Excerpts from Making the Most of One's Mind (1915)



Excerpts from Latitude & The Magnetic Earth (2002)

by Stephen Pumfrey

Chapter 13: Gilbert, Copernicus & the Giant Lodestone

[The] fundamental transformation of astronomy was not the replacement of Ptolemy's geocentric universe with Copernicus' heliocentric one. It was the synthesis of a new science, which we call physical astronomy, out of two traditional disciplines, mathematical astronomy and natural philosophy. Astronomy was different, and inferior to philosophy. This attitude had to change before physical astronomy could take off. Some of those who changed it, like Tycho Brahe, vigorously objected to the Earth's motion. Gilbert was unique—a Copernican who did not want it to change!

The Astronomer's Circles and the Philosopher's Spheres

Gilbert learned the reason why astronomers were inferior at Cambridge. It was one scholastic doctrine he did not entirely reject. Astronomers dealt with planets whose orbits approached the ideal of uniform circular motion, but centuries of observations showed that they deviated from it in a number of small but annoying ways that philosophers could not explain. A few years after Gilbert's death, Johann Kepler made the radical claim that many of the problems disappeared if planets were allowed to accelerate and decelerate in slightly elliptical orbits . . .

[Like their medieval predecessors] Renaissance astronomers were given license to come up with a variety of geometrical constructions, which they called theorics or hypotheses. These had the pragmatic aim of predicting accurately where the heavenly bodies would appear in the sky to an observer on Earth. Unlike the hypotheses of modern physical astronomers, Renaissance astronomers were not concerned whether their theorics were true or not [this approach is known as "saving the appearances"]. Ptolemy's own hypotheses used a geometrical device called an equant, which they knew was physically impossible [and violated the principle of uniform circular motion].

Leaving detail and accuracy to the astronomers, sixteenth-century natural philosophers pontificated about a simpler universe. Their elevated debates concerned the physical causes of the basic heavenly motions. By 1600, only a handful of (primarily mathematical) scholars had converted to Copernicanism. Most philosophers were prompted to restate and develop old reasons for the Earth's immobility . . .

The universe of traditional Aristotelian natural philosophy was a concentric nest of solid spheres, like an onion with the Earth at the center. The heavenly forms gave the property of natural circular motion to the substance of their spheres. Closest to the Earth was the sphere . . . of the Moon . . . Then came the spheres of Mercury, Venus and the Sun, with orbits of a year or less. The sphere of Mars had a period of two years, Jupiter twelve and Saturn thirty. Pressing down on Saturn was the 'sphere of the fixed stars'. This huge sphere rotated every twenty-four hours . . .

Below the boundary of the superlunary [i.e., above the moon] and sublunary [i.e., below the moon] world, of course, a different physics applied—the physics of the four elements that Gilbert detested. Down here, natural motions were rectilinear [i.e., straight lines], not circular. They had beginnings and ends. Endless motion did not befit the Earthly realm of corruption. A fundamental Aristotelian principle was that things only moved 'for the sake of obtaining rest in their natural place'. The only natural motion of elemental earth was a rectilinear motion towards rest at the center of the universe. Ignoring objects like a missile thrown upwards 'against its nature', all earthly things seemed to obey the law.

Can the Earth Move?

In Book VI of De Magnete, Gilbert rehearsed some of the arguments against a moving Earth, which he had found conveniently gathered in Copernicus' book De Revolutionibus Orbium Coelestium published as Copernicus died in 1543. They had their foundations in the doctrine of the single, natural, rectilinear motions of the four elements. They were deployed against the 'semi-Copernican' position that the Earth only rotated diurnally [i.e., daily], which was all that Gilbert defended in De Magnete. They applied equally to Copernicus' annual motion [i.e., yearly], though that raised yet more objections, especially among theologians.

If the Earth rotated on its axis then, even the short time it took for a stone to drop from your hand to the ground, the Earth would have whizzed eastward. The stone would appear to fall in an arc to the west. In any case, if the Earth moved, you would surely be unable to stand. Clouds would race through the sky. Birds would never keep up with their perches when they took off. Buildings would collapse. Indeed, the Earth's entire solid sphere would disintegrate . . .

