If the Earth were a mini-star, with the same temperature as
the Sun, then the ratio of its brightness to that of the Sun
would be
                                                      2
         brightness of Earth        ( 6.37 x 10^3 km ) 
    ---------------------------  =  ( ------------   )  
         brightness of Sun          ( 6.96 x 10^5 km )


                                             -5
                                 =   8.4 x 10