The Problem of the Planets

The night sky as it appears to the naked eye is a generally orderly place. The stars form patterns, move through the heavens in lockstep, and even the complex movement of the sun and moon can be charted. The planets, however, presented a problem to ancient observers.

Five stars moved about in odd motions that simply did not fit well with the movement of everything else -- the Greeks called them "wandering stars"—and one of the great challenges of ancient astronomy was accounting for their movements.

The fellow who did the best job was a Greek working in second-century Alexandria, named Ptolemy. His system placed the earth at the center of the universe and it did a pretty good job of accounting for the movements of the lights up in the sky.

Ptolemy's model of the universe was accepted throughout the Middle Ages, though not without revision. His model was a little ragged at the edges and more accurate observations revealed discrepancies, particularly in regard to the movement of the planets.Uusing tables based on Ptolemy's model, medieval astronomers made predictions regarding the position of this or that planet and the planets did not show up on time.

Even Ptolemy had known that the simplest model, which had each planet moving in a circular orbit about the Earth, led to gross inaccuraces. To compensate, he invented the notion of epicycles; that is, a circular orbit whose center in turn moved in a circular orbit. For example, Venus did not move directly around the Earth, but rather moved in its own orbit. The center of this orbit, however, did move around the Earth. Everything moved in perfect circles, of course, because a circle was a perfect shape and Heaven was a place of perfection.

By the later Middle Ages, increasingly accurate observations had led to increasing elaborations of Ptolemy's systems. Epicycles were added to epicycles until the planets were clanking about in a ludicrous contraption of scores of intersecting circles.

Many among the learned were uncomfortably aware that the situation was downright embarassing. With as many as 200 and more epicycles wheeling about, the whole system was looking less and less divine. And, to make matters worse, the planets still were not showing up on time. The invention of relatively accurate timekeeping devices were, by the 15th century, badly fraying the fabric of the Ptolemaic universe.

One easy way out, long known in the West, was to make the sun rather than the Earth the center of the universe. Such a solution brought its own train of problems, however. For one thing, the mathematical calculations required to create the tables were much more complex. For another, certain passages in the Bible seemed to imply clearly that the sun went around the Earth; the Church had been teaching this as dogma for some time, and a heliocentric theory would have to fight a theological battle as well as a scientific one. Page 2

The man who began the destruction of the Ptolemaic system was Nicolai Copernicus, a Polish astronomer. He created a model of the universe that placed the sun at the center and he worked out many (though not all) the mathematical problems entailed in that change.

Copernicus studied mathematics at the University of Padua, which was one of the leading universities in Europe. He was convinced that Ptolemy was wrong to place the Earth at the center. The system was too complex. God and Nature did not work that way. Moreover, the recovery of Greek scientific writings showed that even some ancient authorities had likewise argued that the sun was the center and not the Earth.

Copernicus moved the Earth into orbit around the sun and proceeded to solve the mathematics involved. He wrote up his ideas and his methods in a book, then carefully put the book away without having it published.

This was, after all, the early years of the Reformation. Issuing a book that so blatantly challenged the authority of the Church was a risky matter that could lead to excommunication or worse. Copernicus was no Lutheran, ready to defy the Catholic Church, so he chose to keep silent.

Copernicus’ book, On the Revolution of the Heavenly Spheres, was not published until 1543, the year Copernicus died. This was done by his own wish: he wanted his book published, but he did not want to risk the wrath of the Church while he still lived.

One hindrance to widespread acceptance was that, while Copernicus greatly reduced the number of epicycles, he found that he could not do away with them completely. He still had to introduce some in order to make the heavens move properly.

But who was to say that his fine-tuning of the Universe was preferrable to any other? At least Ptolemy had the weight of tradition and the approval of the Church.

Even more telling, though, was the fact that the planets still did not show up on time. The very best observers still found discrepancies in Copernican tables, and there was no room for tardiness in a perfect Heaven.

Even so, some were convinced. And they set about defending Copernicus and trying to correct the errors.

The first telescope was made in Holland, by a Dutch lens maker who hit on the idea of putting two lenses at each end of a tube and looking through it. Galileo read about or saw this invention and forthwith built his own. He ground his own lenses, constructed his own tube, and produced a telescope with a power of magnification of about 10 -- more than twice as powerful as the one the Dutch had made.

Galileo built his telescope in 1610 when he was living in Venice. The first thing he did with his invention was try to make money from it. He showed it to Venetian Page 3 businessmen, showing them that with his device they would be able to see ships entering the port of Venice long before anyone else could. By noting the colors and cut of the ship, they would know who owned it and therefore would know its cargo. This would give them crucial market knowledge hours ahead of their competitors.

Galileo soon had orders to build more telescopes. Had he done only this, he would have been known as a great inventor. But he went further. He pointed his telescope up, to the night sky. And what he found there changed the scientific world forever.

