The Doppler shift is the change in the observed wavelength of a wave when its source moves toward or away from the observer, the same effect that lowers the pitch of a passing siren. In astronomy it shifts spectral lines toward blue for approaching objects and toward red for receding ones, turning a spectrum into a speedometer. It measures the wobble of stars tugged by planets, the rotation of galaxies, and the motion of gas around black holes. Cosmological redshift is a related but distinct effect, caused by the stretching of space itself rather than motion through it.