ASTRONOMY IN THE RENAISSANCE

by

Lord Aubrey de Baudricourt

 

"If one admits the motionlessness of the sun, and transfers the annual revolution from the sun to the earth ... it will become apparent that ... the backwards and forward motions of the planets are not motions of these but of the earth, which lends them the appearance of being planetary motions. Finally, one will be convinced that the sun itself occupies the centre of the universe."

--- Copernicus, De revolutionibus orbium celestium, 1543

Introduction

The scholar of the Middle Ages based his knowledge of astronomy on the works of the Greek astronomers and scientists. Aristotle and Claudius Ptolemy were the most important of these. Aristotle said that the Universe was perfect and unchanging. Ptolemy placed the Earth at the center of the Universe with the five planets, the Sun, the Moon and the stars orbiting it. Our scholar knew that the Earth was spherical. For this he had not only the word of the Ancients but also the evidence of his own eyes. As an observer he could see the curved shadow the Earth cast upon the moon during a lunar eclipse. He also knew that when a ship disappeared over the horizon, it was the mast the left sight last, The scholar also knew the approximate size of the Earth because Eratosthenes had measured it some 1500 years before.

During the so-called Dark Ages much of the knowledge of the Greek scientists was lost. Some manuscripts were saved in the vaults of dark monasteries but most went the way of those destroyed with the Library of Alexandria. In these years the Arabs had kept the science alive and even made discoveries of their own. Though he might not want to admit it, our medieval scholar knew the debt that he owed to the infidels. Many of the star names used today are Arabic: Rigel, Deneb, Aldebaran, Antares and Vega, to name a few. Knowledge acquired from the Arabs during the time of the Crusades was vital to relearning what the ancients had known a thousand years before. Finally, the fall of Constantinople to the Turks in 1453 brought to Italy many Greek scholars and their manuscripts. Slowly, Europe reawakened.

Astronomy in the early Renaissance was dominated by John Muller of Konigsberg (1436-1476) During the course of his short and nomadic life Muller, who was better known as Regiomontantus, popularized the use of solid celestial spheres in conjunction with the Ptolemaic universe. In doing so he was to limit the size of the Universe to that of the largest sphere, upon which the stars were affixed.

Leonardo da Vinci (1452-1519) was, in addition to a painter, sculptor, engineer and mathematician, an astronomer as well. His major contribution to the science was his explanation of why, when the moon is at crescent, the areas not lit by sunlight are still faintly visible. Da Vinci correctly surmised that these areas were being lit by "earthshine", that is, sunlight reflecting Off the earth onto the moon. This concept did much to break down the supposed barrier between the earth and celestial objects.

Even with the contributions made by astronomers of the early Renaissance, people of the time still maintained many mistaken ideas about the nature of the Universe. The most important of these, of course, was the siting of the earth as the center of the Universe. Our ancestors had no concept of what kept the planets in orbit. Some believed that the angels pushed the crystal spheres around the Earth. But all believed that the Earth was the center of the Universe. They could see no reason that Ptolemy was wrong. Some discrepancy was noticed between the positions that Ptolemy had predicted for the planets. But tables were revised and no one was terribly concerned with the errors. No one except an obscure Polish astronomer of the late 15th and early 16th century. His new concept of the Universe was to revolutionize not only the science of astronomy but also Man's view of the Universe and of himself.

Copernicus

"The fool wants to overturn the whole art of astronomy!."

--- Luther, conversation referring to Copernicus, C.1529

Nicholas Copernicus was born on February 19, 1473 at Thorn, Poland. By the time of his death, on May 24, 1543, he had totally redesigned the Universe. The publication of his work, "De Revolutionibus Orbium Celestium" (On the Revolution of the Celestial Spheres) was to stand the science of astronomy on its head. Educated at the University of Krakow and in Italy, Copernicus was to place humankind on the first step along the path of understanding the Universe, a path that we still trod today.

