Early life and family
Galileo was born in Pisa (then part of the Duchy of Florence), Italy, on 15 February 1564, the first of six children of Vincenzo Galilei, a lutenist, composer, and music theorist, and Giulia (née Ammannati), who had married in 1562. Galileo became an accomplished lutenist himself and would have learned early from his father a scepticism for established authority, the value of well-measured or quantified experimentation, an appreciation for a periodic or musical measure of time or rhythm, as well as the results expected from a combination of mathematics and experiment.
Three of Galileo's five siblings survived infancy. The youngest, Michelangelo (or Michelagnolo), also became a lutenist and composer although he contributed to financial burdens during Galileo's young adulthood. Michelangelo was unable to contribute his fair share of their father's promised dowries to their brothers-in-law, who would later attempt to seek legal remedies for payments due. Michelangelo would also occasionally have to borrow funds from Galileo to support his musical endeavours and excursions. These financial burdens may have contributed to Galileo's early desire to develop inventions that would bring him additional income.
When Galileo Galilei was eight, his family moved to Florence, but he was left with Jacopo Borghini for two years. He was educated from 1575 to 1578 in the Vallombrosa Abbey, about 30 km southeast of Florence.
Galileo tended to refer to himself only by his given name. At the time, surnames were optional in Italy, and his given name had the same origin as his sometimes-family name, Galilei. Both his given and family name ultimately derive from an ancestor, Galileo Bonaiuti, an important physician, professor, and politician in Florence in the 15th century; his descendants had come to refer to themselves as Galilei in his honor in the late 14th century. Galileo Bonaiuti was buried in the same church, the Basilica of Santa Croce in Florence, where about 200 years later his more famous descendant Galileo Galilei was also buried.
When he did refer to himself with more than one name, it was sometimes as Galileo Galilei Linceo, a reference to his being a member of the Academy of Lincei, an elite pro-science organization in Italy. It was common for mid-sixteenth-century Tuscan families to name the eldest son after the parents' surname. Hence, Galileo Galilei was not necessarily named after his ancestor Galileo Bonaiuti. The Italian male given name "Galileo" (and thence the surname "Galilei") derives from the Latin "Galilæus", meaning "of Galilee", a biblically significant region in Northern Israel. Because of that region, the adjective galilaios (Greek γαλιλαίος, Latin Galilæus, Italian galileo), which means "Galilean", has been used in antiquity (particularly by emperor Julian) to refer to Christ and his followers; after this, the adjective has been adopted as a name with a meaning similar to Jesus or Christian.
The biblical roots of Galileo's name and surname were to become the subject of a famous pun. In 1614, during the Galileo affair, one of Galileo's opponents, the Dominican priest Tommaso Caccini, delivered against Galileo a controversial and influential sermon. In it he made a point of quoting 1:11, "Ye men of Galilee, why stand ye gazing up into heaven?".
Galileo's elder daughter Virginia
was particularly devoted to her father
Despite being a genuinely pious Roman Catholic, Galileo fathered three children out of wedlock with Marina Gamba. They had two daughters, Virginia (born 1600) and Livia (born 1601), and a son, Vincenzo (born 1606).
Because of their illegitimate birth, their father considered the girls unmarriageable, if not posing problems of prohibitively expensive support or dowries, which would have been similar to Galileo's previous extensive financial problems with two of his sisters. Their only worthy alternative was the religious life. Both girls were accepted by the convent of San Matteo in Arcetri and remained there for the rest of their lives.
Virginia took the name Maria Celeste upon entering the convent. She died on 2 April 1634, and is buried with Galileo at the Basilica of Santa Croce, Florence. Livia took the name Sister Arcangela and was ill for most of her life. Vincenzo was later legitimised as the legal heir of Galileo and married Sestilia Bocchineri.