cphil_astro2
ASTRONOMY
CONTINUED
by Ray Shelton
For the next 1400 years after Ptolemy died about A.D. 150, the geocentric system of Ptolemy dominated astronomical thinking. Plato’s geocentric views of the heavens came into the Christian theology of Augustine (354-430 A.D.) by way of Neo-Platonism of Plotinus (205-270 B.C.). Augustine rejected the Plotinian emanation theory of the origin of the world whereby the cosmos eternally issued or proceeds from God by necessity. Augustine held the Biblical teaching that the heavens and the earth are the free act of creation by God out of nothing. This doctrine of the free creation of the world out of nothing is not found in Plotinus’ or Plato’s writings. For Plotinus God does not act freely since such activity of God would mean that there is change in God. Augustine may have thought that Plato had taught creation out of nothing in time in the Timaeus, but this is improbable, since Augustine insisted that God created out of nothing, not out of a pre-existent matter which the Demiurge formed according to the Eternal Ideas or Forms. Augustine, no doubt, accepted the Platonic doctrine of Eternal Ideas as rationes aeternas existing in the mind of God. Augustine also used the Platonic concept of numbers, which goes back to the Pythagoreans; he look on numbers as the principle of order and form, of beauty and perfection, of proportion and law. The Ideas are eternal numbers and material bodies are temporal numbers, the rationes seminales, which unfold themselves in time. This doctrine implied the Platonic geocentric view of the cosmos and that the motions of heavens are circular and uniform.
This Christian Platonism dominated Christian theology until Thomas Aquinas (1225-1274 A.D.) joined Aristotelian philosophy to Christian Augustinian theology, rejecting the Augustinian doctrine of rationes aeternas and rationes seminales. The Christian west got this Aristotelian philosophy from the Arabic translations of Aristotle’s writings. The Moslems had incorporated Aristotelian philosophy into their theology and the Ptolemaic system into their astronomy. As the works of Aristotle became known in western Europe, a shift took place away from the Platonic and Neo-Platonic idealistic ontological philosophy toward the more realistic and empirical philosophy of Aristotle. The Aristotelian astronomical views were incorporated into Christian Theology, so that the view of the cosmos is still geocentric and celestial motion is still considered circular and uniform. The Ptolemaic astronomical system supplied the details and thus became the authoritative and unquestionable teaching of the Church in the Middle Ages. Thomas Aquinas in his voluminous work Summa Theologica published his systematic synthesis of Christian theology and Aristotelian science and philosophy, which became and remains the official theology of the Roman Catholic Church. The earth was the center of the universe, because God had chosen it as the abode of man and as the stage for the whole drama of the salvation of man. The sun, moon and the planets revolved about the earth, each set in “crystalline spheres” concentric on the earth. God and the Savior dwelt in the outermost sphere as the heaven of the heavens. God as the “unmoved mover” kept the crystalline sphere of the stars in motion, and the planets beneath moved in their course by frictional resistance between the nest of concentric spheres. Hell was at the center of the earth and purgatory located at the antipodes opposite Jerusalem. The universe is finite and the sphere of the stars is not very far away. Dante’s epic The Divine Comedy gives a poetic and dramatic picture of this cosmology. In this epic, the poet is able to travel through all the spheres in seven days; the abode of blessed is not far away. Even though the heavens are not far away, they are qualitatively different from the earth; the heavens are incorruptible and unchanging, whereas the earth is corruptible and changing.
