When Galileo left the University of Pisa without taking a degree in the spring of 1585, he was a promising young mathematician with an experimental bent, but there was nothing to foretell his later interest in astronomy. He earned his living by giving private lessons in Florence and Siena, and it is probably at this time that he wrote a short Treatise on the Sphere or Cosmography for the use of his pupils.

This elementary textbook of spherical astronomy is based on the thirteenth-century Sphere of John Holywood, better known under his Latinized name of Sacrobosco. It is conventional in its geocentrism and makes no mention of Copernicus. Galileo may have used it when he became a Professor of Mathematics at Pisa (1589-92) and during the first years of his professorship at Padua (1592-1610).

In Pisa, Galileo made the acquaintance of Jacopo Mazzoni, a philosopher who sought to combine the insights of Plato and Aristotle, and with whom he stayed in touch after he had left Tuscany for the Venetian Republic. It was this friend who, in 1597, provided Galileo with his first opportunity of stating his opinion that the heliocentric theory of Copernicus was more probable than the geocentric system of Aristotle and Ptolemy.