His major contribution was to remove the Earth from its position at the center of the solar system and the Universe. Just as humanist thinkers had replaced God with Man as the measure of all things, so Copernicus replaced the Earth with the Sun at the center of Creation. Copernicus stated that the planets (only Mercury, Venus, Earth, Mars, Jupiter and Saturn were known at that time) and the stars revolved around the Sun. Only the moon remained as a companion of the Earth. It may seem a simple thing that Copernicus had done, removing the earth from the center of the cosmos. But by doing so he removed the earth from the center of creation and overturned all of the existing beliefs about the nature of the universe. If these beliefs were untrue, might it not be possible, some scholars were to reason, that God is not the measure of all things? Perhaps Man might be such.

Copernicus believed that most of the motions that observers noted in the sky were due to the motion of the earth around the sun and to the rotation of the Earth. The Earth, he said, rotated on its axis once a day and revolved around the gun in a circular orbit once a year. Copernicus believed the seasons to be due to the tilt in the Earth's axis. He believed the Earth to be small in comparison to the size of the cosmos. But, like his predecessors, he believed the universe to be spherical and limited in size.

Copernicus arrived at his view of the Universe, in part due to the fact that the Ptolemaic model could no longer predict accurately the proper location of the planets in the sky. Copernicus' new model was meant to do that. Unfortunately, Copernicus did not realize that what caused Ptolemy's model to differ from the observable Universe was not the relationship between the Earth and the rest of the Universe. It was the fact that he caused the orbiting bodies to move in circular orbits, a shortfall that Copernicus retained in his model.

Copernicus' theories were not well received at first. Luther and the Protestants seemed for once in complete agreement with the Pope and the Catholics: Copernicus was wrong. And yet, as we shall see, some scholars were willing to accept it or, at least, give it the benefit of the doubt. One, Giodarno Bruno, was to be burnt at the stake for his steadfast defense of the Copernican system in 1600. Another, Galileo, was to be forced to recant his support of the same system as late as 1633. But by that time, the proof for the sun-centered Universe had already been provided. It only awaited the time when the scholars could convince the clerics and princes.

Tycho Brahe

"...the rehabilitation of astronomy was first conceived and decided upon by Tycho, that phenix among astronomers, in 1564."

--- Johannes Kepler

The only way that existed in the 16th century to prove (or to disprove, for that matter) the Copernican theory of the universe was through direct observation. The greatest observer of the time was a Danish nobleman named Tycho Brahe. Tycho had made some very important observations and conclusions regarding the great Supernova of 1572. His knowledge of the stars and constellations was so good that he was able to spot the intruder without referring to an atlas. Brahe and the astronomer Maestlin of Tubingen argued that the new star (nova) was not a comet or new planet. They showed that it had no motion, remaining in constant position over the course of several weeks. They attempted to measure its distance and found it to be too far away for their methods to reveal how far away it was. It had to be a star. The fact that it was a new star showed that the Universe was not unchangeable after all. So much for Aristotle.

Tycho was the blessed with the best observatory in Europe, constructed at Uranienborg according to his specifications. From his island observatory he charted the locations of over a thousand stars. But Tycho Brahe did not believe in Copernicus' heliocentric universe. None of his observations, made from Europe's most advanced observatory, supported it. And he was quick to decide that one did not need the Copernican theory to explain problems with Ptolemy's earth centered model.

Tycho developed his own model which was a sort of compromise between the Ptolemaic and Copernican models. In Tycho's Universe the other planets orbited the sun. The sun (with the five planets in attendance), the moon and the stars all orbited the Earth, which stood at the center of the Universe, every twenty-four hours. Like all compromises Tycho's new model of the Universe pleased no one and received little, if any, support from anyone.

Tycho Brahe died in October of 1601 convinced that the Earth was the center of the Universe. His observatory was broken up and his instruments taken away by his family. Ownership of his papers was hotly contested between his family and his most brilliant pupil. It was ironic that Tycho was not to live long enough to see this pupil gather support for the Copernican system and use Tycho's own observations to help him prove that the sun was the center of the cosmos.

Johannes Kepler

"Let me know privately, at least, if you do not want to do so publicly, what you have discovered in support of Copernicus."

