The present invention provides a method and device for automatically determining a capture trajectory of a flight trajectory for an aircraft, as well as a procedure and system for automatic guidance of an aircraft. The flight trajectory includes at least one lateral trajectory furnished at least with a plurality of rectilinear stretches. The capture trajectory is a lateral capture trajectory that allows the aircraft to join up laterally with a particular rectilinear stretch of the lateral trajectory of the flight trajectory. The capture trajectory is defined to allow a minimum capture duration. An automatic navigation mode is engaged as soon as requested by the pilot to, if necessary, capture and follow the planned flight trajectory.