Spectral line shift

The spectral line shift or Doppler-Fizeau effect describes the change in wavelength and frequency of light from a source due to its relative motion, which impacts the study of motion and velocity in astronomy. The red shift indicates objects in space receding from the viewer, while the blue shift suggests an approaching motion.
 Armand-Hippolyte-Louis Fizeau France 1848 Spectral line shift

The spectral line shift or Doppler-Fizeau effect describes the change in wavelength and frequency of light from a source due to its relative motion, which impacts the study of motion and velocity in astronomy. The red shift indicates objects in space receding from the viewer, while the blue shift suggests an approaching motion.