This phenomenon discovered by Christian Doppler will be familiar to many with sound waves, for exapmle the siren of an ambulence. As an ambulence approaches us, the pitch of the sound we hear appears to be higher than it really is. Likewise, as the ambulence goes past and speeds away from us, the pitch appears to be lower than it actually is.

This is because as the ambulence approaches, the sound waves from the siren are 'pushed' closer together so that their frequency is increased, and hence the pitch raised. The same is true in reverse as the ambulence receeds from the listener, 'stretching' the sound waves and lowering the pitch.

The same effect can also be observed in light; an object moving towards us at a high velocity will show a spectral shift to the higher-frequency blue end of the spectrum; called a blue shift. If an object moves away from us at high velocity, then a spectral shift to the lower-frequency, or red, end of the spectrum will be observed, called a redshift.

It was through observation of redshift in galaxies around our own, and further afield, that suggested to scientists that the universe is expanding at great speed, implying the Big Bang! theory of the universe. It's most important application within these pages however, is that it can tell an astronomer whether a galaxy is rotating, and if it is, details of the rotation. To illustrate this, take as an example, a spiral galaxy that has three spiral arms, upon one of which an observer stands. Using their equipment, the observer notices that the stars on one "side" are showing a positive redshift; i.e. they are moving away from them. They also notice that the stars on the other "side" are showing a negative redshift, or blueshift; i.e. they are moving towards them. If the observer is taking care to make their measurements in a plane parallel to that of the galaxy disk, then they must conclude that the galaxy is rotating in the same horizontal plane: