How does analyzing spectral lines via the Doppler effect reveal the Sun's rotational velocity?
Light from receding parts is red-shifted, and light from approaching parts is blue-shifted.
The Doppler effect relates observed wavelength shifts to relative motion along the line of sight. For a rotating body like the Sun, the side rotating towards the observer is moving closer, causing the light received from that limb to have its wavelength shortened, resulting in a spectral blue-shift. Conversely, the side rotating away from the observer is moving farther, causing its light to have its wavelength lengthened, resulting in a spectral red-shift. By precisely measuring these subtle shifts in the spectral lines of sunlight, astronomers can accurately quantify the component of velocity induced by the Sun's rotation across different observed latitudes.
