§34. The Starry Messenger
A school of so-called calculatores arose in Ockham's wake, preeminently Thomas Bradwardine, Richard “the Calculator” Swineshead, and Nicole Oresme. They investigated problems of motion and acceleration, including experiments with pendulums.
They tried to test the Peripatetic Law that velocity is proportional to the power of the mover divided by the resistance of the medium (heavier things fall faster), using balls on an inclined plane, as Galileo would later. The work made little progress, in part because it was difficult to connect their primitive calculations and measurements to changes their primitive apparatus could register.It was also difficult to accept that nature might be measured with arbitrary units, or that merely approximate calculations could promote scientific knowledge. No measure is true to nature except God’s own, and it is unknowable. The argument is Platonic rather than biblical. In the Bible, God gave Noah and Solomon the measures they required for their constructions. To Plato the idea is barbaric. A measure “which in any degree falls short of the whole truth is no fair measure; for nothing imperfect is the measure of anything, although persons are too apt to be contented and think that they need search no further.” Calculation with arbitrary units, approximate values, and fallible margins of error is an affront to the dignity of science. An exact science of nature remained out of reach until this ancient ideal of exactitude lost its appealing consistency.86
The universities of north Italy and preeminently Padua were at this time home to secular philosophy. The blend of Aristotle and Christianity characteristic of Thomas and Duns Scotus at Paris had retreated to the monastic orders, and Padua, under Venice from 1404, was the leading anti-papal and anticlerical state. It was also home to the leading scientific school of Europe and a stronghold of Aristotelian empiricism.
Albertus, Cusanus, Witelo, Regiomontanus, Telesio, Copernicus, Cardano, and Harvey all studied at Padua, which was also the leading Renaissance center for the study of medicine, with the largest assembly of medical students and teachers in Europe. Unlike the universities of northern Europe, study in the arts faculty at Padua was not a preparatory stage for aspirants to the higher faculties, but was tied to one specific higher faculty, namely medicine. On the principle that all the arts are necessary to medicine, medical students at Padua were required to study the liberal arts, as well as Aristotle’s natural philosophy, which was often taught by a physician, and emphasized the physical treatises, natural histories, and methodology.87In 1592, when he was twenty-eight years old, Galileo obtained a position at Padua as a teacher of mathematics, where he remained for the next twentyeight years. Galileo’s significance for modern science lies in his discoveries in mechanics and astronomy, but also in his lessons on method in natural philosophy. He favored piecemeal problem-solving, setting aside the elusive goal of completeness. Natural philosophers should learn to live with unsolved problems and be not chary to admit they do not know. The innovation of Galilean natural philosophy is careful measurement. Find ways to reduce the phenomena to number—that is the beginning of all his discoveries. It had long been observed that falling bodies accelerate, but until Galileo the rate was unknown. It had long been observed that projectiles followed a curved path, but until Galileo the parabolic trajectory was unknown. Galileo urged inquirers to set aside explanations in terms of qualities and putative causes— why things behave as they do—and take up the question of how? and especially by how much?
In the summer of 1609 Galileo heard about an optical device from Holland capable of making distant objects appear close. He did not have details but figured that it had to be a combination of concave and convex lenses, and he set out to make one.
By August he had his first telescope. At a demonstration in Venice he was able to descry approaching ships two hours before they were visible to trained observers without the instrument. By December he had a twenty-power telescope and began a program of nightly observations. He soon saw the mountains and craters of the moon, and early in January 1610, four moons orbiting Jupiter. That year he published a small book, Sidereus nuncius (Starry Messenger), in which he described his observations with the still little-understood instrument, and enrolled the discoveries in an argument for the new heliocentric system of Copernicus.88The book made Galileo famous in Italy and eventually in Europe. News of the Dutch invention had spread rapidly and many instruments were constructed, though it did not occur to anybody to direct their enhanced gaze toward the heavens, or if they did they did not publish a book about it. People understood that the new instrument allowed one to see more clearly what was difficult to see, like ships on the horizon, but there was no reason to anticipate seeing things that had never been seen before. Galileo's genius lay in his idea of the telescope as an instrument for a new kind of observation. It is not the same old seeing when you see things that cannot be seen at all without the cooperation of an artifact.89
Within a few months he had discovered more about the heavens than had ever been known before, including unsettling, unacceptable facts. The irregularity of the moon's surface made it earthlike, which violated Aristotle's segregation of heaven and earth (Democritus made the same observation without a telescope). When Galileo detected points of light in an otherwise shadowy portion of the lunar terminator, he conjectured that they were reflections from mountain peaks. Using trigonometry he calculated their height to be about five kilometers. He constantly compares lunar and terrestrial phenomena, driving home the point of similarity.
