A Modern Scientist Considers Smith’s History of Astronomy

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Todd Timberlake for AdamSmithWorks

January 22, 2020
Sometime around the middle of the eighteenth century, Adam Smith began a series of essays demonstrating the principles which lead and direct philosophical inquiry. He only completed one of these essays, The History of Astronomy (hereafter Astronomy), and even that was never published in his lifetime. In spite of the essay’s title, Smith’s Astronomy is not primarily a work of astronomy, or even history. Instead it is best understood as a work of what we would now call the philosophy of science, although no such discipline existed in the eighteenth century.

As a work of philosophy of science, Smith’s Astronomy seems quite modern. When philosophy of science emerged as its own special discipline in the early twentieth century, its primary concern was to provide objective justification for belief in scientific theories (and disbelief in theories deemed unscientific). That is not, however, Smith’s main concern in Astronomy. Instead, he focuses on why new scientific theories are proposed and why we choose one theory over another. Rather than focus on the truth (or not) of the theories themselves, Smith focuses on the human desires that those theories fulfill. It is in this regard that Smith’s work seems modern. He has less in common with the logical positivists of the early twentieth century than with Thomas Kuhn and others in the later twentieth century who took what came to be called the “historical turn” in philosophy of science by emphasizing the development of science and the subjective choices that guided that development.

Smith argues that many natural phenomena are familiar to us and seem to require no explanation, but sometimes unexpected things happen and certain people (namely philosophers) are troubled by these things. To regain their composure natural philosophers re-imagine the world in such a way that the unexpected phenomena become not just unsurprising but actually expected and even necessary. They imagine a chain of connections that link what is familiar to what they found surprising. The best theories link together a wide variety of phenomena by proposing only a small number of imagined, unobserved linkages. Such a system sets the mind of the philosopher at ease, but it also fosters a deeper admiration for the workings of nature (as they are imagined to be). 

Eventually, though, some new observation or new insight generates a fresh surprise, or the existing theory becomes too complicated and the process of re-imagining the world must begin again. To soothe our sense of wonder, each new theory must explain not only the new surprises, but also everything that was explained by the old theory. It must also show that both the new and the old blend together into a single, simple, coherent whole, a grand machine in which all the parts fit together perfectly. Smith judges theories not by their predictive accuracy or their practical benefits, but by their explanatory power. Any theory that makes our minds comfortable with the observed phenomena of nature is a good theory.

It is to lay out this view of the development of science that Smith spends so much time at the beginning of Astronomy discussing surprise, wonder, and admiration. Eventually, though, Smith gives an account of the history of astronomy that is intended to illustrate his philosophy of science. 

His historical account is not without its flaws. Many of those flaws are understandable. For example, his account of the development of spherical astronomy before Eudoxus is more of a “just so story” than a true history, but even today that bit of history is shrouded in the gloom of insufficient documentary evidence. Smith might have been better served to admit ignorance, but he was not a historian in the sense we now understand that term and his goal was philosophical, not historical. He can be forgiven. Likewise, Smith’s account of Eudoxus’ mechanism for retrograde planetary motion is garbled, but even astronomers didn’t understand what Eudoxus had done until Schiaparelli published a reconstruction of the theory in 1877. Smith also claims that Aristotle added additional spheres to the Eudoxan model as a result of “more attentive observation” (59), but in fact Aristotle’s extra spheres produced no visible effects, instead providing a physical mechanism for disentangling the motions of different spheres. Less forgivable is Smith’s misleading account of the Tychonic system, in which he claims that it was more complicated than the Copernican system (it wasn’t), and that it was not widely embraced (it was). 

Occasionally he neglects his fellow Britons, failing to give Thomas Digges credit for promoting an infinite universe long before Descartes, and Thomas Harriot for doing telescopic astronomy a bit before Galileo, but he may have been unaware of their work. He is understandably complimentary of Isaac Newton, but he goes too far in claiming that Newton’s theory of the Moon’s motion was even more accurate than his theory of the planets. About the time Smith wrote Astronomy, French astronomers like Alexis Clairaut were struggling to explain the observed motion of the Moon’s apogee using Newton’s principles.

None of these errors is particularly damning for Smith’s true philosophical enterprise. More troubling, though, are some errors and omissions that relate more directly to the human motivations for doing science and for choosing one scientific theory over another. Smith goes out of his way to show that scientific theories are developed to solve philosophical problems – to allay wonder about the nature of things – rather than to solve practical problems. He doesn’t mention the practical usefulness of the ancient spherical astronomy even though it can explain and predict the seasonal variations of sunlight for any location on Earth. Smith may have felt that these seasonal variations were already so familiar that spherical astronomy contributed nothing of practical, rather than philosophical, worth. 

