IL IX LIBRO DEL DE ARCHITECTURA DI VITRUVIO NEI COMMENTARI DI DANIELE BARBARO (1556-1567)1

Abstract<title> SUMMARY </title>Book IX of Vitruvius's DE ARCHITECTURA is devoted to the Gnomonica, which is the art of making solar clocks, or the second part of architecture, and to the basic construction of the Analemma, an orthographic projection of different circles of the sky - blue sphere.The present paper explains the changes to this Book between the 1556 edition and the 1567 editions of Daniele Barbaro's Commentationes.Indeed, while in 1556 Barbaro reconstructs the Vitruvian Analemma, that he had studied in the De horologiis describendis libellus, in 1567 he will more clearly define the Vitruvian Analemma by considering the Ptolemaic Analemma, after the 1562 study in Federico Commandino's Claudii Ptolemaei liber de Analemmate. Barbaro will also apply this Analemma in detail to the construction of the sun-dial, which was done neither by Vitruvius nor by Ptolemy, but in Commandino's Liber de Horologiorum descriptione.The relevance of the Analemma to the theory of planar projections of meridians is pointed out here. Indeed, even if the work of Barbaro seems traditional from the point of view of an astronomer, its implications for projective geometry are of great importance.