Chapter 4 - Doppler Effect in Astronomy

By Johny Jagannath

The Doppler effect in astronomy appears to be very different from the conventional Doppler effect that is used to determine the velocity of a car, for example, (via radar). In astronomy the spectral lines of our sun are used to compare it with the spectral lines of a distant star light. It turns out that these lines (while identical) are slightly shifted towards the red end or the blue end. In the picture below, the spectral lines are shifted upwards, (towards the red end).
   

Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left)

This shifting of the spectral lines has been understood by current theories as proof that the star must be moving away from us. The reader at this point must rightly be wondering, how can the shifting of spectral lines denote the velocity of the emitting body? I think the shifting of lines, must be a manifestation of classical aberration. which neatly connects velocity with a shifting angle. 

And this shifting angle is the aberration angle, which is proportional to the velocity of the Earth, relative to distant star light. [see the picture below]

Aberration and Spectral lines of star light