Navigating by the stars. 16th-century artwork of a cross-staff being used to determine the altitude of the northern pole star (Polaris, labelled 'norte' at upper right) above the horizon (labelled 'orizonte' at bottom right). This was a standard measurement obtained by mariners and travellers to aid navigation. Artwork from 'Regimiento de navegacion' (Seville, 1563) by Spanish astronomer and geographer Pedro de Medina (1493-1567).