---Kepler, a letter to Galileo, 1598

Tycho's greatest pupil was a young German named Johannes Kepler. Kepler was that impossible mix of astronomer and astrologer that we find common in those days and incredible in these days. His work in astronomy was top rate, vital to the development of the science and sound. Yet his renown as an astrologer often helped him pay the bills and there is every indication that he believed in what he did in both sciences. Kepler was a firm believer in Copernicus and the Copernican model of the Universe. When the forces of the Counter-Reformation came to Graz, Austria (where Kepler resided at the time) the scientist was threatened with imprisonment and torture for his outspoken beliefs. He was forced to flee and was exiled from Graz for life.

In December of 1599 he received an-invitation from Tycho Brahe to come to Denmark and work with him. Kepler seized the opportunity and, after Brahe's death, used Brahe's very precise observation data concerning Mars, to discover why Copernicus' model had problems predicting planetary motion any more accurately than Ptolemy's. He also did observations of his own of the motions of Mars, Venus and Mercury. His conclusions were as simple as they were elegant: the planets did not move in circular orbits! They moved in elliptical ones. Kepler had done what neither Ptolemy, Copernicus nor Tycho Brahe had been able to do--free models of the Universe from the tyranny of the circle. This First Law of Planetary Motion, arrived at in 1604, enabled the Copernican system to accurately predict the movement of the planets and, more importantly, removed the major scientific objection to its validity.

It is ironic that in completing his work in astronomy, Kepler destroyed the framework of his other pursuit, astrology. Throughout his life he was to show no hesitation in his support of the Copernican system. Kepler also encouraged other mathematicians and astronomers to come out in support of the sun centered Universe as well. Among these was an obscure Italian named Galileo Galilei and it is, perhaps, merciful that Kepler died in 1630, without witnessing Galileo's recantation.

Galileo

"I have written many direct and indirect arguments for the Copernican view, but until now I have dared not to publish them, alarmed by the fate of Copernicus himself, our master. "

--- Galileo, a letter to Kepler, 1597

The man destined to be the last martyr for the Copernican system was born in Pisa in 1564 and died at Arcetri in 1642. His father was a well known musician who specialized in the lute and a cloth merchant as well. The younger Galileo attended the University at Pisa but did not take his examinations. It was at Pisa that he had his first exposure to Copernican theory.

As the years passed and he continued his studies into the various types of motion, he became more and more convinced in the validity of the sun-centered Universe. He maintained contact with Kepler and others who held similar views. The Nova of 1604, which like that of 1572 was shown to be outside of the environs of the Earth, gave Galileo the opportunity to expound his view of the Universe.

Galileo is best remembered for his revolutionary discoveries with the telescope. He was one of the first men to see the craters on the moon. He discovered the phases of Venus, the moons of Jupiter and was the first person to view the vast starclouds that make up the Milky Way. He discovered sunspots and was able to show that the Sun, like the Earth, rotates on its axis. But all of this lies just beyond the end of the time period in question, having taken place in 1609. Yet, there are some who believe that the telescope itself may have existed as early as the 13th century. Obscure passages in a work by Roger Bacon refers to the use of lenses to make distant objects appear more close. References to this subject also appear in the papers of Thomas Digges, a late 16th century English astronomer. But even if this is true, it is doubtful that anyone used the telescope for astronomical pursuits prior to the beginning of the 17th century.

Galileo published the results in The Sidereal Messenger. He described his observations. In this and other publications Galileo built a powerful case in support of Copernicus. He had further demolished Aristotle by showing that neither the face of the Sun nor Moon were perfect. He pointed to Jupiter and its system of satellites as proof that the objects of the Universe circles other bodies than the Earth. The phases of Venus, he argued, coupled with its observed motion, could only be explained by the planet orbiting the sun. Finally, in agreement with Copernicus, Galileo stated that the Earth was not motionless and did not stand at the center of the Universe.

Powerful forces within the Church were gathered against Galileo. He was warned for big own good to maintain his silence. But the strength of the proofs that he could gee with his own eyes through his telescopes would not permit this. Finally, and much against its will, the Church was forced to move against the Italian astronomer. Old and broken, he was forced to recant his beliefs in an Earth that was not motionless and was not the center of the Universe. It was 1633.

Astronomy in Elizabethan England

"It is the stars

The stars above us, govern our conditions."

