When the source of some wave is approaching you, the wave you received will be in a higher frequency (shorter wavelength) compared to the case when the source is stationary. If the source is leaving you, you will see lower-frequency wave (longer wavelength). This is called the Doppler e�ect. In astronomy, we will call the EM wave blue-shifted or red-shifted, because blue and red are respectively near the frequency of high and low ends of the visible spectrum. If the speed of the source is not large comparing with c, the ratio of the change in wavelength is given by: (observed wavelength - original wavelength) / original wavelength = v / c where v is the velocity of the source leaving you. By comparing the absorption lines from a star to those obtained in a laboratory, we can tell whether the star is moving away or towards us and how fast the star is moving.