Nikolaus Koppernigk, who used the Latinized form his name, Nicolaus Copernicus, was born at Torun on the Vistula, somewhere in the no-man land between Prussia and Poland, on February 19, 1473. He studied at the University of Cracow. Upon graduation in 1495 his uncle Lucas, who was a Bishop, appointed him Canon of the Frauenberg Cathedral, a job with a good income and virtually no duties. Copernicus immediately left for Italy, where he studied at the Universities of Bologna and Padua for ten years. During his studies he encountered the speculation that the sun, not the earth, was at the center of the universe. But, unlike Ptolemaic system, there were no calculations to back it up. Copernicus apparently decided during his stay in Italy to make this hypothesis of sun-centered universe the basis for new calculations of planetary positions. He pursued this project for 36 years. In 1506 he returned to Poland with the degree of Doctor of Canon Law and with considerable study in medicine and in mathematics. His uncle Lucas freed him from his duties as Canon to make him his private physician and secretary at the bishop’s residence at Heilsberg Castle. His duties left him with plenty of time to pursue quietly his calculations and prepares two manuscripts: one a Latin translation, which was printed, the other an outline of his heliocentric system of the universe, which was never published during his lifetime. It is known as the Commentariolus or Brief Outline. It was written as a letter, in response to a direct inquiry concerning his astronomical work. It was copied by hand by interested people, including himself, and was circulated to a small audience around Europe. It begins with a historical introduction in which Copernicus shows that the Ptolemaic system of astronomy was unsatisfactory because it did not fulfill the ancient basic demand that each planet must move in perfect circles with uniform speed. He points out that in the Ptolemaic system the planets do move in circles but not with uniform speed. He then traces this failure to the use of equants. He says that a systems of this sort are “neither sufficiently absolute or sufficiently pleasing to the mind.” Copernicus then claims that he has constructed a system that corrects this defect. Then he sets forth the seven basic assumptions or postulates of this system of his. The following is a paraphrase of those postulates:
(1) Heavenly bodies do not all move about the same center.
(2) The earth is not the center of the universe; only the moon’s orbit and gravity are centered on the earth.
(3) The sun is the center of the planetary system and therefore of the universe.
(4) Compared to the distance from the earth to the fixed stars, the distance from the earth to the sun is negligibly small.
(5) The apparent daily revolution of the fixed stars are due to the rotation of the earth, not to their actual rotation.
(6) The apparent motion of the sun arises not from the motion of the sun but to the motion of the earth. The earth rotates on its axis once a day and it revolves about the sun once a year, like the other planets.
(7) The apparent retrograde motion of the planets arise not from their motion but from the motion of the earth with respect to them.
Then in seven short chapters the motion of the sun, of the moon and of the planets with their deferents and epicycles are set forth, but without proof or mathematical demonstration, which he reserves for a larger work. But in the last paragraph of his paper he announces the result of that proof or demonstration.
“Then Mercury runs on seven circles in all; Venus on five; the earth on three, and round it the moon on four; finally Mars, Jupiter, and Saturn on five each. Altogether, therefore, thirty-four circles suffice to explain the entire structure of the universe and the entire ballet of the planets.”
Apparently Copernicus thought that the apparent simplicity of his system for astronomical calculations would recommend his system. In 1512, Bishop Lucas died, and Copernicus was obliged to take up official duties as Canon at the Frauenberg Cathedral. In about 1530, he finished his book On the Revolution of the Heavenly Spheres, only to lock it away and work on it from time to time, but he would not publish it. Nevertheless, people knew that he was working on a radically new planetary calculations. In spite of himself, he had a reputation. A young German firebrand, Georg Joachim Rheticus, became his disciple. Having heard a rumor of Copernicus’ work, Rheticus came to Poland to find out about it; he arrived in Frauenberg in the summer of 1539. In November, 1539, Rheticus published his own sketchy account of the Copernican system, entitled the Narratio prima – The First Account, which did not mention Copernicus by name, but refers to him as “my Teacher” and his Book of Revolutions as by “the most learned and most excellent mathematician, the Reverend Father, Dr. Nicolas of Torun, Canon of Ermland”. According to Rheticus, Copernicus was offended and annoyed with the Ptolemaic concept of the equants and sought for a simpler geometry to explain the appearances. Copernicus wanted the circle of the orbits of the heavenly bodies to turn on their own centers rather than eccentrically. By placing the sun at the center of the system, he was able to eliminate the equants. Copernicus also was able to make do with only 34 circles (epicycles and differents) as compare to 79 circles required by Fracastoro revision of the Ptolemaic system. According to Arthur Koestler:
“In fact, Copernicus uses altogether forty-eight epicycles – if I counted them correctly…. In other words, contrary to popular, and even to academic belief, Copernicus did not reduce the number of circles, but increased them (from 40 to 48).”
