| The view of the Universe held by astronomers as little as 500 years ago differed substantially from our modern view. We shall term this earlier worldview the "Old Astronomy". |
| | The Universe of Aristotle and Ptolemy |
| | The celestial sphere that we introduced previously is a convenient fiction to locate objects in the sky. However, the Greek philosopher Aristotle (many of Aristotles works are available at the Internet Classics Archive) proposed that the heavens were literally composed of 55 concentric, crystalline spheres to which the celestial objects were attached and which rotated at different velocities (but the angular velocity was constant for a given sphere), with the Earth at the center. The following figure illustrates the ordering of the spheres to which the Sun, Moon, and visible planets were attached. |
| | (The diagram is not to scale, and the planets are aligned for convenience in illustration; generally they were distributed around the spheres.) There were additional "buffering" spheres that lay between the spheres illustrated. The sphere of the stars lay beyond the ones shown here for the planets; finally, in the Aristotelian conception there was an outermost sphere that was the domain of the "Prime Mover". The Prime Mover caused the outermost sphere to rotate at constant angular velocity, and this motion was imparted from sphere to sphere, thus causing the whole thing to rotate. |
| | By adjusting the velocities of these concentric spheres, many features of planetary motion could be explained. However, the troubling observations of varying planetary brightness and retrograde motion could not be accommodated: the spheres moved with constant angular velocity, and the objects attached to them were always the same distance from the earth because they moved on spheres with the earth at the center. |
| | Epicycles and Planetary Motion |
| | The "solution" to these problems came in the form of a mad, but clever proposal: planets were attached, not to the concentric spheres themselves, but to circles attached to the concentric spheres, as illustrated in the adjacent diagram. These circles were called "Epicycles", and the concentric spheres to which they were attached were termed the "Deferents". Then, the centers of the epicycles executed uniform circular motion as they went around the deferent at uniform angular velocity, and at the same time the epicyles (to which the planets were attached) executed their own uniform circular motion. |
| | The net effect was as illustrated in the following animation. As the center of the epicycle moves around the deferent at constant angular velocity, the planet moves around the epicycle, also at constant angular velocity. The apparent position of the planet on the celestial sphere at each time is indicated by the line drawn from the earth through the planet and projected onto the celestial sphere. The resulting apparent path against the background stars is indicated by the blue line. |
| | Now, in this tortured model one sees that it is possible to have retrograde motion and varying brightness, since at times as viewed from the earth the planet can appear to move "backward" on the celestial sphere. Obviously, the distance of the planet from the Earth also varies with time, which leads to variations in brightness. Thus, the idea of uniform circular motion is saved (at least in some sense) by this scheme, and it allows a description of retrograde motion and varying planetary brightness. |
| | Medieval Aristotelian Astronomy |
| | By the Middle Ages, such ideas took on a new power as the philosophy of Aristotle (newly rediscovered in Europe) was wedded to Medieval theology in the great synthesis of Christianity and Reason undertaken by philsopher-theologians such as Thomas Aquinas. The Prime Mover of Aristotle's universe became the God of Christian theology, the outermost sphere of the Prime Mover became identified with the Christian Heaven, and the position of the Earth at the center of it all was understood in terms of the concern that the Christian God had for the affairs of mankind. |
| | Thus, the ideas largely originating with pagan Greek philosophers were baptized into the Catholic church and eventually assumed the power of religious dogma: to challenge this view of the Universe was not merely a scientific issue; it became a theological one as well, and subjected dissenters to the considerable and not always benevolent power of the Church. |
| | More Sophisticated Epicycles: The Ptolemaic Universe |
| | These ideas concerning uniform circular motion and epicycles were catalogued by Ptolemy in 150 A.D. His book was called the "Almagest" (literally, "The Greatest"), and this picture of the structure of the Solar System has come to be called the "Ptolemaic Universe". |
| | That ancient astronomers could convince themselves that this elaborate scheme still corresponded to "uniform circular motion" is testament to the power of three ideas that we now know to be completely wrong, but that were so ingrained in the astronomers of an earlier age that they were essentially never questioned: |
| | All motion in the heavens is uniform circular motion. |
| | The objects in the heavens are made from perfect material, and cannot change their intrinsic properties (e.g., their brightness). |
| | The Earth is at the center of the Universe. |
| | However, in practice, even this was not enough to account for the detailed motion of the planets on the celestial sphere! In more sophisticated epicycle models further "refinements" were introduced: |
| | In actual models, the center of the epicycle moved with uniform circular motion, not around the center of the deferent, but around a point that was displaced by some distance from the center of the deferent. |
| | In some cases, epicycles were themselves placed on epicycles, as illustrated in the adjacent figure. |
| | The Apparent Motion of Planets on the Celestial Sphere |
| | Two observations concerning the planets were very difficult to explain for astronomers of the Middle Ages: |
| | Both retrograde motion and varying brightness are illustrated in the adjacent animation. |
| | A major reason for the difficulty of explaining these features was the dominance of the Greek philosopher Aristotle on medieval thought. This philosophy held that the heavens were more perfect than the Earth, and that objects in the heavens were unchanging. As part of this philosophy, it was believed that the only motion permitted objects in the heavens was uniform circular motion (motion at constant angular speed on a circle), because such motion brought one back cyclically to the starting point and therefore was (in a sense) unchanging. |
| | This philosophy had no problem with the direct motion of the planets, since that could be explained by uniform circular motion of the planets about the Earth (which, as any fool who looked at the sky could see, was clearly the center of the Universe). However, neither the varying brightness of the planets, nor the occasional retrograde motion of the planets on the celestial sphere, were easily reconciled with this idea of unchanging objects executing uniform circular motion in the heavens. |
| | The usual motion of planets as they "wandered" on the celestial sphere was eastward against the background stars. This is called "Direct" Motion". However, it was observed that at times the planets moved westward for some period on the celestial sphere; this was termed "Retrograde Motion". The episodes of retrograde motion were difficult to explain. |
| | The planets were observed to be brighter at certain times than others. This varying brightness was also a challenge to explain. |
| | Internet Classics Archive |
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