To determine whether a celestial body is approaching or receding from Earth, astronomers employ The Doppler principle.
There is a Doppler effect for both light and sound.
For instance, astronomers regularly measure the amount to which a star or galaxy's light is "stretched" towards the lower frequency, red region of the spectrum to calculate how quickly the object is travelling away from us.
A wave's frequency changes in response to an observer moving in respect to the wave source, which is known as the Doppler effect or Doppler shift.
The Doppler effect results from the fact that as the wave source moves in the direction of the observer, the crest of each succeeding wave emerges from a location that is closer to the observer than the crest of the previous wave.
As a result, each wave travels to the observer slightly faster than the one before it. As a result, the interval between successive wave crests arriving at the observer is shorter, increasing the frequency.
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