Editing Geocentric model (section)

===Observation by Galileo and abandonment of the Ptolemaic model===
With the invention of the [[telescope]] in 1609, observations made by [[Galileo Galilei]] (such as that [[Jupiter]] has moons) called into question some of the tenets of geocentrism but did not seriously threaten it. Because he observed dark "spots" on the Moon, craters, he remarked that the moon was not a perfect celestial body as had been previously conceived. This was the first detailed observation by telescope of the Moon's imperfections, which had previously been explained by Aristotle as the Moon being [[Contamination|contaminated]] by Earth and its heavier elements, in contrast to the [[aether (classical element)|aether]] of the higher spheres. Galileo could also see the moons of Jupiter, which he dedicated to [[Cosimo II de' Medici, Grand Duke of Tuscany|Cosimo II de' Medici]], and stated that they orbited around Jupiter, not Earth.<ref name= "Finocchiaro2008"/> This was a significant claim as it would mean not only that not everything revolved around Earth as stated in the Ptolemaic model, but also showed a secondary celestial body could orbit a moving celestial body, strengthening the heliocentric argument that a moving Earth could retain the Moon.<ref>{{cite web|url=http://www.atnf.csiro.au/outreach/education/senior/astrophysics/galileo.html|title=Galileo and the Telescope|work=Commonwealth Scientific and Industrial Research Organisation|access-date=17 October 2014}}</ref> Galileo's observations were verified by other astronomers of the time period who quickly adopted use of the telescope, including [[Christoph Scheiner]], [[Johannes Kepler]], and Giovan Paulo Lembo.<ref>Lattis, James L. (1995). Between Copernicus and Galileo: Christoph Clavius and the Collapse of Ptolemaic Cosmology, University of Chicago Press, pgs 186-190</ref>

{{multiple image
|footer=In 1610 [[Galileo Galilei]] observed with his telescope that [[Phases of Venus|Venus showed phases]], despite remaining near the Sun in Earth's sky (first image). This proved that it orbits the [[Sun]] and not [[Earth]], as predicted by [[Copernican heliocentrism|Copernican]] and [[Tychonic system|Tychonic]] models, and disproved the Ptolemaic one (second image). 
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|image1 = Phases-of-Venus2.svg|
|image2 = Phases-of-Venus-Geocentric.svg|}}
In December 1610, [[Galileo Galilei]] used his telescope to observe that [[Venus]] showed all [[Phases of Venus|phases]], just [[lunar phase|like the Moon]]. He thought that while this observation was incompatible with the Ptolemaic system, it was a natural consequence of the heliocentric system.

However, Ptolemy placed Venus' [[deferent]] and [[epicycle]] entirely inside the sphere of the Sun (between the Sun and Mercury), but this was arbitrary; he could just as easily have swapped Venus and Mercury and put them on the other side of the Sun, or made any other arrangement of Venus and Mercury, as long as they were always near a line running from the Earth through the Sun, such as placing the center of the Venus epicycle near the Sun. In this case, if the Sun is the source of all the light, under the Ptolemaic system:
{{Blockquote|If Venus is between Earth and the Sun, the phase of Venus must always be [[crescent]] or all dark.

If Venus is beyond the Sun, the phase of Venus must always be [[gibbous]] or full.}}
But Galileo saw Venus at first small and full, and later large and crescent.
This showed that with a Ptolemaic cosmology, the Venus epicycle can be neither completely inside nor completely outside of the orbit of the Sun. As a result, Ptolemaics abandoned the idea that the epicycle of Venus was completely inside the Sun, and later 17th-century competition between astronomical cosmologies focused on variations of the Tychonic or Copernican systems.