I am currently capturing and analyzing spectra. My analysis software includes allowances for Doppler shifts resulting from relative radial motion between source and observer, and also Earth’s rotation. Results are accurate . The basic introduction to the Doppler effect generally starts with waves in water with a static and then a moving wave generator which very clearly shows wave length changes. Sound waves – similar demonstrations with sirens etc,. – also very clear effect. When it comes to light I have a ‘blind spot’. Using the example of an Hα photon (say 6563 Å) emitted from a hydrogen atom (keep it simple - nearby star, ignoring Earth’s rotation). Situation 1: No relative radial motion. Photon measured by observer at 6563 Å. Situation 2: Source ‘static’, observer moves towards or away from approaching photon. Observer measures blue or red shift. Situation 3: Source moves towards ‘static’ observer. Here I need HELP. This photon, once emitted, is traveling at the speed of light – this being independent of the motion of the source. I am currently unable to see how the beginning wavelength would be measured differently by the observer. Regards, Patrick.