The Doppler Shift – how we use it
Atomic energy transitions leave features
in the spectrum whose rest wavelengths are known from laboratory work. We can
measure observed shifts in these wavelengths from astronomical objects,
and
see how fast they are moving (you only get the line-of-sight motion: towards or
away from you).
More subtle analysis can also yield other
motions, like rotation or turbulent motions. These are all direct uses of the
Doppler
shift. It doesn’t matter how far away the source is, either.