To Explain the World: The Discovery of Modern Science (23 page)

There was mention of a certain new astrologer who wanted to prove that the Earth moves and not the sky, the Sun, and the Moon. . . . [Luther remarked,] “So it goes now. Whoever wants to be clever must agree with nothing that others esteem. He must do something of his own. This is what that fool does who wishes to turn the whole of astronomy upside down. Even in these things that are thrown into disorder I believe in the Holy Scriptures, for Joshua commanded the Sun to stand still and not the Earth.”
⁸

A few years after the publication of
De Revolutionibus,
Luther’s colleague Philipp Melanchthon (1497–1560) joined the attack on Copernicus, now citing Ecclesiastes 1:5—“The Sun also rises, and the Sun goes down, and hastens to his place where he rose.”

Conflicts with the literal text of the Bible would naturally raise problems for Protestantism, which had replaced the authority of the pope with that of Scripture. Beyond this, there was a potential problem for all religions: man’s home, the Earth, had been demoted to just one more planet among the other five.

Problems arose even with the printing of
De Revolutionibus.
Copernicus had sent his manuscript to a publisher in Nuremberg, and the publisher appointed as editor a Lutheran clergyman, Andreas Osiander, whose hobby was astronomy. Probably expressing his own views, Osiander added a preface that was thought to be by Copernicus until the substitution was unmasked in the following century by Kepler. In this preface Osiander had Copernicus disclaiming any intention to present the true nature of planetary orbits, as follows:
⁹

For it is the duty of an astronomer to compose the history of the [apparent] celestial motions through careful and expert study. Then he must conceive and devise the causes of these motions
or hypotheses about them. Since he cannot in any way attain to the true cause, he will adopt whatever suppositions enable the motions to be computed correctly from the principles of geometry for the future as well as for the past.

Osiander’s preface concludes:

So far as hypotheses are concerned, let no one expect anything certain from astronomy, which cannot furnish it, lest he accept as the truth ideas conceived for another purpose, and depart from this study a greater fool than when he entered it.

This was in line with the views of Geminus around 70 BC (quoted here in
Chapter 8
), but it was quite contrary to the evident intention of Copernicus, in both the
Commentariolus
and
De Revolutionibus
, to describe the actual constitution of what is now called the solar system.

Whatever individual clergymen may have thought about a heliocentric theory, there was no general Protestant effort to suppress the works of Copernicus. Nor did Catholic opposition to Copernicus become organized until the 1600s. The famous execution of Giordano Bruno by the Roman Inquisition in 1600 was not for his defense of Copernicus, but for heresy, of which (by the standards of the time) he was surely guilty. But as we will see, the Catholic church did in the seventeenth century put in place a very serious suppression of Copernican ideas.

What was really important for the future of science was the reception of Copernicus among his fellow astronomers. The first to be convinced by Copernicus was his sole pupil, Rheticus, who in 1540 published an account of the Copernican theory, and who in 1543 helped to get
De Revolutionibus
into the hands of the Nuremberg publisher. (Rheticus was initially supposed to supply the preface to
De Revolutionibus
, but when he left to take a position in Leipzig the task unfortunately fell to Osiander.) Rheticus had earlier assisted Melanchthon in making the University of Wittenberg a center of mathematical and astronomical studies.

The theory of Copernicus gained prestige from its use in 1551 by Erasmus Reinhold, under the sponsorship of the duke of Prussia, to compile a new set of astronomical tables, the Prutenic Tables, which allow one to calculate the location of planets in the zodiac at any given date. These were a distinct improvement over the previously used Alfonsine Tables, which had been constructed in Castile in 1275 at the court of Alfonso X. The improvement was in fact due, not to the superiority of the theory of Copernicus, but rather to the accumulation of new observations in the centuries between 1275 and 1551, and perhaps also to the fact that the greater simplicity of heliocentric theories makes calculations easier. Of course, adherents of a stationary Earth could argue that
De Revolutionibus
provided only a convenient scheme for calculation, not a true picture of the world. Indeed, the Prutenic Tables were used by the Jesuit astronomer and mathematician Christoph Clavius in the 1582 calendar reform under Pope Gregory XIII that gave us our modern Gregorian calendar, but Clavius never gave up his belief in a stationary Earth.

