1. A method for initializing, in a system comprising an image sensor oriented toward the horizon and a North seeker of gyroscopic or gyrometric type rigidly attached to the image sensor, a value representative of the orientation of the image sensor with respect to the North, in which is determined the offset of two images output by the digital image sensor at distinct first and second instants so as to constitute an item of information representative of an angular movement, and in which this angular movement information is used to discriminate between the movements of the image sensor with respect to the ground and the movements of the image sensor due to the rotation of the Earth.