The 100 Most Influential Scientists of All Time (9 page)

Galileo also had discovered the puzzling appearance of Saturn, later to be shown as caused by a ring surrounding it, and he discovered that Venus goes through phases just as the Moon does. Although these discoveries did not prove that the Earth is a planet orbiting the Sun, they undermined Aristotelian cosmology: the absolute difference between the corrupt earthly region and the perfect and unchanging heavens was proved wrong by the mountainous surface of the Moon, the moons of Jupiter showed that there had to be more than one centre of motion in the universe, and the phases of Venus showed that it (and, by implication, Mercury) revolves around the Sun. As a result, Galileo was confirmed in his belief, which he had
probably held for decades but which had not been central to his studies, that the Sun is the centre of the universe and that the Earth is a planet, as Copernicus had argued. Galileo's conversion to Copernicanism would be a key turning point in the scientific revolution.

After a brief controversy about floating bodies, Galileo again turned his attention to the heavens and entered a debate with Christoph Scheiner (1573–1650), a German Jesuit and professor of mathematics at Ingolstadt, about the nature of sunspots (of which Galileo was an independent discoverer). This controversy resulted in Galileo's
Istoria e dimostrazioni intorno alle macchie solari e loro accidenti
(“History and Demonstrations Concerning Sunspots and Their Properties,” or “Letters on Sunspots”), which appeared in 1613. Against Scheiner, who, in an effort to save the perfection of the Sun, argued that sunspots are satellites of the Sun, Galileo argued that the spots are on or near the Sun's surface, and he bolstered his argument with a series of detailed engravings of his observations.

Following the appearance of three comets in 1618, Galileo entered a controversy about the nature of comets, which led to the publication of
Il saggiatore (The Assayer
) in 1623. This work was a brilliant polemic on physical reality and an exposition of the new scientific method. In 1624 Galileo went to Rome and met with Pope Urban VIII. Galileo told the pope about his theory of the tides (developed
earlier), which he put forward as proof of the annual and diurnal motions of the Earth. The pope gave Galileo permission to write a book about theories of the universe but warned him to treat the Copernican theory only hypothetically.

The book,
Dialogo sopra i due massimi sistemi del mondo, tolemaico e copernicano
(
Dialogue Concerning the Two Chief World Systems, Ptolemaic & Copernican
), was finished in 1630, and Galileo sent it to the Roman censor. Because of an outbreak of the plague, communications between Florence and Rome were interrupted, and Galileo asked for the censoring to be done instead in Florence. The Roman censor had a number of serious criticisms of the book and forwarded these to his colleagues in Florence. After writing a preface in which he professed that what followed was written hypothetically, Galileo had little trouble getting the book through the Florentine censors, and it appeared in Florence in 1632.

In the
Dialogue
Galileo gathered together all the arguments (mostly based on his own telescopic discoveries) for the Copernican theory and against the traditional geocentric cosmology. As opposed to Aristotle's, Galileo's approach to cosmology is fundamentally spatial and geometric: the Earth's axis retains its orientation in space as the Earth circles the Sun, and bodies not under a force retain their velocity (although this inertia is ultimately circular). But in the work, Galileo ridiculed the notion that God could have made the universe any way he wanted to and still made it appear to us the way it does. The reaction against the book was swift. The pope convened a special commission to examine the book and make recommendations; the commission found that Galileo had not really treated the Copernican theory hypothetically and recommended that a case be brought against him by the Inquisition.

He was pronounced to be vehemently suspect of heresy and was condemned to life imprisonment. However, Galileo was never in a dungeon or tortured; during the Inquisition process he stayed mostly at the house of the Tuscan ambassador to the Vatican and for a short time in a comfortable apartment in the Inquisition building. After the process he spent six months at the palace of Ascanio Piccolomini (
c
. 1590–1671), the archbishop of Siena and a friend and patron, and then moved into a villa near Arcetri, in the hills above Florence. He spent the rest of his life there.

Galileo was then 70 years old. Yet he kept working. In Siena he had begun a new book on the sciences of motion and strength of materials. The book was published in Leiden, Netherlands, in 1638 under the title
Discorsi e dimostrazioni matematiche intorno a due nuove scienze attenenti alla meccanica
(
Dialogues Concerning Two New Sciences
). Galileo here treated for the first time the bending and breaking of beams and summarized his mathematical and experimental investigations of motion, including the law of falling bodies and the parabolic path of projectiles as a result of the mixing of two motions, constant speed and uniform acceleration. By then Galileo had become blind, and he spent his time working with a young student, Vincenzo Viviani, who was with him when he died on Jan. 8, 1642.

(b. Dec. 27, 1571, Weil der Stadt, Württemberg [Ger.]—d. Nov. 15, 1630, Regensburg)

G
erman astronomer Johannes Kepler discovered three major laws of planetary motion, conventionally designated as follows: (1) the planets move in elliptical orbits with the Sun at one focus; (2) the time necessary to
traverse any arc of a planetary orbit is proportional to the area of the sector between the central body and that arc (the “area law”); and (3) there is an exact relationship between the squares of the planets' periodic times and the cubes of the radii of their orbits (the “harmonic law”).