--- Shakespeare, "King Lear", 1606

The first astronomer of note in Renaissance England was Robert Recorde (c.1510-1558). Recorde was a mathematician of note, having invented the equal sign. He also wrote Grounde of Artes., which was published in 1543. This scholarly work described both the Ptolemaic and Copernican system but refrained from favoring one or the other.

The first open advocate of the Copernican system in England was John Dee (1527-1608). Dee enjoyed quite a reputation as an alchemist and as the Queen's astrologer. But he was also a serious scientist, having dealings with some of the greatest minds of his time. In 1556 he wrote an introduction for astronomer John Field's 1557 almanac. In it he included a defense of the heliocentric model. Dee theorized that the 1572 Supernova was a star and not a planetary or atmospheric phenomenon.

His most brilliant pupil, Thomas Digges (1546-1591) was the son of the eminent scientist Leonard Digges. Among Digges accomplishments was the possible invention of the telescope a generation before the time of Galileo. His son agreed with both Tycho Brahe and John Dee in regards to the nature of the Supernova of 1572. But his most important contribution to astronomy was the concept of an infinite universe, He took the Copernican universe and removed the stellar sphere that limited its size. Digges argued, instead, that the stars were guns that were a tremendous distance away. These ideas were published in Digges work of 1576 entitled "A Perfit Description of the Caelestiall Orbes according to the most aunciente doctrine of the Pythagoreans, lately revived by Copernicus and by Geometricall Demonstrations approved".

The final English astronomer of the Renaissance that I will discuss is Thomas Harriot (1560-1621). Harriot attended Oxford and traveled to Virginia with Sir Walter Raleigh in 1584. At almost exactly the same time that Galileo was exploring the Universe with his telescope Harriot was doing the same with one of his own. After viewing Galileo's rather inaccurate drawings of the moon in "The Sidereal Messenger" Harriot drew a more accurate map of the moon showing some fifty features. The map was not published in Harriot's lifetime.

Conclusion

The struggle of the sun-centered Universe against the concept that a motionless Earth standing at the center of the

Universe was a long and difficult one. Many scientists were persecuted or broken for their beliefs by the Church and by their disbelieving colleagues. But, outcome was inevitable. Copernicus devised a model that described the state of the Universe far better than that of Ptolemy. Although Tycho Brahe did not believe in the new system his observations made it possible for his pupil Kepler to deduce his Laws of Planetary Motion that provided mathematical proof of the Copernican model. Although he paid with his health and freedom, Galileo placed the final nail in the coffin of the Ptolemaic system by giving scientists the ability to view the wonders of the Cosmos and how they supported Copernicus.

But the forces of reaction were strong. The Copernican system was not to finally triumph until the time of Sir Isaac Newton in the late 17th century. He, and the myriad of astronomers of his time --- Huygens, Roemer, Bradley, Halley, and Bessel, to name a few --- were to finally overthrow the Earth-centered Universe forever.

References

Berry Arthur, A Short History of Astronomy, Dover Publications Inc., New York City, New York, 1961.

Burnham, Robert Jr., Burnham's Celestial Handbook, Vol IIII, Dover Publications, New York City, New York, 1978.

Copernicus, Nicholas, On the Revolution of the Heavenly Spheres, Encyclopedia Britannica, Inc., Chicago, Illinois, 1971.

Fermi, Laura and Bernardini, Gilberto, Galileo and the Scientific Revolution, Basic Books, Inc., New York City, New York, 1961.

Hoyle, Fred, Astronomy, Crescent Books, New York City, New York, 1962.

Kesten, Hermann, Copernicus and his World, Roy Publishers, New York City, New York, 1945.

Lehner, Ernst and Johanna, Lore and Lure of Outer Space, Tudor Publishing Company, New York City, New York, 1964.

Montaigne, Michel de, An Apology for Raymond Sebond, Penguin Books, London, England, 1987.

Wilson, Colin, Starseekers, Doubleday and Company, Garden City, New York, 1980.

THE UNIVERSE ACCORDING TO PTOLEMY

THE COPERNICAN UNIVERSE ACCORDING TO DIGGES