Assume that the maximum separation of the EHT really was the
       diameter of the Earth:  D = 1.2 x 10^7 meters.

   The typical wavelength observed was lambda = 1.3 mm = 0.0013 meters.


   Q:  What was the diffraction limit of EHT?


                     lambda         0.0013 m
         theta  =  ----------  =  ---------------
                       D           1.2 x 10^7 m



                =   1.0 x 10^(-10) radians

        
                =   2.1 x 10^(-5)  arcseconds  =  0.000 021  arcseconds