Rheticus with a help of a friend of Copernicus, Canon Giese, persuaded Copernicus to publish. From the summer of 1540 to September of 1541, Rheticus edited the manuscript and took it to Nuremberg, Germany, to be set in type. But Rheticus had to leave Nuremberg before the printing was finished to take the important position of Chair of Mathematics at Leipzig University. He left the supervision of the book in the hands of the leading Lutheran theologian and preacher of Nuremberg, Andreas Osiander (1498-1552). Osiander was not only favorable to Copernicus, but took an active interest in his work, having corresponded with him in the last two years. He wrote a preface to Copernicus’ book, On the Revolutions of the Heavenly Orbs, which he did not sign. Later Kepler discovered in 1609 that the preface had been written by Osiander. Even though written by Osiander, the preface expresses Copernicus’ view of his work.
The preface was titled “TO THE READER, CONCERNING THE HYPOTHESIS OF THIS WORK.” It started by explaining that the ideas of the book need not be taken too seriously: “For these hypotheses need not be true or even probable”; it is sufficient that they save the appearances. Then the preface went on to demonstrate the improbability “of the hypotheses contained in this work” by pointing out that the orbit ascribe to Venus would make the planet appear sixteen times larger when closest to the earth as when farthest away “which is contradicted by the experience of all ages”. Copernicus knew Osiander’s views. Two years earlier, when Copernicus was still hesitating whether to publish his book, Copernicus wrote to Osiander, asking for his advice. Osiander replied on April 20, 1541, quoted in Kepler’s Apologia Tychonis contra Ursum:
“For my part I have always felt about hypotheses that they are not articles of faith but basis of computations, so that even if they are false, it does matter, provided that they exactly represent the phenomena…. It would therefore be a good thing if you would say something on this subject in your preface, for you would thus placate the Aristotelians and the theologians whose contradictions you fear.”
In his work, Copernicus was careful not to challenge the prevailing view of the universe. He kept the crystalline sphere of the fixed stars and the nest of concentric spheres in which the motion of the planets is circular and uniform. Copernicus departed from the Ptolemaic system in only two ways:
(1) the planets move about the sun as their center rather than the earth. That is, the earth, with the moon moving about the earth, also moves about the sun, along with the other planets;
(2) there are no equants in Copernicus’ system. But he kept the eccentrics and the circles of epicycles and deferents, because he still considered the motion of heavenly bodies as circular and uniform.
Neither did he challenge the Aristotelian concept of terrestrial motion. He presented his work as an alternate method of calculation and did not claim that it was necessarily true. Neither did he draw any implications of his work and certainly did not consider his work heretical. When it was finally published in the year that he died, 1543, it hardly caused a stir. The first copy of his book was delivered to him on his death-bed on the day he died, May 24, 1543.
In spite of Copernicus’ attempt to avoid challenging the current Aristotelian world view, the astronomers of his day did not accept his modifications of it. Contemporary astronomers rejected his idea that the earth was in motion for two major reasons:
(1) the failure to find any stellar parallax, [Parallax is defined as the angular, or apparent, shift in the position of an object due to a change in the position of the observer. Stellar parallax is the annual apparent shift of the stars that results from the earth’s orbital motion. For the nearest stars it is observable with modern telescopes, but it is impossible to see and measure it with the naked eye, because of the great distances of even the nearest stars.] and
(2) that no explanation was provided how such a massive a body as the earth could be kept in motion, or, if it was in motion, why things did not fly off of it or are not left behind. Even though Copernicus tried to avoid drawing any implications of his modifications of Ptolemaic system, others did. The major implication that was drawn was that, if the earth is not the center of the universe, the Aristotelian concept of terrestrial natural motion of bodies to the natural place of the predominate element was wrong. Also the imperfect earth would be placed among the perfect heavenly planets to move in perfect circular and uniform motion, which is impossible in the Aristotelian world view. If Copernicus was right about the location of the earth, the whole of the Aristotelian world view would be destroyed. And since the Aristotelian world view was integrated into Christian theology by the Mediaeval scholastics as well as the Protestant scholastics after Luther and Calvin, the world views of Roman Catholic and Protestant theology was threatened. In particular the place of God in the third heavens was threatened.
