Astronomy 121 Basics
The Doppler Effect
Electromagnetic radiation always travels through diffuse
space at the same speed, the speed of light:
300,000 kilometers/second
The observed speed will not depend on relative motion.
However, the wavelength of the light does change with
relative motion.
Stars moving
away from us appear red-shifted and stars moving
toward us appear blue-shifted.
This can be illustrated
by considering the wave crests of the wave motion.