| Chapter 1: | The New Astronomy |
them compared to the same configuration in the geocentric model. The heliocentric hypothesis also explains other phenomena that the Old Astronomy could not without a slew of ad hoc hypotheses. For example, the apparent directional affinity of Mercury and Venus for the Sun (see figure 1.5) and the diminishing angular sizes of retrograde loops of the planets Mars, Jupiter, and Saturn are readily explained, the latter owing to the fact that apparent changes in the directions of ever more distant objects are progressively less affected by the motion of the Earth.
Copernicus realized that the stars could lie at different distances, but like Ptolemy he opted for a bounding sphere of stars and left it “to the philosophers” to determine the finitude of the Universe (Copernicus 17). However, by adhering to the concept of a revolving Earth, Copernicus had to make the sphere of stars very large because otherwise he would have observed a parallactic shift in their directions (figure 1.3b). If the stars were relatively close by, their angular separations would appear to increase and decrease as the Earth approached and receded from them, whereas the farther the stars are, the less pronounced the effect.
The lack of observed parallax meant that Copernicus either had to abandon the theory of heliocentricism or conclude that stars were much farther away than his predecessors had assumed. To Copernicus, heliocentricism was both mathematically and aesthetically pleasing because it answered questions about the planets efficiently, so the option of placing the stars far away was preferable to abandoning heliocentricism. Copernicus called the void between what was then the outermost planet, Saturn, and the now indeterminately large sphere of stars an immensum. This option faced strenuous opposition from philosophers and religious scholars who believed in the doctrine of First Cause (which stated that the Universe contained all possible existents) because to these sages, the empty space of the immensum served no purpose.
In about 1529, Copernicus circulated a paper titled Commentariolis (“Little Commentary”) stating the essence of his new theory. In 1533, after the paper had made its way to Rome, Church leaders encouraged Copernicus to publish his theory in full, but Copernicus was loath to comply while he still had years to live. In 1531 actors had ridiculed him