One mathematician tried to reconcile this belief with the Copernican theory. In 1568, Melanchthon’s son-in-law Caspar Peucer, professor of mathematics at Wittenberg, argued in
Hypotyposes orbium coelestium
that it should be possible by a mathematical transformation to rewrite the theory of Copernicus in a form in which the Earth rather than the Sun is stationary. This is precisely the result achieved later by one of Peucer’s students, Tycho Brahe.

Tycho Brahe was the most proficient astronomical observer in history before the introduction of the telescope, and the author of the most plausible alternative to the theory of Copernicus. Born in 1546 in the province of Skåne, now in southern Sweden but until 1658 part of Denmark, Tycho was a son of a Danish nobleman. He was educated at the University of Copenhagen, where in 1560 he became excited by the successful prediction of a partial solar eclipse. He moved on to universities in Germany and Switzerland, at Leipzig, Wittenberg, Rostock, Basel, and Augsburg. During these years he studied the Prutenic Tables and was
impressed by the fact that these tables predicted the date of the 1563 conjunction of Saturn and Jupiter to within a few days, while the older Alfonsine Tables were off by several months.

Back in Denmark, Tycho settled for a while in his uncle’s house at Herrevad in Skåne. There in 1572 he observed in the constellation Cassiopeia what he called a “new star.” (It is now recognized as the thermonuclear explosion, known as a type Ia supernova, of a preexisting star. The remnant of this explosion was discovered by radio astronomers in 1952 and found to be at a distance of about 9,000 light-years, too far for the star to have been seen without a telescope before the explosion.) Tycho observed the new star for months, using a sextant of his own construction, and found that it did not exhibit any diurnal parallax, the daily shift in position among the stars of the sort that would be expected to be caused by the rotation of the Earth (or the daily revolution around the Earth of everything else) if the new star were as close as the Moon, or closer. (See
Technical Note 20
.) He concluded, “This new star is not located in the upper regions of the air just under the lunar orb, nor in any place closer to Earth . . . but far above the sphere of the Moon in the very heavens.”
¹⁰
This was a direct contradiction of the principle of Aristotle that the heavens beyond the orbit of the Moon can undergo no change, and it made Tycho famous.

In 1576 the Danish king Frederick II gave Tycho the lordship of the small island of Hven, in the strait between Skåne and the large Danish island of Zealand, along with a pension to support the building and maintenance of a residence and scientific establishment on Hven. There Tycho built Uraniborg, which included an observatory, library, chemical laboratory, and printing press. It was decorated with portraits of past astronomers—Hipparchus, Ptolemy, al-Battani, and Copernicus—and of a patron of the sciences: William IV, landgrave of Hesse-Cassel. On Hven Tycho trained assistants, and immediately began observations.

Already in 1577 Tycho observed a comet, and found that it had no observable diurnal parallax. Not only did this show, again contra Aristotle, that there was change in the heavens
beyond the orbit of the Moon. Now Tycho could also conclude that the path of the comet would have taken it right through either Aristotle’s supposed homocentric spheres or the spheres of the Ptolemaic theory. (This, of course, would be a problem only if the spheres were conceived as hard solids. This was the teaching of Aristotle, which we saw in
Chapter 8
had been carried over to the Ptolemaic theory by the Hellenistic astronomers Adrastus and Theon. The idea of hard spheres was revived in early modern times,
¹¹
not long before Tycho ruled it out.) Comets occur more frequently than supernovas, and Tycho was able to repeat these observations on other comets in the following years.

