Using a gnomon , he measured the Sun 's angle of elevation at noon on the solstice in Alexandria , and found it to be 1\/50th of a circle ( 7 ° 12 ' ) south of the zenith .
He also knew , from measurement , that in Alexandria , the angle of elevation of the sun would be 1\/50 of a full circle ( 7 ° 12 ' ) south of the zenith at the same time .