When you take the Doppler effect into account, it turns out that
the visible light emitted from a source moving away from Earth
will appear to have a wavelength longer than it really has, that is,
shifted toward the red end of the visible spectrum.
This is the only way we have of measuring whether a source of
light is moving away from us, and how fast. We measure the
wavelengths of its light, and measure how much longer they are
than they should be.
The main trick in this whole process is: When you see the light
from a star or a galaxy, how do you know what its wavelength
should be ?
