Re: If the speed of light is a constant, why does the red shift occur?

The Doppler effect does depend on the relative motions of objects, but it doesn't involve the speed of the emitted waves changing. In fact, even for sound, the speed of the waves is fixed by the properties of the medium through which they travel (in this case, air).

So what is the Doppler shift then? It all depends on the wavelength of the light. A wave, of course, has crests and troughs. The distance between two consecutive troughs is the wavelength and determines the color of the light: red light has a longer wavelength than blue light. You can think of an object that emits light as periodically sending out pulses that correspond to these crests. If the object is standing still, the pulses always have the same distance from each other - or wavelength - and the light seen by an observer has the same wavelength. If an object is moving away from the observer, it emits a pulse, moves a bit away, then emits another one. So the wavelength of the light our observer would see is increased by the amount that the object moved in between pulses. This is a tough concept to understand with words, so I recommend checking out these cool animations.