Kepler himself did not call these discoveries “laws,” as would become customary after Isaac Newton derived them from a new and quite different set of general physical principles. He regarded them as celestial harmonies that reflected God's design for the universe. Kepler's discoveries turned Nicolaus Copernicus's Sun-centred system into a dynamic universe, with the Sun actively pushing the planets around in noncircular orbits. And it was Kepler's notion of a physical astronomy that fixed a new problematic for other important 17th-century world-system builders, the most famous of whom was Newton.

Among Kepler's many other achievements, he provided a new and correct account of how vision occurs; he developed a novel explanation for the behaviour of light in the newly invented telescope; he discovered several new, semiregular polyhedrons; and he offered a new theoretical foundation for astrology while at the same time restricting the domain in which its predictions could be considered reliable. A list of his discoveries, however, fails to convey the fact that they constituted for Kepler part of a common edifice of knowledge. The matrix of theological, astrological, and physical ideas from which Kepler's scientific achievements emerged is unusual and fascinating in its own right.

Although Kepler's scientific work was centred first and foremost on astronomy, that subject as then understood—the study of the motions of the heavenly bodies—was classified as part of a wider subject of investigation called “the science of the stars.” The science of the stars was regarded as a mixed science consisting of a
mathematical and a physical component and bearing a kinship to other like disciplines, such as music (the study of ratios of tones) and optics (the study of light). It also was subdivided into theoretical and practical categories. Besides the theory of heavenly motions, one had the practical construction of planetary tables and instruments; similarly, the theoretical principles of astrology had a corresponding practical part that dealt with the making of annual astrological forecasts about individuals, cities, the human body, and the weather. Within this framework, Kepler made astronomy an integral part of natural philosophy, but he did so in an unprecedented way—in the process, making unique contributions to astronomy as well as to all its auxiliary disciplines.

The ideas that Kepler would pursue for the rest of his life were already present in his first work,
Mysterium cosmographicum
(1596; “Cosmographic Mystery”). In 1595 Kepler realized that the spacing among the six Copernican planets might be explained by circumscribing and inscribing each orbit with one of the five regular polyhedrons. If the ratios of the mean orbital distances agreed with the ratios obtained from circumscribing and inscribing the polyhedrons, then, Kepler felt confidently, he would have discovered the architecture of the universe. Remarkably, Kepler did find agreement within 5 percent, with the exception of Jupiter.

In place of the tradition that individual incorporeal souls push the planets and instead of Copernicus's passive, resting Sun, Kepler hypothesized that a single force from the Sun accounts for the increasingly long periods of motion as the planetary distances increase. Kepler did not yet have an exact mathematical description for this relation, but he intuited a connection. A few years later he acquired William Gilbert's groundbreaking book
De Magnete, Magneticisque Corporibus, et de Magno Magnete
Tellure
(1600; “On the Magnet, Magnetic Bodies, and the Great Magnet, the Earth”), and he immediately adopted Gilbert's theory that the Earth is a magnet. From this Kepler generalized to the view that the universe is a system of magnetic bodies in which, with corresponding like poles repelling and unlike poles attracting, the rotating Sun sweeps the planets around.

In 1601 Kepler published
De Fundamentis Astrologiae Certioribus (Concerning the More Certain Fundamentals of Astrology
). This work proposed to make astrology “more certain” by basing it on new physical and harmonic principles. In 1605 Kepler discovered his “first law”—that Mars moves in an elliptical orbit. During the creative burst when Kepler won his “war on Mars” (he did not publish his discoveries until 1609 in the
Astronomia Nova
[
New Astronomy
]), he also wrote important treatises on the nature of light and on the sudden appearance of a new star (1606;
De Stella Nova
, “On the New Star”). Kepler first noticed the star—now known to have been a supernova—in October 1604, not long after a conjunction of Jupiter and Saturn in 1603. The astrological importance of the long-anticipated conjunction (such configurations take place every 20 years) was heightened by the unexpected appearance of the supernova. Kepler used the occasion both to render practical predictions (e.g., the collapse of Islam and the return of Christ) and to speculate theoretically about the universe—for example, that the star was not the result of chance combinations of atoms and that stars are not suns.

Kepler's interest in light was directly related to his astronomical concerns. Kepler wrote about his ideas on light in
Ad Vitellionem Paralipomena, Quibus Astronomiae Pars Optica Traditur
(1604; “Supplement to Witelo, in Which Is Expounded the Optical Part of Astronomy”). Kepler wrote that every point on a luminous body in the field of vision
emits rays of light in all directions but that the only rays that can enter the eye are those that impinge on the pupil. He also stated that the rays emanating from a single luminous point form a cone the circular base of which is in the pupil. All the rays are then refracted within the normal eye to meet again at a single point on the retina. For more than three centuries eyeglasses had helped people see better. But nobody before Kepler was able to offer a good theory for why these little pieces of curved glass had worked.