What astronomical discoveries have resulted from the Doppler effect?

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In astronomy, the Doppler effect for light is used to measure the velocity (and indirectly distance) and rotation of stars and galaxies along the direction of sight. In the spectrum of nearly every star there are wavelengths, characteristic of atoms, that lie near but not quite coincident to the same wavelengths as measured in the laboratory. The small deviations or shifts are generally due to the relative motion of the celestial object and the earth. Both blue shifts and red shifts are observed for various objects, indicating relative motion both toward and away from the earth. Such shifts have been used to measure the orbital velocity of the earth, to detect binary stars and variable stars , and to detect rotation of other galaxies. The Doppler effect is responsible for the red shifts of distant galaxies, and also of quasars , and thus provides the best evidence for the expansion of the universe, as described by Hubble's law . In addition to observations of visible light, the Doppler effect for radio waves is utilized by astronomers to determine the velocities of dust clouds in the spiral arms of the Milky Way galaxy. These observations provided the first direct proof that our own galaxy is rotating. The Doppler shift in radar pulses reflected from the surfaces of Venus and Mercury have been analyzed to obtain new values for their periods of rotation about their axes.