The Story of Astronomy

by Peter Aughton

  Copernicus is remembered for his great work De Revolutionibus Orbium Coelestium (Concerning the Revolutions of Celestial Spheres), in which he argued the case for the heliocentric universe. His idea of the Sun-centered universe was first mooted, however, in a smaller volume called the Hypothesibus Motuum Coelestium a se Constituis Commentariolus (usually simply known as The Commentary). It was only about 20 pages in length and was never published, but in this early document are the main reasons behind Copernicus’ thinking. He specified seven basic assumptions, most of which were heretical in his time:

  1. The celestial circles or spheres do not have a common center. Ptolemy had introduced the idea of an “offset” circle to help produce more accurate motions of the planets. On the Ptolemaic system they all orbited about different centers.

  2. The center of the Earth is not the center of the universe, but only the center of gravity and of the lunar orbit. This is the heretical assumption. But notice that the Earth has not surrendered everything to the Sun. Copernicus had no knowledge of gravity outside the Earth. He had no reason to believe that the other planets had gravity, so he accepted that everything in the universe was drawn to the center of the Earth. It was the center of gravity of his world model.

  3. All the spheres revolve around the Sun, so that the center of the world is near the Sun. Copernicus came to this conclusion when he discovered how much simpler the heliocentric system was than the geocentric system. His other great step forward was to recognize that the Sun must be much larger than the Earth and the other planets.

  4. The distance to the Sun is insignificant when compared with the height of the firmament. Copernicus knew that the distance to the stars was much greater than the distance to the Sun. It was still several centuries before stellar distances could be measured.

  5. The motions appearing in the firmament are not its own motions, but those of the Earth. The Earth performs a daily rotation about its fixed poles while the firmament remains immobile as the highest heaven. A crucial point very well described by Copernicus.

  6. The motions of the Sun are not its own motions, they are the motions of the Earth and out sphere with which we revolve around the Sun just as any other planet does. The Earth moves around the Sun as well as spinning on its axis. The motion of the Sun as seen from the Earth is the result of both factors. The Sun was stationary on Copernicus’ model.

  7. What appears to us as retrograde and forward motion of the planets is not their own, but that of the Earth. The Earth’s motion alone is sufficient explanation for many different phenomena in the heavens. Copernicus showed, using diagrams, how the motion of the planets was sometimes retrograde or backward. The retrograde motion was easy to explain on the heliocentric system.

  There is much repetition in these postulates, but they are consistent with each other and they indicate how Copernicus arrived at his conclusions. The seventh postulate concerns the retrograde motion of the planets; he knew that this effect could be explained very simply if the planets were assumed to orbit the Sun. He also knew that the planets Mercury and Venus were never far from the Sun, and it was obvious to him that they could not orbit around the Earth as Ptolemy had suggested. He also realized that the whole world system would be greatly simplified if the Sun were considered to be at the center of the universe. He observed that Mars was almost as bright as Jupiter when it was near the Earth on the same side of the Sun, but when it reached the far side of the Sun it was very faint and obviously very much further away. He was very scientific in his findings and he concluded from observations of planets such as Mars and Mercury that the Ptolemaic system did not give accurate distances from the Earth to the planets.

  An Orderly System

  Copernicus postulated that if the Sun was assumed to be at rest and the Earth and the other planets were assumed to be in motion around it, then the remaining planets fell into an orderly relationship whereby their sidereal periods increase from the Sun in a relationship to their distance from the Sun. He calculated, very accurately, the periods for the planets to be: Mercury—88 days; Venus—225 days; Earth—1 year; Mars—1.9 years; Jupiter—12 years; Saturn—30 years. This theory resolved the disagreement about the ordering of the planets, but it also raised new problems. To accept the theory’s premises it was necessary to reject Aristotle’s natural philosophy and develop a new theory to explain why heavy bodies fall to a moving Earth. It was also necessary to explain how a body like the Earth, filled with floods, pestilence and wars, could be part of a perfect and imperishable heaven. Copernicus was working with many observations that he had inherited from antiquity and whose reliability he could not verify. In constructing a theory for the precession of the equinoxes, for example, he was trying to build a model based upon very small long-term effects, and his theory for Mercury was left with some incoherencies. Any of these considerations could account for Copernicus’ delay in publishing his work, but he gave another reason in the preface to the book. He claimed that he had chosen to withhold publication, not merely for the nine years recommended by the Roman poet Horace whom he greatly admired, but in fact for 36 years, because of what he knew to be the book’s heretical standpoint concerning a heliocentric universe.

  Copernicus spent many years perfecting his ideas and writing De Revolutionibus Orbium Coelestium. He was certainly working on it by 1515, and as his ideas developed further his manuscript ran to 200 pages. He could not resist telling his ideas to some of his close friends, and during his lifetime news of the Copernican system filtered through to other astronomers. The final version of the book contained a section on the solar system followed by a star catalog. As with his predecessors from the ancient world, he included a chapter on the precession of the equinoxes and a section on the motion of the Moon. He then devoted two sections to the motion of the planets.

