Ask A Scientist© Astronomy Archive

Question:  What is a Red Shift?
kathy e belcher

Answer:  Red shift occurs when the source of radiation is moving AWAY from you.
and refers to the shift in wavelength of the radiation. Blue shift
occurs when the source is moving TOWARDS you. The radiation might be sound
or light or any other electro-magnetic radiation. The classic example
is a train whistle. When the train approaches, the whistle has a higher
pitch than when the train is stationary, and a lower pitch when the
train recedes. Astronomers use measurements of the red shift of stars
and galaxies as a distance indicator. If you would like to know more
about how that works, please ask in another question.
hawley