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 1853, fifty years old, his name already shorthand for relative motion.