B00B7H7M2E EBOK (17 page)

There is another criterion by which theories are judged. For better or for worse, it shows that modern scientists do have a certain kinship with those recalcitrant 17th-century scholars they so disdain. When new theories and the implications of new discoveries disagree with the way a scientist personally feels the universe ought to run, he or she is reluctant to accept them. In the 20th century, Einstein’s resistance to the notion of an expanding universe, as well as many scientists’ discomfort with the possibility that the universe may be far less predictable than previously thought, are examples. One need not be narrow-minded or anti-science to dismiss a theory on the grounds that it offends one’s scientific aesthetic sensibility. ‘It can’t be right, because it feels so wrong.’

These few paragraphs have viewed the competition with the eyes of 20th-century philosophy of science, at the end of a century that has given an extraordinary amount of thought to the question of how we know what we know. But modern
thought
processes didn’t spring out of nowhere. They are rooted in thinking that was already substantially in place even in the ancient world and certainly in Kepler’s and Galileo’s time. Scholars didn’t think in terms of relative motion in the 20th-century sense, but they knew that there could be competing and equally valid geometric explanations. Those who opposed Galileo used this line of argument. When it came to making simplicity a criterion, scholars had no ‘scientific method’ yet to tell them that the most economical and harmonious model that saves the appearances is the best model. But even the ancient astronomer Hipparchus was known as a man who always preferred the least complicated hypothesis compatible with observation, and Neoplatonism brought into late-medieval science a preference for finding simple mathematical and geometric regularities in nature. Copernicus firmly believed in the superior harmony of his arrangement and saw that as a powerful argument in its favour. The search for such harmony was the driving force behind Kepler’s work. Some scholars expressed this preference in religious terms: the work of God and nature is work of great elegance and simplicity, and we must try to understand and explain similarly.

Modern scholars can never know what really happened in the case of Galileo or unravel all the currents and cross-currents of thought (and lack thereof) and behaviour that were part of that chain of events. The surface arguments hid ambiguities, assumptions and personal agendas that would have been impossible to unmask even had we been there in person. Nor can scientists and philosophers of science tell precisely what causes one theory to emerge as ‘scientific knowledge’ while another is consigned to the dustbin. The stories of Ptolemy, Copernicus, Kepler, Tycho Brahe and Galileo should at least discourage simplistic interpretations in terms of proved/unproved, bad science/good science, knowledge/ignorance, pro-Ptolemy/pro-Copernicus, religion/science, open-mindedness/blind
dogmatism
– and caution us not to trust so readily the popular legends that have grown up around historical figures. Indeed, trying to unravel the complex human saga of the Copernican revolution makes explaining the night sky seem a relatively simple, straightforward task.

CHAPTER 4

An Orbit with a View

1630–1900

What a wonderful and amazing scheme have we here of the magnificent vastness of the universe!

Christiaan Huygens

WHEN FRENCH ASTRONOMER
Pierre Gassendi watched the transit of Mercury in 1631, he couldn’t believe what he saw. Gassendi, along with the rest of his generation, had studied the cosmic dimensions given by Ptolemy. The tiny dot on the Sun was surely a sunspot, not the planet. Kepler too had expected a much larger apparent diameter for Mercury. Only when Gassendi realized that the dot was moving much too quickly across the face of the Sun to be a sunspot did he conclude with astonishment that, in spite of its ‘entirely paradoxical smallness’, this was indeed Mercury. In fairness to Ptolemy, he had commented in his
Almagest
that the absence of observed transits of Venus and Mercury might be the result of their apparent smallness. Part of the reason Ptolemy often comes off so badly is that astronomers didn’t read him very carefully!

Ptolemy thought of his cosmic dimension estimates as a
speculative
offshoot of his mathematical astronomy, but it’s evident from both scholarly writing and popular literature that the Middle Ages regarded them as established truth, and Ptolemy’s numbers still had a tenacious hold on the minds of scholars in the early 17th century. However, in the 1630s, when astronomers like Gassendi began using telescopes on a day-to-day basis, and with the dissemination of Kepler’s laws, the ancient distance measurements finally lost credibility, at first among specialists and then among all educated people. Ptolemy’s distances were far too small.

Mercury’s transit in 1631 permitted the first accurate measurement of its apparent diameter (that is, how large its diameter appears from Earth), and eight years later there was a similar opportunity to measure Venus.

The quality of telescopes was improving, and there were also advances in related technology. In the spring of 1610, Galileo had been using a telescope that magnified thirty times. Two years later, astronomer Thomas Harriot had one that magnified fifty times, but at this magnification the area visible through it was frustratingly small. Then in the 1630s, an instrument known as the astronomical telescope came into use. Kepler had first introduced the theory of this telescope in 1611, but his version inverted the image. When changes in the configuration of lenses solved this problem, the astronomical telescope came into its own, providing a much larger field of view at high magnifications.

The astronomical telescope had another great advantage. Yorkshireman William Gascoigne discovered it by accident in the late 1630s when a spider spun a web in his telescope and he saw some of the threads of the web sharply outlined against the background image. What this told Gascoigne was that an object inside an astronomical telescope could appear in sharp focus, superimposed on a distant object under observation. In other words, he could place some sort of ruler inside his telescope. The ruler he built was a ‘micrometer’ (as the name suggests, an
instrument
for measuring extremely small dimensions), with cross-wires that moved across the image with the turning of a screw. Before this invention it was possible only to estimate the apparent size of a body seen through the lens, and these estimates varied widely. Now there was a way to
measure
the apparent size, comparing it directly with an established standard. Sadly, Gascoigne died in the battle of Marston Moor in 1644, and his invention remained unknown to other astronomers until the 1660s. By then French experts had developed a similar idea from astronomer Christiaan Huygens. The Royal Society in England were quick to claim Gascoigne’s prior discovery, but Frenchmen Adrien Auzout and Jean Picard were largely responsible for developing the screw micrometer into a fine precision instrument.

