The Sun-centred or heliocentric theory of the solar system is usually associated with the 16th century Polish astronomer Nicholas Copernicus (1473-1543). It is often called 'the Copernican System', just as the Earth-centred 'Ptolemaic System' is named after Ptolemy. Yet neither Ptolemy nor Copernicus invented the systems named after them. Both formulated coherent mathematical frameworks to explain ideas that were 'in the air' during their lifetimes. The concept of a Sun-centred solar system was known to the ancient Greeks. It predates Copernicus by nearly two millennia and can be traced back several centuries before Ptolemy's pronouncement that the Earth stood fixed and motionless at the centre of the universe.
Ionians and Pythagoreans
In the earliest Greek cosmological systems, the Earth was envisaged as a shield-like disc floating on water and surrounded by the mythical streams of Ocean. This is the view presented around 800 BC in Homer's iliad and Odyssey.
During the 6th century BC, the earliest schools of Greek philosophy emerged. Among them was the Ionian school, inspired by Thales of Miletus (c.624-c.547 BC), who began looking for explanations of the origin and nature of the universe that did not involve the supernatural intervention of the gods of Olympus. The essential ingredients of what later became the Ptolemaic system were first formulated by the Ionian philosophers. Anaximander (c.610-c.547 BC) made the first attempt to explain planetary motion as some kind of mechanism; Anaximenes (fl. 550 BC) suggested that the planets were carried in transparent crystal spheres in their orbits around the Earth; Anaxagoras (c.500-c. 428 BC) postulated an outer sphere of the 'Prime Mover' that was analogous to mind and reason.
Other schools of thought emerged alongside the Ionian philosophers with different theories to account for the origin of the universe and to explain its workings. The most influential of these was the Pythagorean Brotherhood. Even during his own lifetime Pythagoras (c.580-490 BC) was a legendary figure. His most enthusiastic followers declared that he was the son of Apollo. Astronomers in the Pythagorean tradition moved steadily towards an understanding of the solar system. Pythagoras himself is credited with advancing the idea that the Earth is a spherical globe. His pupil Philolaus (fl.500 BC) was the first to visualise it moving through space and simultaneously rotating on its axis. Philolaus took the first step towards a heliocentric theory with his conception of a mystical 'central fire' around which all the celestial bodies, including Earth and Sun, were said to revolve.
Herakleides (c. 380-c.310 BC) developed a hybrid theory half-way between the geocentric and heliocentric systems in which the inferior planets Mercury and Venus were in orbit around the Sun while the Sun itself and the superior planets Mars, Jupiter and Saturn were in orbit around the Earth. Finally Aristarchus of Samos (c.310-c.250 BC), the last of the Pythagorean astronomers, reasoned that the motions of the celestial bodies could all be explained by assuming that the Sun rather than the Earth stood at the centre.
Aristarchus' heliocentric theory was not widely accepted. Greek astronomy of the 3rd century BC was preoccupied with developing an accurate method of predicting planetary movements but the heliocentric theory in itself offered no practical solutions. Furthermore, the concept of a moving Earth seemed to contradict the evidence of the senses as well as breaking every law of physics as then understood. Yet the idea never quite went away. A century after Aristarchus, Hipparchus (c. 190-c.127 BC), generally acknowledged as the greatest astronomical observer of the ancient world, investigated the heliocentric theory. He rejected it because the instruments and techniques at his disposal were not advanced enough to detect any scientific evidence for a moving Earth. But even when Ptolemy wrote his great compendium of astronomy the Almagest around 150 AD, he too felt obliged to present arguments 'proving' that the Earth was stationary at the centre of the universe.
Plato and Aristotle
The persistence of the Earth-centred theory from the time of the ancient Greeks down to the 16th and 17th centuries can be attributed to the tremendous influence of Plato and Aristotle on Greek, Arabic and European philosophy.
Plato (c.427-c.347 BC) taught that behind the world of physical appearances was an archetypal world of forms and ideas. The everyday world was no more than a shadow of the Ideal world; sensory impressions were simply illusions. Consequently, Plato had little interest in the facts and figures of empirical science. His pure world of ideas could only be approached intuitively - through allegory, poetry or myth. This was the spirit in which his pronouncements on the nature of the celestial bodies were made. He declared that the Earth, which possessed a divine essence or 'world soul', was perfectly spherical in shape. It stood fixed and motionless at the centre of creation. The orbits of the Sun, Moon and planets around it were perfectly circular; they moved along their orbits at perfectly uniform speeds, never speeding up or slowing down.
