The invention relates to an actuator having an (auto)synchronous rotary electric motor and a reversible speed-reducing gearbox coupled to the motor to be driven in rotation thereby An outlet shaft is coupled to the speed-reducing gearbox to be driven in rotation thereby A first angular position sensor and a control circuit are connected to the motor. The circuit delivers a motor power supply signal that varies as a function of a position setpoint signal applied to the control circuit and as a function of signals delivered by the angular position sensor. The gearbox has a plurality of speed-reducing each having a pair of gears mounted to rotate about two parallel axes of rotation.