A blueshift is any decrease in wavelength (increase in frequency); the opposite effect is referred to as redshift. In visible light, this shifts the color from the red end of the spectrum to the blue end. The term also applies when photons outside the visible spectrum (e.g. x-rays and radio waves) are shifted toward shorter wavelengths, as well as...
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http://en.wikipedia.org/wiki/Blueshift
A shift in the lines of an object's spectrum toward the blue end. Blueshift indicates that an object is moving toward the observer. The larger the blueshift, the faster the object is moving.
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http://www.seasky.org/astronomy/astronomy-glossary.html
a shift toward shorter wavelengths of the spectral lines of a celestial object, caused by the motion of the object toward the observer.
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https://www.infoplease.com/dictionary/blueshift
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