Placemarker.
Early in this century, astronomers noticed that distant galaxies had peculiar light spectra. Specifically, the galaxies' light spectra were shifted toward the red end of the spectrum. In 1929 astronomer Edwin Hubble compared the galaxies' spectra with their distances, calculated using different methods, and showed that the amount of "red shift" was proportional to distance. Hubble and others realized that the most obvious explanation for the "red shift" was that the galaxies were receding from Earth and each other, and the farther the galaxy, the faster the recession.
If an astronomical object is moving away from the Earth, its light will be shifted to longer (red) wavelengths.
If the same object were moving towards the Earth, its light would be shifted to shorter (blue) wavelengths.