What is it, and how does it happen?

You are stood at the motor racing track and a car approaches from a distance away at high speed. You hear the engine note which is fairly low at first and only gradually becomes sharper and sharper in pitch, reaching a crescendo as the car screams past you and then as it hurtles away into the distance the pitch gradually falls lower and lower and lower. Ever wondered why this should happen? A gentleman called Christian Doppler came up with an answer back in the 19th century; he reasoned that sounds are caused by waves and the shorter the distance between the peaks of the waves, the higher the pitch and conversely when the peaks are further apart the pitch is correspondingly lower. All very simple of course but now we get a little bit more complicated. Just imagine that an object which is moving away from you is emitting sound; there are a certain number of waves between you and the object but since it is moving away from you these waves are becoming stretched so by the time they reach your ear the wavelength is longer and therefore the sound is at a lower pitch. If on the other hand the object was moving towards you the waves would be compressed; the wavelength would therefore be shorter and therefore the pitch would be higher. This raises very interesting possibilities; if we knew what the pitch should be if both the object and the receiver (in this case our own ears) were stationary, or at least remained in the same relative positions, we could calculate the speed of a moving object by measuring the wavelength of not only sound but also of light or other electromagnetic waves which came from it! This principle is used in calculating the movement of objects which are tracked by radar, which has caused headaches for countless motorists who have inadvertently driven through radar speed traps, as well as more socially acceptable uses such as guiding missiles to moving targets; and it is also being used to calculate the movements of the stars in the universe, which is shown that the universe is expanding rapidly as all the stars move away from each other. The Big Bang Theory is a direct result of these observations.

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