Abstract This article is a simple illustration of a series of lessons concerning “horizon astronomy”. In the large terrace above our school, in open air, students observe the local horizon and make a large, circular drawing of the skyline. It becomes an astronomical observatory: on the plotted skyline, the rising and sunset points of the sun, the moon and the stars can be registered in a long-term activity, as well as the ancient astronomers did. The first organisation of space (the orientation) and the first measure of time (a sort of calendar) just started like this.