var dragging = false;

$(function() {

    var target = $('#target');

    var mainTarget = $('#mainTarget');

    var rad = mainTarget.width()/2;

    var elOfs = mainTarget.offset();

    var elPos = {

       x: elOfs.left,

       y: elOfs.top

    };

    target.mousedown(function() {

        dragging = true;

    });

    $(document).mouseup(function() {

        dragging = false;

    });

    $(document).mousemove(function(e) {

      var mPos = {

        x: e.pageX-elPos.x,

        y: e.pageY-elPos.y

      };

      var getAtan = Math.atan2(mPos.x-rad, mPos.y-rad);

      var getDeg = -getAtan/(Math.PI/180) + 135;  //135 = (180deg-45deg)

​

      $('p').text(getDeg);

        if (dragging) {

            mainTarget.css({transform: 'rotate(' + getDeg + 'deg)'});

        }

    });

});