Tycho and Kepler (20 page)

I
N THE EARLY
1580s, having spent considerable money, time, and ingenuity
on producing by far the best instruments in existence, Tycho still knew he lacked the tools necessary for what he hoped to accomplish. As a case in point, his armillary tended to bend and flex unpredictably as the rings were adjusted to different positions. A smaller, lighter armillary would not give him fine enough
readings
, while a larger one would bend and flex even more. Tycho set his mind
to solving these particular problems, and this time, as he had done when he designed his mural quadrant, he thought big and, at the same time, simple. He came rapidly to the conclusion that in order to accommodate a successful design he would need a ground-level observatory beyond Uraniborg’s perimeter wall.

By 1583, Uraniborg was too crowded. Tycho and his assistants were stumbling over one
another. Instruments already in use or in various stages of construction threatened to overflow the space available, and Tycho had not by any means stopped planning new instruments. The large ones he now had in mind would work much better with access to all 360 degrees of the sky, which neither Uraniborg’s large observatories nor the smaller satellites provided. Outside the perimeter wall, with
some excavation, he could have the advantage of being able to build an amphitheaterlike structure around each instrument, allowing an observer to position himself on a level high above the base of the instrument. The shelter of the amphitheater would prevent gusts of winter winds from affecting sensitive readings, not to mention chilling the observer. Tycho also saw the auxiliary observatory as a
way of separating his assistants and establishing better control over the accuracy of their findings. Some would continue to make observations from the castle, others from these new “cellars.” They would not get in one another’s way and would also not compare results and make adjustments to them before he had a chance to study findings that disagreed and think about the implications.

There
was a small rise in the landscape not far beyond the south corner of Uraniborg’s outer wall. Tycho decided that a structure there would not spoil the symmetry of the house or the gardens, and the wall and the house would block only an insignificant low portion of the northern heavens, the least interesting direction. He set the islanders digging again, and he constructed “with no small difficulty
⁶
and expenditure, a subterranean observatory.” He christened it Stjerneborg, “Star Castle.”

Figure 8.2: Tycho’s drawings of Stjerneborg, the partially subterranean observatory that he built outside the perimeter wall of Uraniborg, from
Astronomiae Instauratae Mechanica
.

Stjerneborg (
see color plate section
) was designed with five great cellars or amphitheaters to house a giant armillary that figured largely in Tycho’s observational plans, a revolving quadrant, a zodiacal armillary,
a large steel quadrant, and a four-cubit sextant. Each cellar had a roof that could be removed or swung aside. There was ample storage for other instruments as well, and space to use them. Tycho’s design also did not neglect his own comfort or that of his assistants. There was a “heating installation,” a bed for Tycho “when accidentally there were clouds and we could not enjoy a constant clearness
of the sky,” and a second larger bed to be shared by others.

Despite being purpose-built as an observatory that pushed the boundaries of what such a building should be and what it should allow its users to achieve, Stjerneborg was far from strictly functional
⁷
in design and decor. Tycho and Steenwinkel drew plans with the same attention to symmetry, harmony, and detail that characterized Uraniborg
itself. A significant difference was that Tycho was by this time much more preoccupied with his self-image as a man of stature and wealth, with classical roots, the equal of kings, the greatest of all living astronomers, occupying a preeminent place in history. Stjerneborg was laden with symbolism to convey this image. Above
the
entrance stood three elaborately carved lions with crowns on their
heads. On the back of the portal an inscription in gold letters sang the praises of Tycho and his instruments. Beyond the entrance and several steps down from it was the warming room that gave access to the five round cellars, and this and other subterranean rooms were embellished with poems inscribed in gold letters. The warming room walls displayed seven portraits of astronomers from the ancients
to Tycho, with an eighth portrait of a future astronomer named “Tychonides.” The message was clear: Tycho was the equal of the greatest astronomers in history, and he anticipated that Tychonides would come from his own lineage.

Tycho was diverting some of the attention previously reserved for astronomy to promoting that image. He was eager for the stream of scholars, intellectuals, and highborn
curiosity seekers, who regularly detoured in their travels to visit Hven, to recognize his greatness and also realize that, though his descendants could never be noble Brahes, the mantle of their father could fall onto their shoulders in a far more significant way.

Tycho’s “great equatorial armillary” (
figure 8.3
) was destined to become the most famous of Stjerneborg’s instruments. Like the
great mural quadrant inside the house, it was built into the building’s structure. The foundation for its axis was put in place in December 1584, but it was not until the following summer solstice that the massive instrument was ready for use.

