Galileo Galilei (1564-1642) is perhaps best known as the first astronomer in Renaissance Europe to observe several of the planets in our solar system with a telescope strong enough to allow him to identify them. Beyond that, he made numerous contributions to the study of physics and mathematics. He wrote prolifically on his findings, and published several treatises and books.
The Starry Messenger (1610)
Here Galileo describes the telescope he made and some of his observations regarding the nature of the solar system. Not only does he describe a heliocentric model of the solar system, in which Earth and the other planets revolve around the sun, but he also describes how he observed four moons revolving around Jupiter. He deduced that Earth and Jupiter must be the same kind of object, since they both had moons.
Letter to the Grand Duchess Christina (1615)
This letter was written to explain the independence of science from religion. He disputes that his findings are contrary to the Bible, and states that he believes in both the Bible and what he has discovered through scientific means. According to Galileo, those who say his conclusions oppose Biblical texts misinterpret the Bible. He says that his findings support those of Copernicus, an earlier proponent of the heliocentric model of the solar system.
The Assayer (1623)
This was a book written to disprove a treatise by Orazio Grassi regarding the nature of comets. Science later showed Grassi 's theories to be correct, while Galileo was actually wrong in this instance. Grassi correctly identified comets as real objects, while Galileo was convinced that they were only phenomena of light. He also discusses the differences between tangible properties of substances and properties that can only be experienced in the mind, such as light.
Dialogue Concerning the Two Chief World Systems (1632)
Galileo described and compared the two models of the solar system: the Copernican, or heliocentric model, where Earth and the other planets revolved around the sun, and the traditional Ptolemaic system, in which the universe revolved around the Earth. It was this treatise that led to him being summoned by the Inquisition.