During one of his frequent trips to Venice, Galileo met a young woman named Marina di Andrea Gamba, with whom he entered into a relationship. Marina Gamba moved into Galileo's house in Padua and bore him three children, Virginia (1600), later Sister Maria Celeste, Livia (1601), later Sister Arcangela, and Vincenzio (1606). In none of the three baptismal records is Galileo named as the father. In the case of Virginia, she was described as "daughter by fornication of Marina of Venice," with no mention of the father; on Livia's baptismal record the name of the father was left blank; and on Vinzenzio's baptismal record "father uncertain." [cite] The domestic situation was, apparently, a happy one, except when Galileo's mother, Giulia, visited.

When Galileo left Padua for good to take up his position at the Medici court in Florence, in 1610, he took the two daughters with him but left Marina Gamba behind with Vincenzio, who was then only four years old. Vincenzio joined Galileo in Florence a few years later. In 1613 Marina Gamba married Giovanni Bartoluzzi. It appears that Galileo kept cordial relations with Gamba and Bartoluzzi.

Galileo put his two daughters in a convent. He managed to have Vincenzio legitimated by the Grand Duke of Tuscany. The reason for this unequal treatment is probably that Galileo would not be able to provide sufficiently large doweries for his daughters to allow them to make marriages appropriate to his stature at the Medici court. He would have no such financial obligation to his son.