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EARLY COSMOLOGY TUTORIAL TOPICS Dr. Brian Monson has created this group of pages as a supplement for the unit on early astronomy that is taught in most college astronomy courses and has graciously granted ScienceMaster permission to republish them. It is by no means a complete discussion of this topic. To explore more work by Dr. Monson please visit his Planetary Conjunctions page. |
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Aristotle
Aristotle (384 - 322 B.C.) developed an early model of the cosmos based on the concept of uniform circular motion. To account for the motions of the stars, sun, moon, and the five known planets, his model used 56 spherical shells each centered on the earth. These shells were divided into two regions: the realm of change near the earth and the eternally unchanging heavens. The realm of change consisted of spheres of each of the four classical elements: earth, water, air, and fire. The heavens were made of an unchangeable, transparent material called the ether.
Aristotle divided motion into forced and natural motions. Forced motions require a "push" from something to occur while natural motions do not. In the realm of change, the natural motion of earthy materials was to seek the center of the universe. This is why Aristotle placed the earth at the center of the cosmos. This is also his explanation for why objects fall when dropped. A dropped object is just following its natural tendency to seek the center of the universe. In the eternal realm, the natural motion of a sphere was to rotate. Since the heavenly bodies are all attached to spheres, they would rotate about the center of the universe without any forces applied to them. The stellar sphere rotated once per day and dragged all the other spheres along with it. Each of the planetary spheres rotated slowly from west to east to produce the planets' motions around the ecliptic.
The idea of an earth-centered cosmos may seem alien to you, but given the state of knowledge in those days a geocentric model was a reasonable conclusion. If you go outside and watch the sky, it certainly looks as if everything circles around the earth. Aristotle had some other reasons for placing the earth at the center. Since the earth is made of earthy matter, it would naturally move to the center of the universe. Also, if the earth were not at the center, it would have to revolve around the center and spin on its axis to produce the observed motions of the heavenly bodies. Aristotle reasoned that if the earth rotated about its axis, we should fly off into space. Since we don't, the earth must be stationary. He also reasoned that if the earth revolved about the center of the cosmos, an object that is thrown vertically upward would not fall back to the point from which it was launched. Instead it should fall behind the launch point because the earth would move out from under it. It would be almost 1900 years before Galileo introduced the concepts of gravity and inertia that explain why these effects are not observed even though the earth does move.
Aristotle also reasoned that if the earth moved about the center, an effect called stellar parallax would occur. Parallax is the change in the angular separation of a pair of stars as the earth orbits the sun. When the earth is closest to the stars (Point A at right) , the separation is larger than when we are far away from the stars (Point B). The absence of a detectable stellar parallax led Aristotle to make the earth stationary. In fact, he had grossly underestimated the distances from the earth to the stars and therefore expected the parallax to be much larger than it is. Even the closest stars have parallaxes less than 1 arcsec, much too small to be seen with the unaided eye. The first measurement of a stellar parallax was not made until 1838 by the German astronomer Friedrich Bessel.
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