Doppler's Effect

The illustration shows Doppler's effect which is the change of frequency of a signal when the source of the signal moves either away from us or toward us. Drag the slider to change the radial velocity, i.e. the velocity along the direction from our viewing position to the source. You will see the spectrum change as the spectrum's lines shift away from their "standard" positions. By observing and measuring the changes in the spectrum of a star we can measure its radial velocity.