According to the Doppler effect, objects moving away from Earth would have a cooling appearance. blue shift. red shift. warming appearance.

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?