According to science, position and motion are relative terms.

As we know earth revolves around the Sun but we are on earth. 

We see, Sun is changing direction from east to west in clocking direction.

So, if we keep the earth in centre the Sun will be move as-

In morning - In east

In noon - In south

In afternoon - In west

So, magnetic declination will be equal to

                                                   = T - M_B

                    T_B of sun at noon  = 180°

                                                  = 180°-185°20'

                                                  = -5°20' {west}

 Declination is measured from true north so, magnetic declination = 5°20' west.