  Copernicus’ Vision

  Although Nicolaus Copernicus put the Sun, and not the Earth, at the center of the universe, some minor details on his model were still wrong. He maintained, for example, that the orbits of the planets were perfect circles around the Sun when in fact they are ellipses. We know he was not the first to suggest the idea of the Earth orbiting the Sun, and we know he had access to the work done by Aristarchus long before him. He himself knew that he was not the first astronomer to propose a heliocentric universe either. But Copernicus was a great thinker and a man of bravery and vision. He knew that his theory would create a revolution in astronomy but he also wanted the world to know that he was right and that his system was the truth. In this respect he succeeded in his ambitions, and he takes his place amongst the greatest of the world’s astronomers.

  Great Minds Thinking Alike

  When Copernicus was in his sixties, a 25-year-old professor of mathematics at Wittenburg called Rhaeticus heard about his heliocentric theory. In 1539 he sought out Copernicus, and the two discovered they were in complete agreement about the theory. They enjoyed each other’s company so much that Rhaeticus stayed on for two years. During this time he pressed Copernicus to publish his work. But Copernicus knew there would be a great outcry when his heretical ideas were known, and he intended to delay publication until after his death. The book did indeed remain unpublished until 1543, after Copernicus had died. In the meantime Rhaeticus published a volume called Narratio Prima outlining the Copernican theory. Rhaeticus’ work preceded De Revolutionibus by several years, but he gave full credit for the idea of the Sun-centered universe to Copernicus. There is a tradition that Copernicus held his own book in his hands as he lay on his deathbed—if true, it epitomizes the problems faced in those times by men with heretical new ideas about the universe.

  6

  CHASING THE PATHS OF THE PLANETS

  In the late 16th and early 17th centuries two astronomers strove to calculate the positions of the planets. Tycho Brahe spent many years measuring the paths of the planets across the sky from his observatory on the island of Hven, while Johannes Kepler was able to prove from this data that the orbits of the plan
ets were ellipses.

  It was Christmas 1566, and in the town of Rostock in Germany a university professor was holding a festive gathering for staff and students. At some point during the proceedings, perhaps fueled by wine, a furious row broke out between two of the guests, both of them Danish noblemen. Before anyone could intervene, one of the participants threw down the gauntlet and challenged his antagonist to a duel. A week later tempers had still not cooled off, and so the duel took place at an appointed place in the nearby countryside. One of the noblemen was called Manderup Parbsjerg, and the other was called Tycho Brahe (1546–1601). When the swords were drawn and the blades were flashing in anger it was Parbsjerg who got the better of the encounter. Tycho Brahe’s nose received a severe cut, his face was covered in blood and he was forced to concede. Brahe had his nose patched up with a concoction of gold, silver and wax. The doctor did a good job and, apart from a rather startling appearance that sometimes gave him an advantage, Brahe was hardly troubled by his injury. His relationship with Manderup Parbsjerg was also patched up and the two became good friends.

  Astrological Shortcomings

  The “man with the golden nose” was a very successful astrologer. When he witnessed an eclipse of the Moon just two months before his duel with Parbsjerg, he forecast that the sultan of Turkey was about to die. Soon afterward, news arrived that the sultan had indeed died. This enhanced Brahe’s reputation and it also convinced him of the truth of astrology. Then news arrived that the sultan had died before the eclipse of the Moon and the eclipse was therefore not a valid prediction of the event. This was disturbing, and Brahe soon discovered that there were other problems with his astrological predictions. He had to admit that many of them did not seem to work. He was very confident that his methods were correct, so he concluded that the problem lay with the fact that he was unable to calculate the positions of the planets with sufficient accuracy when he was casting the horoscopes. He had two sets of tables at his disposal, one was based on the well-tried system of Ptolemy, the other was a new set of tables based on the heretical ideas of an upstart Pole called Nicolaus Copernicus, who thought that the Sun was at the center of the universe. Brahe discovered that neither of the two systems, Ptolemaic nor Copernican, gave him accurate positions for the planets. It was bad news for his horoscopes, but he was undaunted and he made a decision to dedicate his life to calculating the positions of the planets as accurately as he could.

  Brahe continued with his astronomical studies. Then he made a discovery that caused him great concern. In 1572 he witnessed a very rare event, in the constellation of Cassiopeia. A very bright new star had appeared. Brahe knew that all the world’s astronomers agreed that the sphere of the stars was fixed; it had been made by God at the creation and it never changed. New stars simply did not appear. He first saw the star in November 1572 when it was brighter than Jupiter. He knew he was not in error, for it was unmistakable. For several months it was the brightest star in the sky. By December its brightness had faded to equal Jupiter. By March it had faded again, but it still ranked with the first magnitude stars. It dimmed steadily through the magnitudes, until by April 1574 it was no longer visible. What Brahe was observing was a supernova—an event so rare that it has happened only three times in our galaxy during the past 1000 years. However, Brahe was more interested in the astrological significance of the new star. It was not good news:

  The star was at first like Venus and Jupiter, giving pleasing effects; but as it then became like Mars, there will next come a period of wars, seditions, captivity and death of princes, and destruction of cities, together with dryness and fiery meteors in the air, pestilence, and venomous snakes. Lastly, the star became like Saturn, and there will finally come a time of want, death, imprisonment and all sorts of sad things.

