The Doppler effect expresses that a source emitting in its rest frame at a
frequency
will appear to emit at a frequency
in the
observation frame, which depends on the velocity of the source in the
observation frame,
. Sections 1.3
and 1.4 will show how this property can be used
to interpret the spectral axis measured in the observatory frame in terms
of the property of the source in the source rest frame.