Galileo saw a number of remarkable things with his new telescope, but he not only saw them, he made careful record of what he saw and then published his findings in a book. The contrast between he and Copernicus is significant. The one feared the Church and suppressed his own book. The other believed so passionately in the truth that it never even occurred to him that there would be a problem and so rushed to press. The one worked in theories and mathematics; the other was a craftsman and an observer.

Galileo was a natural showman, with an instinct for popular appeal, and his book reflects his nature, from the catchy title to the bragging tone to the detailed illustrations. His work immediately captured attention and was soon translated.

The telescope revealed that the moon had a surface that was not smooth and perfect but was covered with hills and valleys like Earth was. This was clear evidence that the objects in the sky were not made of qualitatively different material but were shaped in some sense like Earth.

Looking at Jupiter, Galileo discovered four lesser bodies in orbit around it. This required careful and patient observation, for it is not immediately obvious to the observer that the three or four little lights near Jupiter are not merely other stars. Jupiter therefore had moons even as Earth did.

Galileo had always been an advocate of Copernicus' theory, and in observing Venus he had proof positive that he was right. Venus went through phases, like the moon did. The only way to explain the phases of Venus is that it is in orbit around the sun, that the Earth is in orbit around the sun, and that Venus' orbit lies inside our own. Galileo was giddy with triumph. He knew full well the significance of his discoveries, that he had overturned a theory that was over a thousand years old. He trumpeted his views everywhere, and condemned as a fool anyone who was not immediately persuaded.

Powerful figures within the Church were not persuaded. All through the 1620s, as religious war raged in Europe, Galileo's friends begged him to tone down his rhetoric, but he would not.

He had many friends in the Church who knew he was right, and they were chief among those urging him to be moderate. In the 1640s, a new pope came to power, and Galileo's friends could no longer protect him. His writings were condemned and he was summoned to Rome to answer for his views. Page 4

He was nearly seventy years old when he came before the Roman Inquisition. His writings were heresy, they told him, and he was to recant. At first he refused, but when he was threatened with torture, he could not hold out. He signed the confession and recanted his teachings.

Even with his writings out among the various Philsophers, a thoughtful and open-minded person might still have certain doubts about the Copernican universe. One problem was that Galileo did nothing to make the planets show up on time, for the Copernican model still had the planets moving in circles.

The other problem also related to the movement of the planets. If they moved in orbits about the sun, what kept them on their circular tracks? If the moon was an object like any other object, why did it not fall to the ground (Earth) like any other object? What kept the planets in the sky and in orbit?

Kepler is the man who solved the final mathematical problems that were preventing accurate prediction of the movement of the planets. He was a German, who studied under Tycho Brahe, the famous Danish astronomer.

Kepler was a brilliant thinker, and a masterful mathematician. He was the first to have a law named after him since the ancient Greeks, a tribute to Johannes himself but also a mark of the times, that people believed they were accomplishing feats as great as the ancients had managed. The most significant, for our narrative, was Kepler's description of how a body might move in an elliptical orbit.

The ellipse was the key to making Copernicus' system work. Kepler had inherited the careful observations of Brahe, and these allowed him to work out the mathematics. What he discovered was that a body moved faster as it moved through one end of the ellipse than it did when moving along the other end.

So long as astronomers had the planets moving in perfect circles, their speed was assumed to be constant, and so the tables of predictions were never quite right. Kepler allowed the planets to move in ellipses, and at last the planets began to show up on time.

Kepler needed Copernicus to put the sun in the center of the universe (still no one imagined our Sun to be just another star). He needed Galileo to demonstrate both that Copernicus was right and that the bodies in the heavens were not perfect (and so their motion likewise might not be perfect). And he needed Brahe's precise observations to provide the raw data.

One issue yet remained: if the planets were ordinary bodies moving through the sky, subject to the same laws that applied to moving bodies here on Earth, what kept the planets up there? Everything else falls to the ground. Why not them?

That was the puzzle solved by Sir Isaac Newton. Page 5

Newton was an Englishman. He was accomplished in many fields, and his formulation of the law of gravity was only one of them. He was a brilliant mathematician in his early life -- he created calculus in order to solve certain problems he was working on..

What holds the planets up? Moreover, if they are all whizzing about the sun, what keeps them from flying off into space?

Rene Descartes, a French, argued for vortices, or ether.

Newton's gravity was like make-believe. He was proposing invisible forces that could not be detected. But the model worked. And only a handful of men could even follow the math - he was miles ahead of the others.

He was supported by the newly-formed Royal Society and his views were popularized and spread widely in the press.

His view of the world did not become dominant until 1690s, and even later on the Continent. But it did finally solve the Problem of the Planets completely!

History of Western Civilization E.L. Skip Knox Boise State University http://history.boisestate.edu/WESTCIV/science/