====His telescope and astronomical discoveries====
[[File:Galileo-telescopes.jpg|right|200px|thumb|Early-1900's photograph of Galileo's telescopes]]
His most famous discovery came in June or July of 1609, when he had learned that the Dutchman Hans Lippershey and several others had taken magnifying glasses and constructed the first working telescope. Without knowing the technical details of the construction of the device, he was able to design and create one for himself, and he is credited in being the first to train the new device towards the heavens. Over time he continued to upgrade it. He found mountains on the Moon, viewed the phases of [[Venus]], and discovered four satellites around [[Jupiter]] as well as locating , and with some protection located [[sunspots]].
In 1610 [[Galileo]] used the newly invented [[telescope]] to observe while observing [[Jupiter]] and he discovered four smaller bodies (now called Io, Europa, Ganymede, Callisto) revolving around it, which eventually becoming became one of Galileo's most well-known and important discoveries: [[heliocentrism]]. Galileo's two main evidences in this respect both came from the ability of his telescope to carefully examine the heavens, and his observations of Jupiter and Venus provided strong evidence against [[geocentrism]] (at the time this doctrine was not simply limited to the earth's being the center of the universe, but also that no extraterrestrial orbits existed). <ref> David Filkin, "Stephen Hawking's Universe", David Filkin Enterprises Limited, 1997, pg.44 - 46</ref>
[[NASA]] scientist Ron Baalke wrote
He published his finding in a paper titled "The Starry Messenger" which he dedicated to Grand Duke Cosimo II. His discoveries and publication led him to be appointed as Grand Duke court mathematician, a position he had coveted.
== Advocacy of the Copernican system and conflict with Church authorities ==
Until the sixteenth century, the prevailing view was that the [[Sun]], Moon, [[star]]s and [[planet]]s circled the [[Earth]] (the [[geocentrism|geocentric]] system, based on the second century work of [[Ptolemy]]). Puzzled astronomers noticed that [[Mars]], [[Jupiter]] and [[Saturn]] sometimes seemed to move backwards, but their motions were well within the predictions of Ptolemy's theory. The moon was considered to have more freedom of movement.