First edition of the first printed star atlas.

Piccolomini (1508-1579) was a humanist from a prominent noble and scholarly Sienese family. Two of his ancestors were popes. He produced translations of classical texts, poetry, and commentaries, as well as his astronomical works, 'De La Sfera del Mondo' and 'De Le Stelle Fisse'. Both are works of mathematical astronomy, rather than observation.

'De La Sfera del Mondo' deals with the structure of the universe. In it, Piccolomini defends the Ptolemaic system, with an immobile earth at the centre of the universe. His opinion would only change nearly two decades later, when new observations of planetary movements forced him to reframe the Ptolemaic model as a useful tool for the astronomer rather than gospel.

'De Le Stelle Fisse' was the first printed star atlas and "the first handbook for stargazers" (Gingerich), identifying each of the 48 Ptolemaic constellations. Piccolomini was the first to use a lettering system to identify the prominent stars in each constellation, a practice later copied by the German astrologer Johann Bayer, whose work forms the basis of the system of star nomenclature we use today. Piccolomini used Ptolemy's system of star magnitudes, although he reduced it to four rather than five, and assigned different symbols to each one. The charts show only the shape of the constellations, rather than overlaying them with a pictorial map. The constellations are often not oriented to the north, but shown in their most recognisable position, making it easy for the amateur astronomer using the book. Any myths associated with the constellation are added in the accompanying text.

Unusually for scientific works of the time, they are both written in Italian rather than Latin. While at university in Padua, Piccolomini had been a member of the Society of Infiammati, which strove to promote the vernacular, and had helped to found the Society of Intronati in Siena with a similar aim. It is also exceptional in that it is dedicated to a woman, Laodomia Forteguerri, whereas women were more commonly the dedicatees of poetry or tracts on feminine virtues. Forteguerri, however, was herself exceptional: a Sapphic poet who led a team of women to help build defences when Siena was besieged by Charles V. Piccolomini admired her sonnets and delivered a lecture on them to the Infiammati, the first secular Italian female poet to have her work discussed in an academic setting. He supposedly dedicated 'De la Sfera del Mondo' and 'De Le Stelle Fisse' to her after she complained that she could not study astronomy because she was a woman. It is certainly true that traditional academic texts would be barred to anyone without a classical education (which most women did not have), which adds weight to his decision to publish in the vernacular to reach a wider audience.