Christian Doppler

Christian Andreas Doppler was born in Salzburg on 29 November 1803, trained as a mathematician and physicist in an Austria still finding its scientific footing. He spent years working through the geometry of waves — sound, light, anything that propagates — and in the early 1840s he published the principle that would outlive him: the frequency you observe depends on whether the source is moving toward you or away. It was abstract at first, a thought experiment about starlight and color shifts. Then the trains came, and every passing whistle proved him right. Doppler died in Venice on 17 March 1…