The figure shows the positions of the solar and neutral hydrogen regions A to D rotating in a circular orbit relative to the center of the galaxy. The sun and A to D assume a Kepler rotation.
What is the correct explanation for the observation of the 21cm wave emitted from A to D as selected in <Example>?
b. The wavelength of D is 21cm.
Is b true even when considering the horizontal doppler effect? And what will be the observed wavelength if b is incorrect?