Tycho and Kepler (4 page)

Figure 1.2: On this carousel the horses are attached to smaller rotating disks that ride on the perimeter of the large, rotating floor. As the carousel turns, the horses not only make their way all the way around the large circle but also move in smaller circles, chasing their tails.

By the same token, in a stroke of insight, ancient astronomers realized that if the planets moved continually
in smaller circles centered on the rim of a larger circle centered on Earth, the result would be the regularly occurring retrograde motion they were observing.

The technical term in Ptolemaic astronomy for the small circle in which a planet moved was
epicycle
. The larger circle on which the epicycles turned (in
figure 1.2
, the inner circle—the radius at which the minicarousels are bolted to
the floor) was the
deferent
. By adjusting the size, direction, and speed of the epicycles, astronomers could explain many irregularities they observed in the way the planets, Sun, and Moon move. A planet traveling in its epicycle would sometimes be closer to Earth and sometimes farther away, which explained apparent variations in its brightness. In Ptolemaic astronomy a planet’s sphere was just
large enough for the planet to cartwheel along on its epicycles.

Tycho, Kepler, and their peers at university also learned the use of the
eccentric
. With an eccentric, the planet (perhaps simultaneously traveling in an epicycle) orbited Earth, but the orbit wasn’t
centered
precisely on Earth. Its center was a point a small distance away from Earth.

Figure 1.3: Devices of Ptolemaic astronomy: eccentric orbit, deferent, epicycle, and equant.

Epicycles, deferents, and eccentrics were devices Ptolemy refined from earlier astronomy, but another, the
equant
, was probably his own invention. Many astronomers were uncomfortable with it, for it was not only complicated to use but also seemed to cheat a bit on the requirement of uniform motion.
The equant was an imaginary point that Ptolemy used to rationalize a planet’s apparently slowing down and speeding up as it wheeled in its epicycles around the deferent. It was possible to establish mathematically that if one were able to view the heavens from the equant, the velocity of a planet would
appear
to be uniform, though from Earth or the eccentric center of the orbit it would appear
to vary.

Ptolemy combined these devices in a complex and highly successful model of heavenly motion. Without removing Earth from its position as unmoving center, his astronomy could, with a surprising degree of accuracy, predict and account for the changing positions of the Sun, the Moon, and the five planets that were known at that time. Fulfilling the hopes of centuries of scholars before
him, Ptolemy was able to accomplish this feat entirely in terms of circles, spheres, and uniform motion.

Students and scholars of Tycho Brahe’s and Johannes Kepler’s generations were also steeped from childhood in a worldview that far pre-dated Ptolemy: Nineteen centuries removed from its origin in ancient Greece, Aristotelian philosophy and cosmology still had an enormously strong hold on
the thinking of scholarly and religious Europe. This worldview held that everything below the orbit of the Moon was subject to change, degradation, and decay, while the heavenly spheres beyond the Moon were a realm of unvarying, eternal perfection. Both experience and observation gave weight to these ideas. Before the telescope, there was little evidence to challenge the perfect immutability of the
heavenly spheres. Nor was it possible to deny that things were different on Earth.

This dichotomy had entered the thinking of European Latin-speaking scholars when the first Latin translation of Aristotle appeared in the twelfth century. These men knew nothing of Ptolemy, though the heritage from his astronomy was still flourishing in Islamic parts of the world. They came to revere Aristotle,
instead, as the final authority on science and cosmology, and Aristotle’s cosmology, filtered through the understanding of these scholars, merged with medieval Judeo-Christian thought. Somewhat later, when Ptolemaic astronomy arrived in Latin Europe, there was a second merger. Scholars, who in both instances were all clergymen, put prodigious effort and much debate into reconciling the Bible first
with Aristotle and later with Ptolemaic astronomy. To do so, they began to give the Scriptures a less literal, more metaphorical interpretation. What emerged over time was a coherent body of philosophical, scientific, and religious thought, with astronomy giving a visual, geometric structure to abstract medieval Judaism and Christianity. Aristotle’s picture of the degraded, changing, decaying
nature of Earth and the pristine perfection of everything beyond the Moon was consistent with the Judeo-Christian view of fallen, lost humanity on Earth and the eternal, holy realms above.

From the thirteenth century until the sixteenth, most educated Europeans accepted this worldview as reality. For them, astronomy seemed capable not only of describing and predicting planetary movement and
providing a map of the cosmos but also of accurately describing the human condition, with men and women pitifully torn between the passions of the squalid, death-ruled Earth and the lure of the deathless, sacred heavens. In the fourteenth century, Dante gave this worldview eloquent poetic expression in his
Divine Comedy
, describing a journey downward through nine circles of hell toward the center
of Earth, the most debased point in the universe, and a journey upward through the celestial spheres of the planets to reach the throne of God. It is no wonder that Melanchthon believed that the study of astronomy was essential to the clergy.

Both young Tyge and the young Kepler found this picture compatible with all they knew of Earth and heaven. However, though both men remained devoutly
religious all their lives and found no contradiction between their science and their belief, they would leave this primitive worldview in tatters.

fn1
Information about Tycho’s childhood and youth is found in Tycho’s own later accounts and in Victor E. Thoren’s splendid scholarly treatment,
The Lord of Uraniborg: A Biography of Tycho Brahe
. Thoren was the leading Tycho Brahe scholar of the
twentieth century.

fn2
The significance of this event for Tycho was recorded by his first biographer, the early-seventeenth-century astronomer Pierre Gassendi.