The Copernican astronomy implied that the universe was very large. Since the stellar parallax was not observed, and, if the earth was in motion in an orbit about the sun, the base line for observing the stellar parallax would be the distance between the positions of the earth on opposite sides of its orbit six months apart, the calculations of the distance to the stars on this bases indicated that the stars must be very far away. Also Copernican view implied that, since the earth rather than the celestial sphere of the stars are in motion, the stars did not have to move. And if the stars did not move, then they did not have to be embedded in a rotating celestial sphere. That would mean that stars could be distributed throughtout space, the less bright stars being farther away. Thus the size of the universe might be infinite, since it could no longer be said to be finite. The possibility of an infinite universe put the place of God and angelic hosts in the heavenly sphere, even the very existence of God, in doubt. The Italian philosopher, Giordano Bruno (1548-1600), who was executed by the Inquisition on February 17, 1600 for denying the Trinity, drew the conclusion from the Copernican astronomy that the universe was infinite and that the sun also is not at center of the universe, because the universe has no center. Bruno’s views drew the attention of Roman Catholic Church to the Copernican theory.
Soon after the Roman Catholic Church became alarmed at the spread of Copernicanism, especially after Galileo Galilei published in 1610 his Siderus Nuncias, Star Messager or Messager from the Stars, in which he announced some of early observations of the heavens with the telescope and seemed to support the Copernican astronomy. In 1616, Copernicus’ book was condemned by the Roman Catholic Church for teaching that the earth moves, contrary to the teaching of the Church and Scriptures, placing it on the Index Expurgatorus in the year 1616 where it remained until 1620, when the “corrections” were published. From then on, any Catholic publisher was free to reprint the corrected Book of Revolutions, but no Catholic or Protestant publisher did for nearly three hundred years.
Not all the astronomer immediately after Copernicus accepted the Copernican astronomy. Tycho Brahe (1546-1601), the great Danish observational astronomer, rejected the Copernican heliocentric system and proposed his own system in which he modified the Ptolemaic system, while keeping the earth at the center of the universe with the moon and sun revolving around it, he had all the planets revolving around the sun. It was a compromise between the Ptolemaic and Copernican systems. On his deathbed Tycho pleaded with his assistant, Johannes Kepler (1571-1630), to support his modified system and give up his interest in the Copernican system, which Tycho considered as obviously absurd. As late as 1622, the English philosopher, Francis Bacon (1561-1626), who is sometimes called “the father of scientific empiricism,” dismissed the Copernican system, even after hearing of the telescopic evidence; he criticized Copernicus of inventing “fictions” which are not based on sound philosophical foundations but purely for the purpose making the calculations come out right.
The so-called attack on Copernicus by the German Protestant reformer Martin Luther (1483-1546) with the words that “this fool wishes to reverse the entire science of astronomy; but the sacred scripture tells us (Joshua 10:13) that Joshua commanded the sun to stand still, and not the earth,” was among the off-hand remarks of Luther made at the dinner table in 1539, some four years before Copernicus’ book was published, and recorded by some of his students who ate with Luther; these Table-Talks, as they were called, were not published until 1566, twenty years after Luther’s death. On the basis of textual criticism by Heinrich Bornkamm, it is doubtful whether Luther even made these after-dinner remarks. It is certainly believable that Luther could have made this remark; Luther, like his contemporaries, did believe in the Ptolemaic system, since there was no alternative to it at that time. But is also possible that it came from the hand of later editor, who for reasons of opposition to Copernicus, or on the basis of hearsay, may have incorporated it. In any case, it must be remembered that this is the only reference to Copernicus in Luther’s many writings, and that the Table-Talks did not come from the hand of Luther, but from notes taken by a student eating with Luther. Definitely Luther was not planing an all-out attack on the Copernican system and was not responsible for later Lutheran opposition to new physical sciences, as is the implication of countless works on the history of science. The Genevan Protestant reformer, John Calvin (1509-1564) also accepted the Ptolemaic system, but he never attacks the Copernican system in his voluminous writings; in fact he never refers to Copernicus, by name or otherwise. The later attacks in the late 17th and 18th century on the Copernican system by Protestant theologians was not instigated by the Protestant Reformers. Protestant Scholasticism integrated Christian theology with the Aristotelian world view, which contained the Ptolemaic Astronomy. The rejection and attack on the Ptolemaic system by the philosophers of the new science were interpreted by the Protestant theologians as attacks on the Christian faith and theology, which in some cases it was.
