Final answer:
A spectroscope is the instrument used to measure the red shift by sorting and analyzing light according to its wavelength.
Step-by-step explanation:
The instrument used to measure the red shift is a spectroscope. Astronomers use spectroscopes to sort and analyze light according to its wavelength. Spectroscopes can separate the light into its full rainbow of colors, allowing astronomers to measure individual lines in the spectrum and determine the red shift of celestial objects.