change in relation to their position with the Earth.The Doppler Effect is used in astronomy, radar and navigation.