Editing Concentric spheres (section)

==Origins of the concept of concentric spheres==
[[File:Animated Hippopede of Eudoxus.gif|thumb|Animation depicting Eudoxus' model of retrograde planetary motion. The two innermost homocentric spheres of his model are represented as rings here, each turning with the same period but in opposite directions, moving the planet along a figure-eight, or hippopede]]
[[Eudoxus of Cnidus]] was the first astronomer to develop the concept of concentric spheres. He was originally a student at Plato's academy and is believed to have been influenced by the cosmological speculations of [[Plato]] and [[Pythagoras]].<ref>{{cite journal|last1=Goldstein|first1=Bernard|title=A New View of Early Greek Astronomy|journal=Isis|date=September 3, 1983|volume=74|issue=3|pages=332–333|jstor=232593|doi=10.1086/353302 |s2cid=144808083}}</ref><ref name=Gale>"Eudoxus of Cnidus." Complete Dictionary of Scientific Biography. Vol. 4. Detroit: Charles Scribner's Sons, 2008. 465–467. Gale Virtual Reference Library. Web. 2 June 2014.</ref> He came up with the idea of homocentric spheres in order to explain the perceived inconsistent motions of the planets and to develop a uniform model for accurately calculating the movement of celestial objects.<ref name=Gale/> None of his books have survived to the modern day and everything we know about his cosmological theories comes from the works of [[Aristotle]] and [[Simplicius of Cilicia|Simplicius]]. According to these works, Eudoxus’ model had twenty-seven homocentric spheres with each sphere explaining a type of observable motion for each celestial object. Eudoxus assigns one sphere for the fixed stars which is supposed to explain their daily movement. He assigns three spheres to both the sun and the moon with the first sphere moving in the same manner as the sphere of the fixed stars. The second sphere explains the movement of the sun and the moon on the ecliptic plane. The third sphere was supposed to move on a “latitudinally inclined” circle and explain the latitudinal motion of the sun and the moon in the cosmos. Four spheres were assigned to [[Mercury (planet)|Mercury]], [[Venus (planet)|Venus]], [[Mars]], [[Jupiter (planet)|Jupiter]], and [[Saturn (planet)|Saturn]] which were the only known planets at that time. The first and second spheres of the planets moved exactly like the first two spheres of the sun and the moon. According to Simplicius, the third and fourth sphere of the planets were supposed to move in a way that created a curve known as a [[hippopede]]. The [[hippopede]] was a way to try and explain the [[Apparent retrograde motion|retrograde motions]] of planets.<ref>{{cite journal|last1=Yavetz|first1=Ido|title=On the Homocentric Spheres of Eudoxus|journal=Archive for History of Exact Sciences|date=February 1998|volume=52|issue=3|pages=222–225|jstor=41134047|bibcode = 1998AHES...52..222Y|doi=10.1007/s004070050017|s2cid=121186044}}</ref> Many historians of science, such as Michael J. Crowe, have argued that Eudoxus did not consider his system of concentric spheres to be a real representation of the universe but thought it was merely a mathematical model for calculating planetary motion.<ref>{{cite book|last1=Crowe|first1=Michael|title=Theories of the World from Antiquity to the Copernican Revolution|date=2001|publisher=Dover|location=Mineola, NY|isbn=0-486-41444-2|page=23}}</ref>