• Reduction in the wavelength of radiation caused by the observer and source approaching each other. This manifestation of the Doppler effect is seen far less often than the redshift in astronomy, since the latter is widely caused by the expansion of the universe. Blue and red shifts are used together in astronomy for measuring rotations of planets and stars, since the blueshift is observed at the limb approaching the observer and the redshift at the limb receding from it.

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