By 1675 observers were beginning to agree on the apparent sizes of the planets. However, consensus about their distances, now that Ptolemy was no longer to be trusted, was slower in coming. Kepler’s third law of planetary motion had established a relationship among the planets having to do with their orbital periods and their distances from the Sun, but Earth-bound observers were still little better off in terms of absolute measurements than if they had been standing at the foot of a ladder leading up into the sky, knowing that the second rung was twice as far as the first, and the third three times as far, and so forth, but having no clue how far up the first rung was.

In the southern wall of Bologna’s Cathedral of San Petronio, there is a small opening high up through which a shaft of sunlight penetrates to the cathedral floor below. There, a gnomon – a protrusion like the raised part of a sundial – makes possible the measurement of the shifting image of the Sun. In the mid-17th century, alterations to the cathedral made the old gnomon useless, and Gian Domenico Cassini of the University of Bologna was given the task of setting up a new one to replace it. Cassini was a good choice for the job, for he was keenly
interested
in the Sun and its movements. Cassini’s gnomon still stands in the cathedral.

In 1669, when Cassini was 44, Jean Baptiste Colbert, an influential minister in the court of King Louis XIV of France and member of the recently founded Academy of Sciences in Paris, invited him to come to Paris to work at the new Royal Observatory. Louis XIV was the king who built Versailles, engaging another eminent Italian, architect and sculptor Giovanni Bernini, for that task. Louis cultivated a public image of himself as the Sun King and surrounded his sumptuous court with appropriate symbolism. Naturally, his chief astronomer should be not only one of the best in Europe but also an expert on the Sun.

There were additional, scientific, reasons for bringing Cassini to Paris. The Academy were already deeply interested in solar theory and eager to gain a better understanding of the Sun, the planets and the ‘refraction’ of light by the Earth’s atmosphere – that is, the way the atmosphere bends and smears light rays passing through it. It was their intention to bring astronomical tables into line with new measurements provided by telescopes and micrometers. Cassini accepted the invitation and became head of the Observatory. The French version of his name was Jean-Dominique Cassini.

All over Europe at this time telescopes were getting longer and longer. Some experts even voiced the extravagant hope that a long enough telescope would allow astronomers to study the animals on the Moon. One of Cassini’s first undertakings was to make sure that the new Observatory acquired some of the finest instruments available. In 1671, he brought from Rome a 17-foot telescope manufactured by his good friend and former colleague Giuseppe Campani, one of the most skilled telescope builders in Europe. Colbert, the courtier at whose invitation Cassini had come to Paris, was particularly taken with this telescope, and his enthusiasm prompted the King to present the Observatory with a still larger Campani telescope, a 34-footer,
and
later, in 1684, a 100-footer. The old wooden Marly water-tower was trundled over to the Observatory to serve as a support for these giant instruments, with steps built to the top and a rail added to prevent astronomers and their assistants from toppling off on dark nights. Among the discoveries Cassini made with these telescopes were several moons around Saturn and the dark division in Saturn’s rings. His most famous achievement, however, was the first successful measurement of distances within the solar system.

Cassini and other astronomers knew that in August and September of 1672, Mars would be at its nearest proximity to the Earth, and this would provide optimum conditions for measuring its distance. The method Cassini planned to use was an old mapmaker’s trick, known as triangulation, in which two observers in two widely separated locations measure the position, against the background, of the same distant object at the same time. Because they are far apart, the two observers have different viewing angles.

Human beings and other animals use this technique instinctively to judge distances. Our two eyes are the two ‘observers’. The view from the right eye is not exactly the same as the view from the left. The demonstration is familiar: hold a finger upright close in front of you. Shut one eye, then the other, and the finger appears to change position relative to the background, although it actually hasn’t moved. The closer your finger is to your face, the greater the shift will appear to be. It doesn’t occur to us to use geometry to measure the distance from our finger to our face from how great the apparent shift is. Our brains do it automatically with a precision that is adequate for most everyday purposes.

The apparent shift in the position of an object against the background, as seen from two different locations, is called a parallax shift. To understand the use of this shift for measuring distances: suppose you are driving along a road across a desert. The desert stretches in all directions to the horizon, where a
string
of mountains is just barely visible. There is a solitary cactus not far from the road. The problem at hand is how to measure the distance from the road to the cactus without leaving the road.

Begin by constructing two imaginary triangles. The first triangle has the road as one of its three sides and a line drawn straight to the cactus as a second side. See
Figure 4.1a.
The letter X designates the point at which the line to the cactus meets the road. Standing at X, find a landmark on the horizon that is directly behind the cactus. Fortunately, there is one, a snowcapped peak that stands out clearly from the others. From X, move a little to the left along the road. Look again at the cactus and the mountain peak. They will no longer appear to be lined up. The peak will have moved out from behind the cactus. Call the location where you have stopped to take a second look ‘Y’. (To associate this analogy with the finger exercise above: X is like the view from your right eye, and Y is like the view from your left.)