Answer:What one believes about the nature of the universe and one's own place in it is not a trivial matter, and it has profound consequences for how an individual thinks about all aspects of life. Through the teaching that I have done over many years at Knox College (especially in the now defunct First-Year Preceptorial course on "Nature and Culture" and in "History & Systems of Psychology"), I have developed a strong interest in the history of cosmologies, especially the worldview of Medieval Europeans. I am especially interested in how their beliefs about the nature of the physical universe evolved from earlier Greek philosophies and how these beliefs became entangled with Christian Theology through a series of historical coincidences. [See Plato, Aristotle, Christianity, & Knowledge for more details.]
From the early through the late Middle Ages, Europeans moved from a disorganized, almost mystical way of thinking about the universe to an acceptance of a well-ordered, geocentric universe based upon the ideas of Greek philosophers such as Ptolemy and Aristotle. In this universe, the Earth was at the center and other heavenly bodies rotated around it in a series of concentric spheres (see the diagram to the left). The entire system was powered by the primum mobile, or "Prime Mover," which was the outermost sphere set in motion directly by God.
It took some very creative thinking to make this universe work well. For example, the retrograde motion of the planets in which they sometimes seemed to be changing directions and moving backwards was explained by way of "epicycles" (see the diagram on the right below). Specifically, it was proposed that the planets rotated around a center point fixed in place on the sphere of that planet, causing the apparent change in the direction of planetary motion.
Since medieval Europeans had no conception of a vacuum, it was believed that the heavens were filled with a celestial fluid that flowed as the spheres of the universe rotated, thus sustaining the motion of the planets. Furthermore, all of this motion created a beautiful "music of the spheres" which could not be detected by humans (at least not until after they died and went to heaven), but which provided pleasure for angels and other supernatural beings.
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