The decrease in wavelength of electromagnetic radiation from a source moving toward the observer, causing the light to appear shifted toward the blue end of the spectrum. This is the opposite of redshift and indicates approach rather than recession.
Coined in the mid-20th century as the counterpart to 'redshift,' combining 'blue' (representing shorter wavelengths) with 'shift.' The term emerged as astronomers developed more sophisticated spectroscopic techniques to measure stellar motions.
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