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November 29th is Christian Andreas Doppler's birthday. Doppler was an Austrian physicist best known for the Doppler effect. Doppler wrote in a paper titled "On the Colored Light of Double Stars and Certain Other Stars of the Heavens" an explanation for the apparent color change of the frequencies of light from stars. He explained how the change in frequency was due to the relative motion of the source and the observer.

The easiest demonstration of the Doppler effect uses sound waves. When a noise approaches a stationary observer, the pitch of the sound (the frequency) increases. As the source passes, the pitch decreases as it speeds away from the observer. This effect applies to all forms of waves, including light. Stars are all made up of mostly hydrogen and spectral analysis shows this to be true, but the spectral lines are moved uniformly towards the red end of the spectrum, or lower frequencies. This suggests the stars are moving away from our solar system and is part of the evidence that the universe is expanding.

Find out more about Doppler and what else occurred on this day in science history.

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