Galileo later formulated something like our response [and Newton would further develop answers to the problem of a moving Earth]. . .

Before Gilbert, no Copernican had systematically reworked basic matter theory in order to answer the powerful Aristotelian objections [to a moving earth]. Copernicus had made a very unconvincing start. He was an innovative astronomer, but a traditional natural philosopher. He simply stated that if the Earth moved, then earthly things had to move in natural circles. . . It was just as well that Copernicus primarily rested his case upon mathematical not physical arguments. He believed that heliocentric astronomy was more accurate, economical and harmonious. . .

Copernicus was the first physical astronomer of modern times. In his preface to On the Revolutions, he expressed his disgust with the purely predictive role given to contemporary astronomy. He thought it the duty of astronomers to play a part in discovering the true way that God had created the cosmos. A big factor in his revolt was his exposure to Italian Neoplatonism, according to which the Creator had designed the cosmos using geometrical principles. Copernicus eliminated Ptolemy's equant device because it offended those principles [i.e., uniform circular motion]. [For Copernicus] heliocentrism was not a mere hypothesis, it was the physical truth. Mathematicians could determine natural philosophical questions. . .

Unfortunately for the dying Copernicus, a colleague called Osiander inserted an anonymous foreword to On the Revolutions. It contradicted Copernicus' revolutionary vision of the disciplines. It advised readers not to be startled by the book's references to the Earth's motion. . . Osiander's foreword encouraged people in Gilbert's era to treat Copernicanism as just another hypothesis. Some astronomers preferred it. It promised to be more accurate, the equant had been a pig of a calculating device, and Copernicus's treatment of technical problems . . . They called Copernicus 'the Restorer of Astronomy', but they didn't think that the Earth moved. That was physically absurd. And they didn't argue about it, because physics was not their business.

The Rise of Real Copernicanism

Nevertheless, the first people to break ranks and argue publicly that the Earth moved were mathematicians like Copernicus. Men like the young Kepler and Galileo, Simon Stevin in Holland and Thomas Digges in England had the ability to understand his mathematical astronomy. They also shared his belief in the power of mathematics to discover physical truths . . .

The small camp of public Copernicans had something else in common. They did not work in universities, where the subordination of mathematics to natural philosophy was maintained, and reflected in salaries and status. . . [The] first Copernican mathematicians found positions as clients of European nobles and princes. . . .

There was one exception—William Gilbert. In his history of astronomy, Gilbert had praised mathematicians for being clever, hard-working people doing a difficult job. It was so difficult that they were forced to deal only in useful fictions [i.e., "saving the appearances"]. Every time natural philosophers had made their fictions real, as they had done with the heavenly spheres, disastrous errors had ensued. Gilbert believed that astronomers and natural philosophers should stick to their disciplinary roles. When Gilbert praised Copernicus on the very last page of De Magnete as the 'restorer of astronomy', he meant the traditional fictionalist astronomy of Osiander's preface.

Thus Gilbert was the only true Copernican who denied that Copernicus as a revolutionary astronomer. He did not like mathematical arguments for the Earth's motion. And, uniquely among the early Copernicans, he did not need them. He did not need to counter the unanswered arguments of Aristotelian physics with the mathematical superiority of heliocentrism. He had replaced Aristotelian theories of matter and motion with experimental magnetic philosophy. His magnetic philosophy of the Earth gave him all the proofs of the Earth's motion he needed and, as a natural philosopher, all that he would accept. [159-166]

QUESTIONS:

1) What does the author mean when he says that the fundamental transformation in astronomy “not the replacement of Ptolemy's geocentric universe with Copernicus' heliocentric one”? Explain.

2) In what specific ways did William Gilbert differ from other early Copernicans of his age? Explain.

3) Why would a presentist account of these changes in astronomy actually distort what really happened? Explain.

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