From 1583 on, Tycho worked on a new theory of the planets, based on the idea that the Earth is at rest, the Sun and Moon go around the Earth, and the five known planets go around the Sun. It was published in 1588 as
Chapter 8
of Tycho’s book on the comet of 1577. In this theory the Earth is not supposed to be moving or rotating, so in addition to having slower motions, the Sun, Moon, planets, and stars all revolve around the Earth from east to west once a day. Some astronomers adopted instead a “semi-Tychonic” theory, in which the planets revolve around the Sun, the Sun revolves around the Earth, but the Earth rotates and the stars are at rest. (The first advocate of a semi-Tychonic theory was Nicolas Reymers Bär, although he would not have called it a semi-Tychonic system, for he claimed Tycho had stolen the original Tychonic system from him.)
¹²

As mentioned several times above, the Tychonic theory is identical to the version of Ptolemy’s theory (never considered by Ptolemy) in which the deferents of the inner planets are taken to coincide with the orbit of the Sun around the Earth, and the epicycles of the outer planets have the same radius as the Sun’s orbit around the Earth. As far as the
relative
separations and velocities of the heavenly bodies are concerned, it is also equivalent to the theory of Copernicus, differing only in the point of view: a stationary Sun for Copernicus, or a stationary and nonrotating Earth for Tycho. Regarding observations, Tycho’s theory had the advantage that it automatically predicted no annual stellar
parallax, without having to assume that the stars are very much farther from Earth than the Sun or planets (which, of course, we now know they are). It also made unnecessary Oresme’s answer to the classic problem that had misled Ptolemy and Buridan: that objects thrown upward would seemingly be left behind by a rotating or moving Earth.

For the future of astronomy, the most important contribution of Tycho was not his theory, but the unprecedented accuracy of his observations. When I visited Hven in the 1970s, I found no sign of Tycho’s buildings, but there, still in the ground, were the massive stone foundations to which Tycho had anchored his instruments. (A museum and formal gardens have been put in place since my visit.) With these instruments, Tycho was able to locate objects in the sky with an uncertainty of only
¹
/
₁₅
°. Also at the site of Uraniborg stands a granite statue, carved in 1936 by Ivar Johnsson, showing Tycho in a posture appropriate for an astronomer, facing up into the sky.
¹³

Tycho’s patron Frederick II died in 1588. He was succeeded by Christian IV, whom Danes today regard as one of their greatest kings, but who unfortunately had little interest in supporting work on astronomy. Tycho’s last observations from Hven were made in 1597; he then left on a journey that took him to Hamburg, Dresden, Wittenberg, and Prague. In Prague, he became the imperial mathematician to the Holy Roman Emperor Rudolph II and started work on a new set of astronomical tables, the Rudolphine Tables. After Tycho’s death in 1601, this work was continued by Kepler.

Johannes Kepler was the first to understand the nature of the departures from uniform circular motion that had puzzled astronomers since the time of Plato. As a five-year-old he was inspired by the sight of the comet of 1577, the first comet that Tycho had studied from his new observatory on Hven. Kepler attended the University of Tübingen, which under the leadership of Melanchthon had become eminent in theology and mathematics. At Tübingen Kepler studied both of these subjects, but became more interested in mathematics. He learned about the theory of
Copernicus from the Tübingen mathematics professor Michael Mästlin and became convinced of its truth.

In 1594 Kepler was hired to teach mathematics at a Lutheran school in Graz, in southern Austria. It was there that he published his first original work, the
Mysterium Cosmographicum
(
Mystery of the Description of the Cosmos
). As we have seen, one advantage of the theory of Copernicus was that it allowed astronomical observations to be used to find unique results for the order of planets outward from the Sun and for the sizes of their orbits. As was still common at the time, Kepler in this work conceived these orbits to be the circles traced out by planets carried on transparent spheres, revolving in the Copernican theory around the Sun. These spheres were not strictly two-dimensional surfaces, but thin shells whose inner and outer radii were taken to be the minimum and maximum distance of the planet from the Sun. Kepler conjectured that the radii of these spheres are constrained by an a priori condition, that each sphere (other than the outermost sphere, of Saturn) just fits inside one of the five regular polyhedrons, and each sphere (other than the innermost sphere, of Mercury) just fits outside one of these regular polyhedrons. Specifically, in order outward from the Sun, Kepler placed (1) the sphere of Mercury, (2) then an octahedron, (3) the sphere of Venus, (4) an icosahedron, (5) the sphere of Earth, (6) a dodecahedron, (7) the sphere of Mars, (8) a tetrahedron, (9) the sphere of Jupiter, (10) a cube, and finally (11) the sphere of Saturn, all fitting together tightly.