Q:  What is the speed of Arcturus toward/away from us?


  A:  One of the absorption lines in the Sun's spectrum has a wavelength
         of about 883.8 nm.  In the spectrum of Arcturus, that same
         line appears at 884.0 nm.  So the shift is

                observed lambda - rest lambda  =  884.0 - 883.8  =  0.2 nm

      We can use this shift to compute the speed of Arcturus relative
         to the Sun:

                     v              0.2 nm
                   -----     =    ----------   =   0.000 226
                     c             883.8 nm

         So

                     v       =   0.000 226 * c  =  0.000 226 * (3 x 10^8  m/s)

                             =   67,800  m/s   


     Because the observed wavelength is longer than the rest wavelength,
         Arcturus must be moving AWAY from us at 67.8 km/s.