Doppler Effect (1842)

Doppler suggested that it would be possible to determine the radial component of a star's velocity by observing the shift in its color. For double star systems the star approaching us should be shifted to the blue whereas the star receding from us should be shifted to the red. The typical velocities involved would not allow one to detect this by the naked eye. It required the discovery of discrete stellar spectral lines and their corresponding earthly counterparts to observe this effect.