A rotary steerable system for use in a drill string for drilling a deviated well is disclosed. The system utilizes a mechanical gravity reference device comprising an unbalanced weight which may rotate independently of the rotation of the drill string so that its heavy portion is always oriented toward the low side of the wellbore and which has an attached magnet. A magnetic switch that rotates as the drill string rotates is activated when its axis coincides with the axis of the magnet, and this activation results in a thrust member or pad being actuated to "kick" the side of the wellbore.