The acceptance of the heliocentric system was not complete until after the publication of the Principia Mathematica (1687) by Isaac Newton (1642-1727). But the heliocentric system of Newton was quite different from the heliocentric system of Copernicus. Major modification by Kepler and the telescopic observations of the heavenly bodies made the heliocentric system of Newtonian mechanic quite different. Gone is the celestial spheres of the stars and of the planets with their circular and uniform motion. But more fundamentally gone was the terrestrial natural motion based on the four elements, being replaced by the Newtonian mechanics and theory of gravitation. The four elements will not be finally removed and replaced by the modern view of the chemical elements until the end of the 18th century. Newton’s work will remove the dualism of a perfect heavenly realm and the imperfect terrestrial realm and replace it with one realm of heaven and earth governed by a single set of laws. Unfortunately the Aristotelian thinking and philosophy will still remain even until 20th century, even though the Aristotelian view of the world with the Ptolemaic astronomical system is gone.
Tycho Brahe was a great Danish observational astronomer. He was born of aristocratic Danish family at Knudstrup, Denmark, three years after the death of Copernicus. He studied at Copenhagen, Leipzig, Rostock and Augsburg. He went to Copenhagen to study law to prepare to be a diplomat. But near the end of first year he saw a partial eclipse. Learning that the astronomers had predicted the eclipse, he was so impressed by such knowledge, “something so divine that men could know the motion of stars so accurately that they were able to predict a long time beforehand their place and relative position.” He decided to devote his life to this study of stars, astronomy. In 1563 the conjunction of Jupiter and Saturn was missed by a whole month because the Alphonsine Tables predicted the event wrongly. The Copernican tables missed the event by a few days. As a result Tycho dedicated his time and energy to achieving more accurate observations. Tycho’s uncle Jorgen had died while rescuing the young King of Denmark, Fredrick II, from drowning. Fredrick offered to support Tycho’s research, partly to repay the debt, partly to keep the increasing famous Tycho in Denmark, partly for nationalistic pride to make Denmark famous. Fredrick gave him the island of Hveen and he constructed, between Elsinore and Copenhagen, a castle dedicated wholly to astronomy called Uraniborg, the castle of the heavens. Each tower and turret was equipped with the most advanced instruments ever made for naked-eye astronomical observation. They were nearly all of Tycho’s design to satisfy his passion for accuracy. Tycho remained on the island for twenty-one years, observing the heavens, entertaining lavishly, and oppressing the peasants. Finally in 1597 as a result of an irreconcilable disagreement with Fredrick’s successor, Christian IV, over finances, Tycho left Denmark in search of generous patron. In 1600 he settled in Prague, becoming the official royal mathematician to the Holy Roman Emperor, Rudolph II. Less than two years later he died of the plague. The nova of 1572 made Tycho’s reputation. The word “nova”, in Latin, means “new”; in astronomy it means a new star. The nova suddenly appeared in the constellation of Cassiopeia during November, 1572; it gradually grew dimmer and disappeared from sight in eighteen months. This event caused much excitement among European astronomers and was of general interest and amazement, because according to the Aristotelian world view the heavens are perfect and unchanging. A new star could not appear in the celestial sphere and once it had appeared it could not die out. According to the Aristotelian world view, the heavens were immutable; here was a fact that contradicted that view. Unless this nova was not a star and was below the celestial sphere of stars. Tycho had no difficulty in showing that it was beyond the sphere of the moon. The moon had a parallax due to its daily rotation of the earth which shifted its position against the background of the stars. Because Tycho’s instruments were so superior to those of anyone else in Europe, he was able to measure the parallax to a few minutes of arc and clearly showed that the nova was not only farther away than the moon, but that it was much farther away. It definitely was a star beyond the spheres of the planets. Novas had been observed before and were explained as supernatural events, not natural to the celestial sphere of the stars. Hipparchus had reportedly observed a new star in 125 B.C. and an exceptionally bright new star, a supernova, was visible during the day for over three weeks and at night for over three weeks in 1054 A.D. But it had not cause the stir that the of one of 1572 did. Tycho also observed a comet that appeared in 1577 and was able show that it was beyond the moon, confirming the idea that change was possible beyond the moon in the celestial spheres. The comet was not a ball of fire in the region between the earth and the moon. His calculation showed it was beyond the orbit of Venus and that its orbit was “oblong… like an oval”; this was the first time an astronomer ever said that the orbit of heavenly body was not a circle. The determination of motion of the comet by Tycho raised a another problem: how could the comet move so freely in the space between the planets which was supposed to be filled with a crystalline material. Tycho was unable to solve this problem and it would not be solved until Newton.
