This paper examines the extent to which English seamen in the later sixteenth century began to employ astronomical observations and mathematical calculations in navigation instead of relying largely on practical experience. It discusses the manuals of seamanship that were published in increasing numbers at this period and also looks at the availability of instruction in the necessary mathematics. The popularity of some texts including Waghenaer's theMariner's Mirrour, a book combining charts and sailing directions with the tables needed to use astronomical observations, are, it is suggested, an indication of the progress made in this respect by the early seventeenth century.