“Its supposed immaculacy must yield to observation,” he insisted two years later, in his Letters on Sunspots. “They will philosophize better who give assent to propositions that depend upon manifest observations, than they who persist in opinions repugnant to the senses and supported only by probable reasons.”90Galileo’s telescopes were unruly artifacts, difficult to construct. He needed one strong concave lens and one weak convex or concave lens. Artisan grinders did not routinely produce such pairs and were ill-disposed to accommodate a fussy customer, so Galileo had to grind his own lenses, a task demanding both skill and labor. However, by grinding his lenses he was able to assemble a pair that greatly enhanced then-possible magnification, from about 3X to 20-30X. He also added aperture stops to his objective lens, reducing interference caused by the brightness of celestial objects. The type of telescope he made had a narrow field of view, which made it especially sensitive to movement and required a steady mount. Something as distant as Jupiter would be difficult to find and difficult to keep in view.
The scene in that lens of a night would be tiny distant lights against blackness, frantically slipping in and out of focus, to which we can add the constant need to wipe fog off the lenses during Galileo’s winter observations early in 1610. The optical quality of these first telescopes was necessarily poor, there being as yet no correction for chromatic aberration. In passing through the lens light breaks up into the colors of the spectrum, which focus at different lengths, making images of bright objects like stars ill-defined and haloed in prismatic color. Combined with jitters and slipping in and out of focus, one can understand how the whole performance smacked of optical illusion (familiar from magic), especially when the instrument was directed on things no one had ever seen under “normal” conditions.
Conflict with naked-eye observation prompted Galileo to seek reasons for trusting his telescope though his skeptics urged the same points as reason to dismiss the Starry Messenger.
For instance, in the telescope both Venus and Mars show pronounced variations of brightness invisible to the unaided eye. Opponents used such discrepancies to argue that the telescope produces illusions, but Galileo argued the other way, that it is the unaided eye that produces illusions which the telescope eliminates.91He observes how features of the eye produce distortions, for instance, by the moisture that covers the pupil, and by reflections from the edges of the lids that shine on the pupil. By enlarging the image to fill the eye, the telescope excludes these adventitious rays. Stars in the telescope seem not to be magnified only because with the naked eye their image receives an illusory magnification by stray fringes of light. The telescope eliminates these and does enlarge the apparent size of the star if we discount the incorrect impression produced by naked-eye observation. Galileo experimentally confirms that some of the fringes causing the apparent enlargement of fixed stars are in the eye, not the object. He squints and compresses his eye and notes changes to the fringe; he tilts his head and observes that the orientation of the fringe changes with no change in the rays from the object. The fringes cannot belong to the object because they change with changes in the observer’s eye, a mode of argument as old as Theaetetus.
So the telescope does more than show things that are too distant for naked-eye observation. It corrects the misleading appearances that things can have for that ancient way of seeing. The idea that the senses are subject to error is an old one, but not the idea that once this is recognized we can construct instruments to correct them. With that we begin to think of the senses themselves as instruments that operate in ways analogous to instruments we construct. The telescope “operates upon the stars in such a way [as to circumvent] the irradiation which disturbs the naked eye and impedes precise perception.” It is better than an eye.
The eye is not an immediate source of information, but is instead an instrument whose output must be interrogated, evaluated, even corrected.Kepler reached that conclusion some years earlier, when using a camera obscura to observe an eclipse. Astronomers must recognize that “some deception of sight intrudes [into their observations], arising partly from the art of observing [e.g., small apertures]... and partly from simple vision itself; and this deception, so long as it is not taken into account, creates great difficulties for investigators and detracts from the ability of the art to judge [properly].” Therefore, astronomers, “take note of this: that one must not trust the sense of sight.”92
Modern astronomical research may seem to attenuate the visual in favor of evidence remote from the naive observation of the night sky. With astrophotography, spectroscopy, radio astronomy, and satellite and high-altitude balloon observations, the objects of astronomy have become laboratory constructions. Nevertheless, optical telescopes remain crucial to this science and still provide more information than any other technique, for the simple reason that “stars radiate most of their energy as visible light and the earth’s atmosphere is transparent to this radiation.”93