Figure 8.3: The great equatorial armillary
⁸
as illustrated in
Astronomiae Instauratae Mechanica
. To find the position of a star, an observer stood on a tier of the amphitheater, behind the half circle (
O
) representing the celestial equator. Through the movable sight (
R
), which could be positioned anywhere along this half circle, he peered toward the axis pole (
B
) and moved the sight along
the half circle until, looking through the slits of the sight, he saw the star on both sides of the axis pole. Rulerlike markings on the half circle indicated, from the new position of the sight, the right ascension of the star (distance in degrees east of the prescribed meridian established by the position of the Sun at the vernal equinox). In order to compare this finding with the right ascension
of another star whose position was already known, two observers sighted from the half circle, one for each star. They learned the difference between the stars’ right ascensions by noting the distance on the arc between the new positions of the sights.

To find a star’s declination (distance above the celestial equator), assistants pivoted the armillary on its axis until the large complete ring
had one edge toward the observer and the other toward the star. There were two alidades—the “fan blades” that met in the center at a cylinder (
E
) with their other ends on the large ring. The drawing shows those ends at two positions on the ring, (
F
). Like hands on a clock, the alidades were fixed at the center and moved along the ring. Assistants moved one until, sighting along the alidade, the
observer saw the star on both sides of the central cylinder (
E
). The new position of the sight indicated the declination of the star.

To double-check an observation, Tycho gave the entire apparatus a half turn on its axis and used the other alidade for the same measurement. As he informed his readers, “the two values found
⁹
should agree with each other.”

It was difficult to see that the
instrument
was
an armillary, for most of the familiar rings (compare with
figure 8.1
) seemed to be missing. Except for brass alidades (the two “fan blades” that ran from the center at
E
on Tycho’s drawing to the outer edge, marked
F
) and graduation strips to act as rulers on the rings, the great equatorial armillary was made of wood for easier handling and to minimize the distortions caused by
the weight of metal. It pivoted on a hollow steel axle (the pole marked
B
in the drawing). At first glance, the entire apparatus looks to be strangely skewed from plumb, but take a globe of the world and tilt it so that Denmark is on “top” and the skew makes sense.

Tycho supported the giant armillary at its lower pivot point with a half-buried stone pillar (partially visible at the lower left
corner of the picture) topped by a globe supported by a splendidly carved figure of Atlas. A wishbone-shaped stone structure supported the upper pivot point, and the two legs of the wishbone flanked the door to the passage to the warming room and the crypt beneath the armillary.

After the completion of the mural quadrant and the great equatorial armillary, Tycho and his shop went on to produce
a revolving wood quadrant and to revamp the old 1581 “large quadrant” by giving it a stronger base and a pivot at the top to stabilize it. It was
henceforth
known as the “great steel quadrant.” This done, Tycho finally felt that he no longer needed to use his older, less successful instruments to check daily observations. When observations with his newer, larger instruments agreed with one another,
he was confident that he had achieved an extremely high degree of accuracy. He had at last made enormous strides toward fulfilling his dream of instruments to create a new astronomy.

fn1
The engraving also shows a man in the lower right-hand corner noting the time of the observation, while another opposite him transcribes the observation into a log book. These figures were not part of the
mural itself. They were part of the engraving of the mural as it appeared in Tycho’s book.

9

C
ONTRIVING
I
MMORTALITY

1581–1588

IT WAS NOT
only in its design and symbolism that Stjerneborg was a direct bid for immortality. The urgency with which Tycho went about his instrument development and the construction of the new observatory stemmed in large part from a decision he had made as early as 1581 about his future research.

If Copernican astronomy or his own evolving
Tychonic system was correct, the planet Mars came closer to Earth than the Sun did. If the venerable Ptolemaic system was correct, Mars never came as close as the Sun. Hence there was a way of deciding the contest between the systems: Find out whether Mars does, in fact, come closer than the Sun. One way of doing so was to measure Mars’s parallax and compare it with that of the Sun. However, Mars’s
parallax had never been measured. No observation had ever been able to show that Mars even
had
a parallax. Tycho took on the challenge, a project that would require all his ingenuity and the finest instruments in the world.

At this point, Tycho had not yet conceived in full his “Tychonic system of the world,”
¹
but he had come a good way in his thinking about how one might devise a compromise
between Copernicus and Ptolemy. Three years after the Copenhagen lecture series, in which
he
had attempted to retain the essentials of the Ptolemaic system while eliminating the need for an equant, he had been considering the possibility, suggested by others, that Venus and Mercury orbit the Sun, while the Sun and the outer planets orbit the unmoving Earth.

In 1580 a young scholar named Paul
Wittich had spent three or four months at Uraniborg and shared his own attempts to solve the same problems. Their conversations were a great stimulus to Tycho. However, although he would later claim to have done so earlier, it probably was not until 1584 that he finally arrived at the full Tychonic system in which
all
the planets orbit the Sun, while the Sun and Moon orbit Earth—an arrangement
later embraced by Jesuit scholars who opposed Galileo. This was the beloved intellectual child that Tycho would spend the rest of his life defending, and that he would guard with self-destructive paranoia. Though it retained an unmoving Earth, it was, in fact, the geometric equivalent of the Copernican system. (
see figure 9.1
)