Letter to the Grand Duchess Christina
The Letter to The Grand Duchess Christina was an essay written in 1615 by Galileo Galilei.
Christina, daughter of Charles III of Lorraine and granddaughter of Catherine de' Medici, was the widow of Ferdinando I de' Medici, Grand Duke of Tuscany 1587-1609, who had appointed Galileo to the professorship of mathematics at the University of Pisa in 1588.
In 1611 Galileo was informed by a friend, Cigoli, “ill-disposed men envious of your virtue and merits met [to discuss]... any means by which they could damage you.” The number of scholars who disapproved with his Discourse on Floating Bodies, or were simply ill-spirited toward Galileo grew, but other than one letter from Niccolo Lorini there was not much discussion about the issue for the remainder of the year.
Late in 1613, Galileo's former student Benedetto Castelli, a Benedictine monk, wrote to Galileo about the events at a recent dinner with the Grand Duke Cosimo II de' Medici. In the course of conversation at the dinner Cosimo Boscaglia, a professor of philosophy, argued that the motion of the Earth could not be true, being contrary to the Bible. After dinner ended, Castelli was called back to answer scriptural arguments against the motion of the Earth from Christina. The monk took on the role of theologian in response, and convinced everyone there except the Duchess (who he thought was arguing mainly to hear his answers) and Boscaglia (who said nothing during this dialogue). 
Contents of Letter
Galileo replied with a long letter giving his position on the relation between science and Scripture. By 1615, with the controversy over the Earth's motion widespread and increasingly dangerous, Galileo revised this letter and greatly expanded it; this became the Letter to the Grand Duchess Christina. The letter circulated in manuscript but was not printed until much later, when the Inquisition had condemned Galileo. It appeared in Strasbourg in 1636 with both Italian and Latin text, as seen in the top right image. It was suppressed in Catholic jurisdictions, as were all works of Galileo at that period which dealt with that subject.
In this letter was a very direct paragraph in which Galileo wrote "I hold the sun to be situated motionless in the center of the revolution of the celestial orbs while the earth rotates on its axis and revolves about the sun. They know also that I support this position not only by refuting the arguments of Ptolemy and Aristotle ... especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it."  Galileo stated that the Copernican theory was not just a mathematical calculating tool, but a physical reality. This caused some, including Dominican friar Niccolo Lorini, to complain to the Inquisition, which tried and eventually accused Galileo of suspicion of heresy.
- http://www.inters.org/galilei-madame-christina-Lorraine Galileo's Letter to the Grand Duchess in 1615
- Stillman Drake (translator and editor) (1957). Discoveries and Opinions of Galileo. New York: Doubleday Anchor Books. ISBN 0-385-09239-3. This has the full text of the letter, with commentary, as well as other short works of Galileo.
- Maurice A. Finocchiario (1989). The Galileo Affair. University of California Press. ISBN 978-0-520-06662-5. This compilation of relevant original documents also includes a 43-page introduction by the author.