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The Copernican Revolution

The 16th century finally saw what came to be a watershed in the development of Cosmology. In 1543 Nicolas Copernicus published his treatise De Revolutionibus Orbium Coelestium (The Revolution of Celestial Spheres) where a new view of the world is presented: the heliocentric model.

It is hard to underestimate the importance of this work: it challenged the age long views of the way the universe worked and the preponderance of the Earth and, by extension, of human beings. The realization that we, our planet, and indeed our solar system (and even our galaxy) are quite common in the heavens and reproduced by myriads of planetary systems provided a sobering (though unsettling) view of the universe. All the reassurances of the cosmology of the Middle Ages were gone, and a new view of the world, less secure and comfortable, came into being. Despite these ``problems'' and the many critics the model attracted, the system was soon accepted by the best minds of the time such as Galileo

Copenicus' model, a rediscovery of the one proposed by Aristarchus centuries before (see Sect. 2.4), explained the observed motions of the planets (eg. the peculiar motions of Mars; see Fig. 2.13) more simply than Ptolemy's by assuming a central sun around which all planets rotated, with the slower planets having orbits farther from the sun. Superimposed on this motion, the planets rotate around their axes. Note that Copernicus was not completely divorced from the old Aristotelian views: the planets are assumed to move in circles around the sun (Fig. 3.3).




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Figure 3.3: The page in Copernicus' book De Revolutionibus Orbium Coelestium outlining the heliocentric model. 
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It must be noted that Copernicus not only put forth the heliocentric idea, but also calculated various effects that his model predicted (thus following the steps outlined in Sect. 1.2.1). The presentation of the results was made to follow Ptolemy's Almagest step by step, chapter by chapter. Copernicus' results were quite as good as Ptolemy's and his model was simpler; but its predictions were not superior (since the planets do not actually move in circles but follow another - though closely related - curve, the ellipse); in order to achieve the same accuracy as Ptolemy, Copernicus also used epicycles, but now in the motion of the planets around the Sun. The traditional criticisms to the heliocentric model he answered thusly,

Copernicus was aware that these ideas would inevitably create conflicts with the Church, and they did. Though he informally discussed his ideas he waited until he was about to die to publish his magnum opus, of which he only printed a few hundred copies. Nonetheless this work was far from ignored and in fact was the first (and perhaps the strongest) blow to the Medieval cosmology. His caution did not save him from pointed criticisms, for example, Luther pointed out (from his Tabletalk)

There was mention of a certain new astrologer who wanted to prove that the Earth moves and not the sky, the Sun, and the Moon. This would be as if somebody were riding on a cart or in a ship and imagined that he was standing still while the Earth and the trees were moving [*]. So it goes now. Whoever wants to be clever must agree with nothing that others esteem. He must do something of his own. This is what that fellow does who wishes to turn the whole astronomy upside down. Even in these things that are thrown into disorder I believe the Holy Scriptures, for Joshua commanded the Sun to stand still and not the Earth. [*]

The Pope Paul III was not very critical, but his bishops and cardinals agreed with Luther and the model was condemned by the Church.

The heliocentric model was eventually universally accepted by the scientific community, but it spread quite slowly. There were several reasons for this, on the one hand there certainly was a reticence to oppose the authority of the Church and of Aristotle, but there was also the fact that the heliocentric model apparently contradicted the evidence of the senses. Nonetheless the model became better known and was even improved. For example, Copernicus' version had the fixed stars attached to an immovable sphere surrounding the Sun, but its generalizations did and assumed them to be dispersed throughout the universe (Fig. 3.4); Giordano Bruno even proposed that the universe is infinite containing many worlds like ours where intelligent beings live.

In fact it was Bruno's advocacy of the Copernican system that produced one of the strongest reactions by the Church: Bruno advocated not only the heliocentric model, but denied that objects posses a natural motion, denied the existence of a center of the universe, denying even the Sun of a privileged place in the cosmos. Bruno was executed by the Inquisition in 1600.




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Figure 3.4: The heliocentric model of Thomas Digges (1546-1595) who enlarged the Copernican system by asserting that the stars are not fixed in a celestial orb, but dispersed throughout the universe.  
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The slow progress of the heliocentric model was also apparent among part of the scientific community of the time; in particular Tycho Brahe, the best astronomer of the late 16th century, was opposed to it. He proposed instead a ``compromise'': the earth moves around the sun, but the rest of the planets move around the Earth (Fig. 3.5). Brahe's argument against the Copernican system was roughly the following: if the Earth moves in circles around the Sun, nearby stars will appear in different positions at different times of the year. Since the stars are fixed they must be very far away but then they should be enormous and this is ``unreasonable'' (of course they only need to be enormously bright!)




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Figure 3.5:  Brahe's model of the universe: a central Earth around which the sun moves surrounded by the other planets [From the Compendio di un trattato del Padre Christoforo Borro Giesuita della nuova costitution del mondo secondo Tichone Brahe e gli altri astologi moderni (Compendium of a treatise of Father Christoforo Borri, S.J. on the new model of the universe according to Tycho Brahe and the other modern astronomers) by Pietro della Valle, Risalah- i Padri Khristafarus Burris Isavi dar tufiq-i jadid dunya.
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next up previous contents
Next: Aristotle in the 16th Up: From the Middle Ages Previous: The Middle Ages.

Jose Wudka
9/24/1998