Christian Doppler was in Prague in 1842 when he wondered if he might be able to explain the colours of binary stars. He came up with an idea that we now call the Doppler effect, to do with the perceived frequency of a wave and the way the relative velocities of the source and the observer affect the observed frequency.
|This picture has nothing to do|
with this story: I just fell prey to
the pictorial imperative.
If somebody drives past sounding their horn, the pitch drops in the same way. It happens with sirens as well, but it's harder to spot, because the siren is varying anyhow.
Then in 1845, a clever young Dutch physicist with the curious (to our eyes) name of Buys Ballot said that it was all a load of old cobblers, but he reckoned there was something in the idea itself, even if it didn't cause the colours of binary stars.