used to measure the speed of distant objects by measuring their movement, which affects radiation they emit
You can compare how much the radiation has shifted, compared to radiation on earth, and thus determine the velocity relative to earth
When an object is moving away from the earth, the wavelength gets longer and the frequencies get shorter - red shift
When an object is moving towards the earth the opposite happens and the radiation undergoes blue shift
v/c = Δλ / λ
The equation is valid only if v is much less than c
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