It is harder, though, to explain his failure to mention astrology as a motivation for the development of accurate planetary theories. From the classical to early modern periods, astrology served as the practical side of astronomy. Astronomers may well have been motivated to develop new astronomical theories in order to satisfy philosophical needs, but they also worked at these new theories in hopes of casting more accurate horoscopes. Perhaps it was just the Enlightenment distaste for astrology that led Smith to neglect this aspect of the history of astronomy, but it may also be that astrological motivations did not fit into Smith’s philosophical narrative. (To be fair to Smith, he never claims that scientific theories are proposed ONLY to solve philosophical problems – but he is only concerned with that aspect of science.)

Smith also makes a bit too much of the divide between natural philosophers and astronomers. That divide did exist, and played an influential role in the development of astronomy, in medieval and early modern Europe. But when Smith claims that Georg Puerbach was the first to try to unify Aristotelian spherical cosmology with Ptolemaic astronomy, he is mistaken. Ptolemy himself attempted such a unification in his Planetary Hypotheses and a few Arabic astronomers made similar attempts. 

Smith also exaggerates this divide when it comes to the reception of  Copernicus's heliocentric theory. Smith claims that astronomers were eager to embrace a new theory (“when you have convinced the world, that an established system ought to be corrected, it is not very difficult to persuade them that it should be destroyed” (71) and that they readily adopted the Copernican system, while natural philosophers resisted  Copernicus's new theory because they could not accept the idea of a moving Earth. In fact, very few astronomers adopted  Copernicus's heliocentrism. Many astronomers did eagerly adopt  Copernicus's mathematical techniques, his observations, and his data tables, but they rejected the motion of the Earth just like the natural philosophers did. 

In fact, very few astronomers really believed in the reality of  Copernicus's theory when it was first introduced, and we can use Smith’s philosophy of science to understand why. There were no new and surprising phenomena that the Ptolemaic theory failed to account for but which Copernicus could explain. Smith correctly notes that the Copernican theory was only slightly more accurate than the Ptolemaic, and even that advantage stemmed from  Copernicus's new observations rather than from advantages inherent in his new theory. The Ptolemaic theory could have been updated with the new observations, and it would have been just as good as the Copernican theory. Smith implies that the Ptolemaic theory had gotten too complicated, but in fact it was no more complicated than the Copernican theory. Rather, it was a subtle incoherence in the Ptolemaic theory, recognizable only to those trained in astronomy, but of importance only to those who were concerned with natural philosophy, that led some to embrace  Copernicus's radical idea of a moving Earth. These few philosophical astronomers could appreciate the greater explanatory power of the Copernican system to the extent that they were willing to believe in seemingly impossible motions of the Earth. 

In spite of these historical inaccuracies, Smith’s Astronomy is a much better historical account than we might expect for the eighteenth century. His emphasis on the subjective aspects of science leads him to judge scientific theories according to how they functioned in their historical context, rather than by how closely they match our current scientific knowledge. As a result, Smith’s Astronomy is much more sympathetic to the past than are some other Enlightenment historical accounts. For example, he gives a very good explanation of why natural philosophers were initially so opposed to the Copernican theory. He shows that these philosophers were fully justified in rejecting heliocentrism at that point in history because it directly contradicted the accepted (Aristotelian) physics of the time.

Smith’s sympathetic view of the past leads him to some excellent historical insights. For example, he notes that the development of spherical astronomy led to the notion of a spherical Earth. Indeed, the early arguments for a spherical Earth, such as those found in Aristotle’s writings, are based mostly on spherical astronomy. Smith also recognizes that theories need only be as accurate as the data they are intended to explain. Without accurate observational data, small errors in a theory will not be noticed and thus astronomers will not be troubled to devise a better theory. Smith raises this point in his discussion of ancient astronomy before Eudoxus, but it is no less important for understanding the transition from Eudoxus to Ptolemy, or from Copernicus to Kepler.

The historical development of science is not a straight-line march from ignorance to perfect knowledge. There are plenty of errors and dead ends along the way, and steps that we now judge as progress might have been judged quite differently when they were first taken. Although the elliptical astronomy of Kepler was, in hindsight, a great step forward, Smith is careful to point out that Copernicus himself would not have approved of Kepler’s new astronomy. Moreover, Smith correctly states that Kepler’s new astronomy was too abstract to be easily pictured in the mind. Thus it was not until Giovanni Domenico Cassini’s observations of the moons of Jupiter and Saturn, which showed that these moons followed the same laws that Kepler had proposed for the planets, that Kepler’s new astronomy was fully embraced. Smith also correctly notes that it was Descarte’s system of natural philosophy that really won widespread acceptance for  Copernicus's heliocentric idea, even though the Cartesian system was unsatisfying for astronomers because it could not reproduce Kepler’s laws. What the Cartesian system did was to provide a mental picture of the invisible linkages that could cause the planets to orbit the Sun. That mental picture was able to transform wonder into admiration more readily than Kepler’s abstract mathematical astronomy.

