Galileo Galilei was an Italian mathematician, astronomer and philosopher.
In 1609, Galileo was in Venice to show the Doge his telescope when he encountered the First Doctor, Steven Taylor and Vicki Pallister, who had come to Venice responding to a mysterious invitation. Since the Doctor required Galileo's assistance at the same time he was scheduled to demonstrate the invention, Steven had to impersonate Galileo by means of a holographic disguise. The Doctor also helped Galileo improve his telescope design. (PROSE: The Empire of Glass)
The Doctor warned Galileo that he should remain quiet about some of his ideas that would be seen as heretical. For his belief in heliocentrism, the Catholic Church put Galileo on trial for heresy and he was sentenced to house arrest in Arcetri. His daughter, Virginia, died in 1634, after which Fortuna took her place. Although he was aware of this, he was pleased to have his daughter back.
In 1637, Galileo was blinded when he used his telescope to view what he named Phaiton. Fortuna gave him a celestial projector which he used to spy on her and Cleaver, who made him record a message for the Doctor to lead him into a trap. When the Eighth Doctor arrived, Galileo offered him what help he could and encouraged Helen Sinclair to continue travelling with him. (AUDIO: The Galileo Trap)
In the first parallel universe that Rose Tyler visited in her quest to warn the Doctor of the reality bomb, Galileo had observed several stars disappearing in the sky as a result of the bomb. (AUDIO: The Endless Night)