2

A
RISTOCRAT BY
B
IRTH
,
A
STRONOMER BY
N
ATURE

1562–1571

TYCHO CELEBRATED HIS
fifteenth birthday in December 1561. He had been at university for three years, and it was time to begin a new phase of his education. As the scion of a noble family, presumably destined for public life, he needed to become familiar with the history, music, art, literature, and architecture not only of Denmark
but of the whole of Europe. He also needed to be able to speak other languages besides Danish and the Latin, Greek, and Hebrew he had already studied, and to experience the idiosyncrasies of foreign courts and their rulers and learn something of military science and political theory.

Traditionally, a young Danish aristocrat began to acquire this sophistication by serving as a page in the household
of a kinsman and then as squire to a foreign nobleman. At the age of twenty-one, he would move up to the level of a courtier or a knight and finally come back to Copenhagen to serve the king and his court, the last stage of training that equipped him and gave him the credentials to take on the governance and defense of a royal fief. Tycho’s four younger brothers followed that path. Two of
them eventually became members
of
the Rigsraad. Had Tycho grown up in his father’s castle, or had his aunt not been from the intellectually inclined Oxe family, he probably would have done the same.

The paths of the young men in Inger Oxe’s family, like those of the Brahe family, had led them abroad, but not to foreign courts and wars. They had gone to foreign universities. Inger’s older brother
Peder Oxe had traveled for five years with a tutor from university to university, and this alternate form of education had not prevented his succeeding in public life. Indeed, though by the time Tycho left Copenhagen Peder had suffered a severe downturn of political fortune, there had for a while been no man in Denmark whose career had been more spectacularly successful. In 1548, when Tycho
was still a child, Peder led the entourage of Princess Anne of Denmark when she wed the duke of Saxony, and that was the start of a meteoric rise to power that brought Peder, at the age of thirty-two, to a position in the Rigsraad.

Jørgen and Inger chose the University of Leipzig,
¹
in Saxony, as the place for young Tycho to begin his foreign experience. They had traveled to Saxony themselves
for Princess Anne’s wedding, and Inger still corresponded regularly with Anne, who was now Electress Anne of Saxony. The language there was that spoken at the Danish court, a “pure” form of High German. Furthermore, Saxony was the birthplace of Lutheranism.

A fifteen-year-old wasn’t sent abroad alone. Jørgen and Inger carefully chose Tycho’s “preceptor,” Anders Sørensen Vedel. Four years older
than Tycho, he came from a respectable middle-class background and had excelled at the University of Copenhagen. As a preceptor, Vedel’s duties combined those of companion, chaperon, and tutor. In return for the payment of his expenses, and supposedly adhering to instructions in letters sent from home, he was to supervise Tycho’s university studies, act as his spiritual and moral guide, see that
he received language instruction and lessons in fencing, riding, and dancing, and manage the purse. All of these responsibilities added up to a formidable assignment, but being a preceptor was nevertheless a good way for a young man of modest means to support his own education abroad and make invaluable contacts.

Tycho and Vedel left Denmark on February 14, 1562, traveling, for safety and
companionship, as part of a caravan. The journey took five weeks, first by ship across the icy Baltic and then on horseback along the roads beside the Elbe and Saale Rivers (see map, Tycho and Kepler’s Europe,
here
). Many Danish students attended the University of Wittenberg, but Tycho and Vedel’s travels took them two days beyond that, to Leipzig.

Germany, then, was still three centuries
from unification, and Saxony was not nearly so significant or wealthy a power as Denmark. Leipzig was, however, the site of one of Europe’s largest and most important universities. Though there were few Danes in residence, one of Vedel’s classmates back home had a brother there, and there were similarities to the University of Copenhagen that made it less alien to Tycho and Vedel. Instruction was
in Latin, as it was in
all
European universities, and education went according to the pattern and philosophy of the Philippists, with many Philippists among the faculty. At Leipzig, Tycho studied classical languages and classical culture. He also continued, at first clandestinely, with astronomy.

Tycho later wrote that he had “bought astronomical books
²
secretly and read them in secret.” He
studied the constellations from maps drawn by Albrecht Dürer. He began to keep track of the planets by a rough method of lining up a planet and two stars, holding up a “taut piece of string,” and then figuring the positions of the planet from the locations of the two stars on a little globe that he owned, “no bigger than a fist.” Flawed as this method inevitably was, Tycho was nevertheless able
to come to a conclusion that impressed him deeply. Neither the Alfonsine Tables, which had been calculated in the thirteenth century using Ptolemy’s Earth-centered model, nor the Prutenic Tables, based on Copernicus’s Sun-centered astronomy and drawn up much more recently, were dependable in their predictions of planetary positions.
^fn1

When sixteen-year-old Tycho began to keep a logbook of
his own astronomical observations, in August 1563, during the second summer of his stay in Leipzig, the first record he made in it was of an observation of Mars, and the second was of a conjunction of the planets Jupiter and Saturn. A conjunction is the coincidence of two or more heavenly bodies at the same celestial longitude. To an earthly observer, one appears to pass the other. This happens with
Jupiter and Saturn once every twenty years. Tycho found that neither the predictions of the conjunction based on Ptolemy nor those based on Copernicus were correct. The discrepancies were great enough to show up clearly even with his amateur efforts. The Copernican tables
weathered
the test slightly better than the Ptolemaic ones, which were off by an entire month. The cocky sixteen-year-old concluded
that someone ought to produce better tables, and he began to think of himself as the person destined to “rectify this sorry state of affairs.”
³