The Story of Astronomy (7 page)

In the 16th century the astronomer Nicolaus Copernicus shook the world with his heretical assertion that it was the Sun, not the Earth, that lay at the center of the universe. Such was the expected weight of opinion against this theory that Copernicus' views were only published after his death.

Ptolemy's
Almagest
was translated into Arabic in the ninth century, but a Spanish version of his work did not appear until the 12th century. It was translated into Latin in the time of Frederick II of Denmark (reigned 1559–88) and thus became available to the majority of European scholars. In the early Middle Ages there was still much interest in astronomy, although there were few active observers. In the monasteries the primary task of the monks was to copy the gospels into Latin and other languages. They occasionally came across scientific manuscripts, however,
and sometimes these were also copied. Toward the end of the first millennium we sometimes find chronicles of historical events. The
Anglo-Saxon Chronicle
is a good example: it contains copious references to astronomical events, and when an eclipse or a comet is mentioned it is usually possible to put an exact date to the sighting.

Thus in the year 540:
“The Sun darkened on June 20th, and the stars showed fully nearly an hour past nine in the morning.”
In 678:
“there appeared the star called a comet, in August; and it shone for three months each morning like a beam of the Sun.”
The chronicle records that in 1066:
“it happened that all through England such a sign as the heavens was seen as no man had seen before. Some men said it was the star ‘Comet,' that some men called the long-haired star. It appeared first on the eve of Letania maior, April 24th, and so shone all seven nights.”

The “long-haired star” was literally woven into the fabric of English history when it appeared prominently on the Bayeux Tapestry. Six hundred years later Edmond Halley identified the object as a comet that reappeared every 76 years. A new star appeared a few years before the comet. It was first seen on July 4, 1054, and it was so bright that
for several months it was visible in broad daylight. Chinese astronomers recorded the event, but there is no mention of it in European records. Nine hundred years later it became identified with a remnant of a supernova (an exploding star) in the Crab Nebula, and for a time it became the most intriguing object in the whole of the night sky.

Comets and exploding stars are rare events, but changes to the face of Moon are so uncommon that they are virtually unknown. One summer evening in 1178 five English monks were relaxing and staring at the night sky. There was a new Moon, and as they gazed at it they noticed something very strange. A great explosion appeared to be taking place on the Moon before their eyes. The monks knew that the face of the Moon never changed, and so they realized they had witnessed a very unusual event. They did not understand what had happened, but they felt that the event must be a message of some kind from the heavens. They decided to report their findings to a higher authority, and so they made their way to Canterbury where they gained an audience with the archbishop. They swore the truth of their story under oath, and we know a little about the event because it was recorded by the chronicler Gervase of Canterbury (
c
.1141–
c
.1210):

There was a bright new Moon, and as usual in that phase its horns were tilted towards the east. Suddenly the upper horn split in two. From the mid point of the division, a flaming torch sprung up, spewing out fire, hot coals and sparks.

The Moon is covered with craters—the result of being regularly struck by objects from space in its long history. It is probable that the monks actually witnessed a large object such as a meteorite striking the Moon. Recent research suggests that the crater named after Giordano Bruno may be the result of the impact witnessed in 1178. (The Earth has suffered similar attacks in the past, but over time the weather has worn all but the largest craters away so that they are far less obvious.)

The Middle Ages are dotted with astronomical observations, but there are no radical new ideas about the structure of the universe recorded in this period. Astrologers were common, but few could also be called astronomers. The anecdotes can still be of interest, however. Geoffrey Chaucer (
c
.1343–1400), better known as the founder of English literature, wrote a treatise on the astrolabe, the oldest astronomical instrument. Chaucer had a son, Lewis, who it seems was more interested in mastering numbers
than following in his father's footsteps and mastering words. Chaucer decided to write a treatise on the astrolabe for the benefit of his son. For one who did not have any formal education in mathematics and the sciences, Chaucer's mastery of the instrument must be admired:

Litel Lowis my sone, I have perceived wel by certeyne evidences thyn abilite to lerne sciencez touchinge noumbres and proporcions; and as well considere I thy bisy preyers in special to lerne the Tretis of the Astrolabie. Than, for as mechel as a philosophre seith, “he wrappeth him in his frend, that condesendeth to the rightful preyers of his frend,” ther-for have I given thee a suffisaunt Asrolabie as for oure orizonte, compowded after the latitude of Oxenford; op-on which, by mediacion of this litel treatis, I purpose to teche thee a certain nombre of conclusions apertening to the same instrument.

In Chaucer's time, Britain was seen as backward and uncivilized compared with places like Italy and Spain. So when the first glimmerings of a scientific revolution appeared, it was in the cultured cities of Italy. However, it was in a most unlikely eastern European town that the astronomical knowledge of 13 centuries first came into question. Nicolaus Copernicus, the son of a well-to-do
merchant, was born on February 19, 1473, in Torun, a city on the River Vistula in north-central Poland. Nicolaus was the youngest of four children. After his father's death, around 1484, Nicolaus' uncle, Lucas Watzenrode (1447–1512), took him and his three siblings under his protection. Watzenrode, who later became bishop of the Chapter of Warmia, provided for young Nicolaus' education and helped him to make his future career as a church canon.

For two years from 1491, Copernicus studied liberal arts, including a smattering of astrology, at the University of Cracow. He left the university before completing his degree, but in 1493 he resumed his studies in Italy at the University of Bologna. He stayed in Bologna for four years, studying law. For a while he lived in the same house as the principal astronomer at the university, Domenico Maria de Novara (1473–1543), who held the post of official astrologer for the city. It seems certain that Novara introduced Copernicus to Ptolemy's work—not to the original writings but to one of the later versions containing certain corrections and critical expansions of the models of the planetary orbits. These corrections were of great interest to Copernicus, and they may have suggested to him the ideas that led toward formulating his famous heliocentric hypothesis.

In 1501 we find Copernicus at Frombork in Poland, but soon he returned to Italy to continue his studies, this
time at the University of Padua, where he had changed his direction of study from law to medicine. Copernicus' astrological experience at Bologna was actually a better training for medicine than we might imagine, for at that time there was so much faith in astrology that the stars were thought to influence parts of the body, and a good horoscope was considered a very valuable aid toward a diagnosis. In 1503 Copernicus received his doctorate, not in astrology or in medicine but in canon law, for he had changed his direction of study yet again.

When he returned to Poland his uncle arranged a sinecure for him at Cracow. Copernicus' duties were largely administrative and medical. He collected rents from church-owned lands; he secured military defenses; he oversaw chapter finances; he managed the bakery, brewery and the mills; and he cared for the medical needs of his uncle and the other canons. His astronomical work took second place to his other duties, but it occupied all of his spare time.

By 1514, at the age of 41, Copernicus was regarded as a competent astronomer. In that year he was invited to offer his opinion at the church's Fifth Lateran Council on the problem of the reform of the calendar. The civil calendar then in use was the one produced 15 centuries
earlier by Sosigenes under Julius Caesar. Over the centuries since it was instigated it had fallen about ten days out of alignment with the stars, and the church was concerned that it cast doubts over the true dates of crucial feast days—Easter in particular. It is unlikely that Copernicus ever offered any views on how to reform the calendar, however, because there is no record that he ever attended any of the council's sessions. In time Copernicus went to live again at Frombork. He took up residence in a tower of the cathedral house, a high observatory from where he was free to continue his astronomical studies. His revolutionary ideas had probably been forming in his mind for many years, but it was at Frombork that we first learn about them.

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.

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.