The idealised Platonic universe of spheres and circles came to be regarded as axiomatic. The principle of circular motion was inviolable because the heavens were believed to be perfect and immutable. The messy processes of generation and decay, death and rebirth, prevailed only in the earthly 'sub-lunar' regions. Later generations of astronomers and mathematicians were saddled with the task of devising a theory of planetary motion that could explain such awkward anomalies as retrograde motion and the apparent variations in the brilliance of the planets, yet remain within the prescribed requirements of a fixed central Earth with circular planetary orbits and uniform speeds.
Aristotle (c.384-c.322 BC) was a disciple of Plato but later rejected his abstract idealism in favour of a more pragmatic approach, insisting that all speculation must be based upon observation, analysis and systematic research. He left a vast body of written works that provided a first foundation for several modern sciences. Classical astrology
- from the Greeks to the 17th century masters - owes its theoretical foundation to the doctrines of Aristotle. In astronomy Aristotle followed Plato's geocentric concepts, but unlike Plato he attempted to explain the laws that made the planets physically revolve.
Plato's pupil Eudoxus (c.400-c.355 BC) had devised a model of planetary motion in which the planets revolved in several spheres, one nestling within another and all rotating independently. This construct explained all the variations of planetary motion by combinations of simple circular movements, thus demonstrating that Plato's conception of circular motion and uniform speeds was mathematically possible - and mathematical truth was close to the Ideal in Platonic thought. Aristotle took Eudoxus' scheme literally as a working model of the universe and set about improving it in the light of contemporary physics. He attempted to explain retrograde motion with a system of 'working' and 'neutralising' spheres. While this idea never really caught on, it led eventually to the development of the elaborate theory of planetary epicycles or wheels-within-wheels, a theory first proposed by Appolonius of Perga (fl. 200 BC), developed by Hipparchus and perfected by Ptolemy (c. 100-178 AD). The Earth-centred 'Ptolemaic' universe, founded upon the doctrines of Plato and Aristotle, remained the last word in astronomical theory for 1500 years.
An Arabic Interlude
With the collapse of the Roman Empire in 476 AD, knowledge of classical science was lost to Europe for several centuries. The Church eventually established the monastic system as a means of promoting learning but the philosophers of pre-Christian times were regarded with suspicion by the Church fathers. In early Christendom, astronomy regressed into Old Testament fundamentalism.
In 622 AD the Prophet Mohammed launched his holy war against the infidel. Within a century, the Islamic Empire extended eastwards across northern India to the borders of China, and westwards across Asia Minor, north Africa and
- with the Arabic conquest of Spain and Sicily - into western Europe. Alexandria, the centre of classical learning, fell to the Arabians in 642. When the period of military expansion was over, Islamic scholars became enthusiastic students of classical philosophy. Many important manuscripts were translated from Greek into Arabic. In the world of medieval Islam, Aristotle and Ptolemy were the supreme authorities in matters of natural science and astronomy.
In 999 AD, Gerbert of Aurillac, the most accomplished mathematician, musician, astronomer and classical scholar in Europe, ascended to the papal throne as Sylvester II, known as 'the magician Pope' to his contemporaries. His papacy, at the symbolic date 1000 AD marks the turning point of the European 'dark ages'. Contact was established with Arabic centres of learning in Spain, where Muslim, Christian and Jewish scholars congregated. During the following centuries, Hebrew and Arabic versions of ancient Greek texts were translated into Latin and began to circulate in Europe. The works of Aristotle were translated from around 1200. The translation of Ptolemy's Almagest into Latin in
1175 re-vitalised European astronomy. King Alfonso X of Castile (122-184) commissioned new astronomical tables calculated according to Ptolemy's theory with Arabic mathematical refinements. Completed in 1252, the Alphonsine Tables remained the best astronomical tables available in Europe for the next three centuries. The complexities of the Ptolemaic system exasperated King Alfonso however. When the intricacies of epicycles, deferents and equants were explained to him Alfonso 'the Wise' is said to have remarked that if the Almighty had consulted him on the matter, he would have recommended something a little simpler...
The Copernican Revolution
The heliocentric theory proposed by Aristarchus also found its way into Europe through translations of Arabic texts. It was discussed in learned circles as it had been in ancient Greece, but the Ptolemaic system gave the only explanation of planetary motion that could be put to practical use. The heliocentric theory remained speculative because observing instruments were not good enough to detect any evidence for it. In an attempt to rectify this, Regiomontanus (1436-76) set up the first European observatory, which he established at Nuremburg in 1471.
