Christian Doppler was an Austrian mathematician and physicist, born on November 29, 1803. He is best known for his discovery of the Doppler effect, which describes the change in frequency or wavelength of waves in relation to an observer moving relative to the source of the waves. This phenomenon is commonly observed in sound and light waves. Doppler's work laid the foundation for various applications in science and technology, including radar, sonar, and astronomy. The Doppler effect is crucial for understanding how the movement of stars and galaxies can be measured, helping scientists determine their speed and distance from Earth.