What is the name of the effect, named after a 19th Century Austrian physicist, which causes the sound of a moving object to change its pitch as it approaches and then passes you?

You've almost certainly experienced this phenomenon while waiting for an ambulance or fire truck to pass. The siren seems to climb to a higher pitch as it races toward you, and then suddenly drops to a lower pitch as it speeds away. This happens because the sound waves an object emits get compressed in front of it as it moves, increasing their frequency and creating a higher pitch. As the object passes and moves away, the sound waves are stretched out, lowering their frequency and the corresponding pitch.

This auditory illusion is named after Christian Doppler, the Austrian physicist who first described it in 1842. Interestingly, his original paper wasn't about the sound of moving trains or sirens, but about the light from distant stars. He correctly theorized that the same principle should apply to light waves, causing the color of a star to shift slightly depending on whether it was moving toward or away from Earth.

Doppler's theory about light proved to be incredibly important for astronomy (Deals). The "redshift" of light from distant galaxies, where light waves are stretched out, is the primary evidence that our universe is expanding. The effect is also used in more down-to-earth applications, from the radar guns police use to measure a car's speed to the weather radar systems that track storms.