Regiomontanus (Johann Muller) was born at Konigsburg in Prussia. He was a child prodigy who went to Leipzig university at the age of 11, published his first almanac at the age of 12 and at age 15 was casting horoscopes for the Hapsburg Emperor Frederick III. Regiomontanus became famous as a mathematician. He made important advances in spherical trigonometry and developed the system of astrological house division that bears his name. He also wrote a celebrated summary of the Almagest but grew dissatisfied with Ptolemy's explanation of planetary motion and impatient with his fellow astronomers' unquestioning acceptance of it. He was interested in the alternative theories of Herakleides and Aristarchus, but realised that better observational data would be needed before any advances could be made. He also recognised the limitations of contemporary astronomical instruments and with the help of a wealthy patron equipped his observatory with improved instruments of his own design. Regiomontanus' preparations for what could have become a major reform of astronomy were cut short by his untimely death at the age of 40. Yet his influence played its part in shaping the ideas of Doctor Copernicus.
Born at Torun on the border between Prussia and Poland, Copernicus studied astronomy and mathematics at the university of Cracow, then went to Italy where he studied canon law at Bologna and medicine at Padua, finally obtaining his degree at Ferrera in 1503. His principal teachers in astronomy, Brudzevsky at Cracow and Novara at Bologna, both described themselves as students of Regiomontanus. The heliocentric theory that came to be associated with the name of Copernicus was a regular topic of discussion and debate during his student years.
Copernicus was not particularly interested in observing the sky but he was devoted to Pythagorean mathematics. He believed that the harmony of the universe revealed itself through the perfect geometry of planetary orbits. A technical imperfection in the Ptolemaic scheme forced him to formulate his Sun-centred theory. Ptolemy had been a little devious in the matter of uniform planetary speeds. In his system each planet would appear to move at a constant rate (as Plato decreed it should) only if it could be seen from a hypothetical point in space called its equant. Most philosophers were content to accept this device but it irritated the perfectionist Copernicus. He concluded that the only way to 'save the phenomena' of perfect circles and uniform speeds was to place the Sun at the centre of the solar system and let the planets revolve around it, as Aristarchus had suggested. Since Copernicus assumed that the orbits of the planets are circular his scheme still needed epicycles to make it work, but the simulation was precise. For the first time, tables of planetary motion could be calculated from heliocentric principles.
Furthermore, these tables proved more accurate than those based on the Ptolemaic system.
Copernicus was reluctant to commit his theory to print. Around 1512 he wrote the Commentariolus ('brief commentary') in which he outlined the new system. This was circulated in manuscript form amongst a few selected scholars. By 1530 he had completed the text of his major work De Revolutionibus ('On the Revolutions of the Heavenly Spheres') but he kept the manuscript locked away and made no plans to publish it. This was not through fear of religious persecution as is often supposed. At least during the early part of the 16th century, a climate of intellectual tolerance prevailed in Europe. Cardinal Schonberg, a close advisor to three successive popes, urged him to publish but Copernicus had no desire to draw attention to himself. He suspected that he would be ridiculed as his ideas became known outside rarefied academic circles.
It was the Protestant astronomer Georg Joachim von Lauten (15 14-74), known as Rheticus, who persuaded Copernicus to publish. Rheticus was professor of mathematics and astronomy at the university of Wittenberg, the newly-established centre of Protestant learning. Although Martin Luther and other Protestant theologians argued against the heliocentric theory Rheticus was given permission to visit Copernicus, in Catholic Frauenburg, in order to discuss it. After lengthy negotiations he obtained Copernicus' permission to publish De Revolutionibus and it finally appeared in print in May 1543. Copernicus died within a few hours of receiving the first copy.
The book's initial impact was negligible. Few people bothered to read it. Of those who did, most regarded the Copernican system as a useful calculation device rather than a serious theory of the structure of the solar system. The literally earth-shaking implications of the Copernican revolution did not begin to emerge until the work of Galileo and Kepler at the beginning of the 17th century.
David Plant is a respected scholar of the history and traditional practice of astrology. He is also an expert on the English Civil War period and the life and work of the 17th century astrologer William Lilly. He runs two very reputable websites: the
English Merlin site, which is devoted to all aspects of the life and times of
William Lilly and his contemporaries; and the British Civil Wars and Commonwealth site, which explores the turmoil of the Civil Wars and Interregnum, and the constitutional experiments of the Commonwealth and Protectorate period of the 1650s.
Both sites are leading points of reference for their fields and a visit is strongly recommended.
© David Plant