By the time Smith wrote Astronomy, however, the Cartesian system had been overthrown by Newtonian physics. Yet, Newtonian physics is just as abstract and mathematical as Kepler’s astronomy. Newton never provided a clear mental picture of invisible linkages to explain the motion of the planets. Instead he showed that a single universal gravitational principle, expressed as a mathematical law, could reproduce not only the motion of the planets but also the motion of comets, the fall of bodies near the Earth, the tides, and other phenomena. Smith argues that Newton’s physics serves to satisfy the mind because it leads to strict mathematical laws that agree with observations in a wide variety of contexts, because it deals with a phenomenon (gravity) that is already very familiar to us, and because it works in a way that is analogous to other things we know (like the propagation of light). Cartesians like Leibniz criticized Newton for introducing “occult qualities” by proposing that gravity can act at a distance over empty space, but Smith dismisses this criticism by saying that the Cartesians had simply gotten attached to thinking about the world in a certain way. By the time Smith wrote Astronomy the scholarly world had adapted itself to Newtonian thinking.

In today’s terms Smith might be considered an anti-realist because of his emphasis on the subjective aspects of theory choice, but, in his discussion of Newton, Smith ventures close to realism. After his discussion of Newtonian physics, Smith remarks: 
And even we, while we have been endeavouring to represent all philosophical systems as mere inventions of the imagination, to connect together the otherwise disjointed and discordant phaenomena of nature, have insensibly been drawn in, to make use of language expressing the connecting principles of this one, as if they were the real chains which Nature makes use of to bind together her several operations. Can we wonder then, that it should have gained the general and complete approbation of mankind, and that it should now be considered, not as an attempt to connect in the imagination the phaenomena of the Heavens, but as the greatest discovery that ever was made by man, the discovery of an immense chain of the most important and sublime truths, all closely connected together, by one capital fact, of the reality of which we have daily experience. (105)

Smith’s skepticism falters in the face of the Newtonian achievement, but perhaps he should have stuck to his skeptical stance. After all, the twentieth century would show that Newton’s physics was not the final word, and that even Newton’s notion of universal gravitation could be improved upon and perhaps put in a form that would have been more acceptable to Leibniz and the Cartesians. Although Einstein’s General Relativity is even more abstrusely mathematical than Newton’s physics, it does provide a mental picture for the operation of gravity, in the form of distorted spacetime, that Newton never supplied.

Smith’s endorsement of Newtonian physics is one of the few places in Astronomy where he makes a positive claim about reality rather than providing a historical account of the actions and choices of others. However, there are a few other places in his essay where Smith’s skepticism fails him and he makes positive claims that don’t hold up well today. He makes the interesting claim that spots like those seen on the Moon were discovered on each planet, and that observations of these spots demonstrated that each planet rotated on its own axis. It is true that the planets do rotate, and the aforementioned Cassini had used observations of spots on Jupiter and Mars to estimate their rotational periods. However, in spite of some earlier claims, efforts to observe the rotation of the other planets (including Uranus and Neptune, both of which were discovered after Smith wrote Astronomy) were not successful until the twentieth century. That argument by analogy, though, was persuasive. If Earth, Mars, and Jupiter rotate then so must the other planets.

In a similar vein, Smith accepts the idea that the density of the planets decreases with distance from the Sun. This idea goes back at least to Kepler and it was given support in Newton’s Principia, where Newton was able to estimate the relative densities of Earth, Jupiter, and Saturn to show that Earth was the most dense and Saturn the least. However, Newton’s method could only be used for planets that had orbiting Moons and thus it could not be applied to Mercury or Venus (which have no moons), nor to Mars (whose moons were not discovered until 1877). We now know that the densities of the planets do not follow any such pattern. Smith may have been motivated to accept this idea because it seemed to fit with the notion, widely accepted in the mid-eighteenth century, that all planets in the solar system were habitable and likely inhabited, although probably by beings of somewhat different composition. Smith’s contemporary Immanuel Kant made the decrease of planetary density with distance from the Sun a key part of his argument for the habitability of the planets in his 1755 Allgemeine Naturgeschichte und Theorie des Himmels.

What, then, are we to make of Smith’s Astronomy? We should view it with the same sympathy that he gives to astronomical theories of the distant past. The important historical and philosophical insights in Smith’s essay should not be overshadowed by his occasional errors or omissions. His account of the development of astronomy is largely correct, and it serves well to illustrate his philosophy of science. Most impressively, Smith’s emphasis on the subjective aspects of theory choice mirrors the positions of some of today’s most prominent philosophers of science. As a scholarly work on the history of astronomy, Smith’s essay would soon be completely supplanted by, for example, the work of Delambre. Even so, philosophers as late as the early twentieth century would have done well to heed Smith’s insights into the nature of scientific inquiry.