  On the Island of Hven

  In time, Brahe decided to move from Denmark to Germany, but when King Frederick II heard about this he became alarmed to think that he might lose the services of such a wise man. So the king made Brahe a generous offer. He was to have his own private observatory. It would be built on the island of Hven, an isolated but inhabited piece of land in the straits between Denmark and Sweden. Brahe would become landlord of Hven, and by collecting the rents from the local farmers he would have financial independence over and above his royal patronage as well as his own small kingdom. Tycho Brahe could not refuse such an offer. He began to build the most magnificent observatory the world had seen.

  Brahe’s observatory looked like a magical fairy-tale palace. Built in the Flemish style, it rose to 12 meters (40 ft) in height, and was surmounted by domes, spires and pinnacles sufficient to grace a cathedral. It had two semicircular observing bays on the north and south walls. It was also a luxurious home with running water in the bedrooms. It even had a jail—a useful facility for tenants who could not, or would not, pay their rent! The observatory became known as the Palace of Uraniborg. Brahe kept a dwarf called Jep to enhance his importance, and he also acquired two large dogs, presented to him on a royal visit by King James VI of Scotland.

  Brahe was an artist as well as a scientist and craftsman, and everything he undertook or surrounded himself with was innovative and beautiful. He imported Augsburg craftsmen to construct the finest astronomical instruments. He established a printing shop to produce and bind his manuscripts in his own individual way. He induced Italian and Dutch artists and architects to design and decorate his observatory, and he invented a hydraulic pressure system to provide one of the great luxuries of the time—sanitary lavatory facilities. Uraniborg fulfilled the hopes of Brahe’s king and friend, Frederick II of Denmark, that it would become the center of astronomical study and discovery in northern Europe.

  The greatest and most unusual feature of the building was the many astronomical instruments it contained. There were quadrants, sextants, armillary spheres, parallactic rules, astrolabes and clocks. On the island of Hven there was every astronomical instrument known to mankind, all made by skilled craftsmen and fashioned to the highest quality. The largest instrument was the great mural quadrant that could measure the positions of the planets to within a few minutes of arc.

  Nightly Vigils in the Sky

  Night after night Brahe and his assistants searched for the planets and carefully measured their positions in the night sky. Month after month the positions of more and more stars were added to a great catalog of 777 stars—all located with greater accuracy than ever before. Night after night and year after year for 20 years the lonely vigil was kept on the island of Hven. The data were collected for what would turn out to be the last, and greatest, of the catalogs created using observations made with the naked eye, and everything was carefully recorded. But where was it all leading? What was the purpose of this great enterprise? The main object was to create a set of tables to record the positions of the stars more accurately than ever before, but it also involved plotting the positions of the planets whenever they were visible—this was in some ways a greater task, for they changed their positions nightly. But Brahe wanted to do more than simply record the nightly positions of the planets. He wanted to predict what would happen in the future as well as what had happened in the past. What he really needed more than anything was a mathematician to study the data and to formulate a new theory that could predict the planetary positions in the future. Such knowledge would be a great bonus to the science of astrology.

  Then Brahe received a great setback to his ambitions. In 1588 King Frederick II died, and it became clear that his son and successor Christian IV was not prepared to patronize the eccentric astronomer any longer. Brahe was forced to continue his work funded by his own resources. He soldiered on for nearly a decade in this way, but in 1597 he finally left Hven and by 1599 he had moved to Prague. It was there that he was fortunate enough to obtain a second royal patron in the person of Rudolf II, the Holy Roman Emperor, who was prepared to pay for the publication of his works. It was also in Prague that Brahe met a much younger man called Johannes Kepler (1
571–1630). As we shall see shortly, the meeting between Brahe and Kepler was a famous and significant one, for Brahe found in Kepler someone who was capable of formulating a mathematical theory that would fit his data to the motion of the planets.

  The Hypochondriac Mathematician

  Our story moves to Germany, where Johannes Kepler was born in 1571 in the town of Weil de Stadt. Kepler was a small, frail man. He was near-sighted, and he was a hypochondriac. He was always plagued by fevers and stomach ailments. He was also a strange and mystical character who was very interested in astrology, and at least 800 of his horoscopes are still preserved. When he was casting horoscopes for his family he described his grandfather as “quick tempered and obstinate,” his grandmother was “clever, deceitful, blazing with hatred, the queen of busybodies,” his father Heinrich was “criminally inclined, quarrelsome, liable to a bad end” and his mother was “thin, garrulous and bad-tempered.” In later life he spent many months trying to clear his meddlesome mother of a charge of witchcraft.

  In 1597 Kepler married Barbara Muller. She had been twice married before and she was twice widowed. When he first met her she “set his heart on fire,” but unfortunately when they tied the matrimonial knot he did not consult his horoscopes or he would have realized that the planets were in the wrong signs of the zodiac for such an event to take place. The marriage produced two children, but both died very young. Kepler became disillusioned with his wife when she told him in no uncertain terms that his precious astrology was nothing more than nonsense. He responded by accusing her of being “fat, confused and simple-minded.”