Editing Definition of planet (section)

=== Earth ===
When Copernicus's [[heliocentric model]] was accepted over the [[geocentric]], Earth was placed among the planets and the Sun and Moon were reclassified, necessitating a conceptual revolution in the understanding of planets. As the [[historian of science]] [[Thomas Kuhn]] noted in his book, ''[[The Structure of Scientific Revolutions]]'':<ref>Thomas S. Kuhn, (1962) ''The Structure of Scientific Revolutions'', 1st. ed., (Chicago: University of Chicago Press), pp. 115, 128–9.</ref>

<blockquote>The Copernicans who denied its traditional title 'planet' to the sun ... were changing the meaning of 'planet' so that it would continue to make useful distinctions in a world where all celestial bodies ... were seen differently from the way they had been seen before... Looking at the moon, the convert to Copernicanism ... says, 'I once took the moon to be (or saw the moon as) a planet, but I was mistaken.'</blockquote>

Copernicus obliquely refers to Earth as a planet in ''De Revolutionibus'' when he says, "Having thus assumed the motions which I ascribe to the Earth later on in the volume, by long and intense study I finally found that if the motions of the other planets are correlated with the orbiting of the earth..."<ref name=koper/> [[Galileo Galilei|Galileo]] also asserts that Earth is a planet in the ''[[Dialogue Concerning the Two Chief World Systems]]'': "[T]he Earth, no less than the moon or any other planet, is to be numbered among the natural bodies that move circularly."<ref name=gal>{{cite web|title=Dialogue Concerning the Two Chief World Systems|work=Calendars Through the Ages|url=http://www.webexhibits.org/calendars/year-text-Galileo.html|access-date=June 14, 2008}}</ref>