Tycho’s method of collection of the planetary data was radically different from Ptolemaic method. The Ptolemaic astronomy required that only certain observations of the planets were needed to be collected to fix the calculation of the orbits of the planets: those at conjunction for Venus and Mercury and at opposition for the rest of the planets (Only three points are necessary to determine the circumference of circle). Tycho did not follow this method; instead he took as many observations that he could and these were good to four minutes of arc. This was the first real advance in observational astronomy since the Babylonian astronomer-priests. And this data became extremely important to Kepler when he tries to determine the orbits of the planets and in particular the orbit of Mars.
Tycho had difficulties with the Copernican system. First of all, he did not observe the stellar parallax which should be observed, if the earth is in motion. Since no shift of the nearest stars could be observed within the observation accuracy of 1 minute of arc, it seemed best to assume that the earth was not in motion.
Secondly, he thought that his observations showed that the stars were not very far beyond the orbit of Saturn. After visually measuring the apparent diameter of the brightest stars, he concluded that they were several minutes of arc in size. Now he reasoned that if a star the diameter of the sun were to be seen with an angular diameter of several minutes of arc, then it could not be very far away in space. Of course he was wrong, but for these observational reasons, he rejected the heliocentric system of Copernicus, thinking that his observations called for a stationary earth with the stars not very far away. But he did find some advantages to the heliocentric system: it explained why the planets Venus and Mercury are always close the sun. By putting their orbits between the earth and the sun, these planets would always be observed from the earth near the sun. To incorporate this explanation into a geocentric system like Ptolemaic system, Tycho revived the proposal of Heracliedes in which Venus and Mercury orbit about the sun but the sun, the moon and the rest the planets orbit about the earth. Thus Tycho thought he had the best of both the Ptolemaic and Copernican system, the perfect compromise. Not being a mathematician, he was not able to fit his observational data to his model of the universe.
King Fredrick died in 1588 and was succeeded by Christian IV, who found Tycho too expensive and arrogant. And there was also the way Tycho treated the peasants on the island of Hveen. When King Christian reduced his financial support, Tycho packed up his instruments and records and left Hveen around Easter, 1597. The lavish Tychonic caravan of twenty persons and baggage which included, not only his instruments (except the four largest), but a printing press, library, furniture, and paintings, traveled first to Copenhagen, and next to Rostock from where, having left Danish territory, he wrote a rather impertinent letter to King Christian, complaining about the treatment he had received from the King and his country. This burned his bridges and for two years he wandered about Europe, to Wandsbeck Castle near Hamburg, to Dresden, and to Wittenberg. He spent the winter of 1598 in Wittenberg, where he received Kepler’s little book, Cosmic Mystery. He was impressed with Kepler’s value from the book and determined to add Kepler to his retinue. He wrote a warm letter to Kepler both of appreciation and criticism and invited the young man to join him.
At last on June, 1599, Tycho’s caravan arrive in Prague, the residence of the Holy Roman Emperor Rudolph II, who appointed him Imperial Mathematicus. He was given the choice of three castles and a salary of three thousands florins a year (Kepler’s salary at Gratz had been two hundred). He took possession of the castle of Benatek, which was twenty-two miles, six hours’ journey, north-east of Prague, in August, 1599. On February 4, 1600, Tycho and Kepler met face to face at the castle. This was a historic meeting. Each man needed the other. Tycho possessed a wealth of accurate planetary observations which Kepler needed and Kepler had the mathematical skills and the drive to seek the cosmic order that laid in those observations, which Tycho hoped would support his model of the universe. Tycho assigned him to work on the Mar’s observations. A year and half later Tycho died on October 24, 1601, and a few days later on November 6, 1601, Kepler was appointed to his position. Kepler inherited Tycho’s observations which he later published in 1627 as the Rudophine Tables (after the death of Rudolph II).