| Chapter 1: | The New Astronomy |
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Here, “stars” refer to the “fixed stars,” which are fixed relative to one another and which form the backdrop against which the “wandering stars,” or Ancient Planets, move.
The Ancient Planets are called “Wanderers” because they seem to wander relative to the stars in the sky. No Ancient Planet moves at a steady rate or follows a circular path. Most egregious of all is that relative to the stars, five of them—Mercury, Venus, Mars, Jupiter, and Saturn—routinely reverse their direction of travel. The Sun and Moon always move from west to east relative to the stars—a fact easily observed in the case of the Moon over a span of an hour or more—but the five renegade wanderers sometimes move in the opposite direction. Accounting for this “retrograde motion” remained the premier problem of astronomy into the seventeenth century (figure 1.2).
Aristotle argued against the notion of a moving Earth on several grounds. If the Earth moved in either rotation or revolution, an observer looking at two stars lying in a particular direction would see them sometimes closer together and sometimes further apart (figure 1.3a). The effect is like that which enables creatures with two or more eyes to see their immediate surroundings in three dimensions. One-half the difference between two viewing directions is called the parallax angle.1 In the case of the Earth, the parallax angle would be larger and easier to
Figure 1.2. Retrograde motion of Saturn relative to background stars.
