2.2 The Geocentric Universe
|Figure 2.4 Planetary Motion Most of the time, planets move from west to east relative to the background stars. Occasionallyroughly once per yearhowever, they change direction and temporarily undergo retrograde motion before looping back. The main figure shows an actual retrograde loop in the motion of the planet Mars. The inset above depicts the movements of several planets over the course of several years, as reproduced on the inside dome of a planetarium. The motion of the planets relative to the stars (represented as unmoving points) produces continuous streaks on the planetarium "sky." (Museum of Science, Boston)|
Like the Moon, the planets produce no light of their own; instead, they shine by reflected sunlight. Ancient astronomers correctly reasoned that the apparent brightness of a planet in the night sky is related to its distance from Earthplanets appear brightest when closest to us. However, the planets Mars, Jupiter, and Saturn are always brightest during the retrograde portions of their orbits. The challenge facing astronomers was to explain the observed motions of the planets and to relate those motions to the variations in planetary brightness.
The earliest models of the solar system followed the teachings of the Greek philosopher Aristotle (384322 B.C.) and were geocentric, meaning that Earth lay at the center of the universe and all other bodies moved around it. (Figures 1.7 and 1.10a illustrate the basic geocentric view.) (Sec. 1.2) These models employed what Aristotle, and Plato before him, had taught was the perfect form: the circle. The simplest possible descriptionuniform motion around a circle with Earth at its centerprovided a fairly good approximation to the orbits of the Sun and the Moon, but it could not account for the observed variations in planetary brightness or their retrograde motion. A more complex model was needed to describe the planets.
Figure 2.5 Geocentric Model In the geocentric model of the solar system, the observed motions of the planets made it impossible to assume that they moved on simple circular paths around Earth. Instead, each planet was thought to follow a small circular orbit (the epicycle) about an imaginary point that itself traveled in a large, circular orbit (the deferent) about Earth.
However, as the number and the quality of observations increased, it became clear that the simple epicyclic model was not perfect. Small corrections had to be introduced to bring it into line with new observations. The center of the deferents had to be shifted slightly from Earths center, and the motion of the epicycles had to be imagined uniform with respect to yet another point in space, not Earth. Around A.D. 140, a Greek astronomer named Ptolemy constructed perhaps the best geocentric model of all time. Illustrated in simplified form in Figure 2.6, it explained remarkably well the observed paths of the five planets then known, as well as the paths of the Sun and the Moon. However, to achieve its explanatory and predictive power, the full Ptolemaic model required a series of no fewer than 80 distinct circles. To account for the paths of the Sun, Moon, and all nine planets (and their moons) that we know today would require a vastly more complex set. Nevertheless, Ptolemys text on the topic, Syntaxis (better known today by its Arabic name Almagest"the greatest"), provided the intellectual framework for all discussion of the universe for well over a thousand years.
Figure 2.6 Ptolemys Model The basic features, drawn roughly to scale, of the geocentric model of the inner solar system that enjoyed widespread popularity prior to the Renaissance. The planets deferents were considered to move on spheres lying within the celestial sphere that held the stars. The celestial sphere carried all interior spheres around with it, but the planetary (and solar) spheres had additional motions of their own, causing the Sun and planets to move relative to the stars. To avoid confusion, partial paths (dashed) of only two planets, Venus and Jupiter, are drawn here.
Actually, history records that some ancient Greek astronomers reasoned differently about the motions of heavenly bodies. Foremost among them was Aristarchus of Samos (310230 B.C.), who proposed that all the planets, including Earth, revolve around the Sun and, furthermore, that Earth rotates on its axis once each day. This, he argued, would create an apparent motion of the skya simple idea that is familiar to anyone who has ridden on a merry-go-round and watched the landscape appear to move past in the opposite direction. However, Aristarchuss description of the heavens, though essentially correct, did not gain widespread acceptance during his lifetime. Aristotles influence was too strong, his followers too numerous, his writings too comprehensive. The geocentric model went largely unchallenged until the sixteenth century A.D.
The Aristotelian school did present some simple and (at the time) compelling arguments in favor of their views. First, of course, Earth doesnt feel as if its moving. And if it were, wouldnt there be a strong wind as we move at high speed around the Sun? Then again, considering that the vantage point from which we view the stars changes over the course of a year, why dont we see stellar parallax? Nowadays we might be inclined to dismiss the first two points as merely naïve, but the third is a valid argument and the reasoning is essentially sound. We now know that there is stellar parallax as Earth orbits the Sun. However, because the stars are so distant, it amounts to less than 1", even for the closest stars. Early astronomers simply would not have noticed it. We will encounter many other instances in astronomy where correct reasoning has led to the wrong conclusions because it was based on inadequate data.