Copernicus Suggests a Heliocentric Cosmology

The Historical Context

• A sun-centered Solar System had been proposed as early as about 200 B.C. by Aristarchus of Samos was quickly forgotten  (see below).
• The Ptolemaic model held sway for about 1500 years, until the Renaissance–at least, this was the case in Europe).
• This lack of progress reflects the stagnation which prevailed in Europe through the Dark Ages.
• Some of the varied developments and breakthroughs around 1500 AD or after include:
  • The development of the printing press (allowing new ideas to be widely disseminated ).
  • Columbus's discovery of the New World, and the inauguration of a new age of exploration.
  • The revolutionary thinking of Martin Luther (considered heretical by the Catholic Church).
  • The artistic creations of Michaelangelo, Leonardo da Vinci, Shakespeare, and others.
  • New technological breakthroughs, such as the development of the telescope.

In short, the time was right for free-thinking scientists to reject the old hypotheses which had long been accepted merely because some earlier authority had espoused them. This encouraged speculation about new models of the universe (which, in those days, meant the Solar System: the stars were seen as merely the background realm against which everything else happened). On the other hand, this was the time of the Spanish Inquisition and general religious intolerance, especially of scientific theories which sought to repudiate the doctrine that we were in a divinely chosen place at the center of the universe. This threatening atmosphere certainly had an effect on the pace and style of astronomical development.

Aristarchus of Samos

The idea of Copernicus was not really new! A sun-centered Solar System had been proposed as early as about 200 B.C. by Aristarchus of Samos (Samos is an island off the coast of what is now Turkey). However, it did not survive long under the weight of Aristotle's influence and the "common sense" prevailing at the time:

1. If the Earth actually spun on an axis (as required in a heliocentric system to explain the diurnal motion of the sky), why didn't objects fly off the spinning Earth?
2. If the Earth was in motion around the Sun, why then didn't it leave behind the birds flying in the air?
3. If the Earth were actually on an orbit around the Sun, why wasn't a stellar parallax effect observed?

The first two objections were not valid because they represent an inadequate understanding of the physics of motion (gravity, inertia, etc.) that would only be corrected in the 17th century. The third objection is valid, but failed to account for what we now know to be the enormous distances to the stars. Thus, the heliocentric idea of Aristarchus was quickly forgotten and Western thought stagnated for almost 2000 years as it waited for Copernicus to revive the heliocentric theory.

Copernicus revives the heliocentric idea

• Copernicus rejected the Ptolemaic theory basically because he found it too contrived, reasoning that there had to be a simpler hypothesis which could explain everything to his satisfaction.
• This sentiment had already been expressed in a remark attributed to Alfonso X (1221-1284), the King of Castille and Leon. He is said to have remarked on having the Ptolemaic system explained to him: "If the Lord Almighty had consulted me before embarking upon Creation, I should have recommended something simpler."'
• Though not the first to suggest the heliocentric cosmology, Copernicus was the first to lay a serious foundation for it, which can be summarized as follows:
  1. The Earth's center is not the center of the universe.
  2. The center of the universe is near the Sun.
  3. In anticipation of the objection that his model should lead to the discovery of stellar parallax, he simply stated that the stars are enormously far away (which would render the parallactic shift too small to detect). Indeed, the parallax of stars was not measured conclusively until the year 1838.
  4. The rotation of the Earth on its axis accounts for the apparent daily cycles (i.e., rising and setting of the Sun and stars).
  5. The apparent annual cycle of movements of the Sun is caused by the Earth revolving around the Sun.
  6. The apparent retrograde motion of the planets is an illusion related to the motion of the Earth-based observer.
• Copernicus still imagined (incorrectly) that the planets move in perfect circles (rather than ellipses), so in the end he had to use epicycles anyway to explain some of the details of the motions.

By banishing the idea that the Earth was the center of the Solar System, the Copernican model immediately led to a simple explanation of both the varying brightness of the planets and retrograde motion:

1. The planets vary in brightness because they are not always the same distance from the Earth.
2. The retrograde motion could thus be explained in terms of geometry and a faster motion for planets with smaller orbits, as illustrated in the following animation. A similar construction can be made to illustrate retrograde motion for a planet inside the orbit of the Earth.

Retrograde motion in the Copernican System

Copernican model not completely correct

There is a common misconception that the Copernican model did away with the need for epicycles. This is not true. While Copernicus was able to rid himself of the long-held notion that the Earth was the center of the Solar system, but he did not question the assumption of uniform circular motion. In fact, the orbits of the planets are ellipses, not circles. As a consequence, the Copernican model, with its assumption of uniform circular motion, still could not explain all the details of planetary motion on the celestial sphere without epicycles. The difference was that the Copernican system required many fewer epicycles than the Ptolemaic system because it moved the Sun to the center.

As noted earlier, 3 incorrect ideas held back the development of modern astronomy from the time of Aristotle until the 16th and 17th centuries:

1. The Earth was the center of the Universe
2. Uniform circular motion in the heavens.
3. Objects in the heavens were made from a perfect, unchanging substance not found on the Earth.

Copernicus challenged idea 1, but not idea 2. We may also note that the Copernican model implicitly questions idea 3 that the objects in the sky were made of special unchanging stuff. Since the Earth is just another planet, it might not be unreasonable to suppose that other planets in the sky are made of the same material.

His ideas remained rather obscure for about 100 years after his death. But, in the 17th century, the work of Kepler, Galileo, and Newton would build on the heliocentric Universe of Copernicus and produce the revolution that would sweep away completely the ideas of Aristotle and replace them with the modern view of astronomy and natural science. This chain of events, arguably the greatest revolution in thinking that Western civilization has seen, is commonly called the Copernican Revolution.

Copernicus was a reluctant revolutionary

While Copernicus's revolutionary ideas found a quick acceptance in a scientific society ready to question theories of long standing, the following suggests that he may not have been too eager to take credit.

• Copernicus's book was published in highly technical Latin and was thus not accessible to a wide audience.
• Consequently, the book had little impact on the general population. Koestler (see below) describes it as one of the worst-selling books of all time.
• His book was published almost literally as he lay on his deathbed. It is believed by many that he hesitated for years because he believed his ideas would be considered heretical. The Church had elevated the ideas of Aristotle to the level of religious dogma.
• A man named Osiander, who had been supportive of Copernicus, encouraged the publication but prepared an anonymous prologue which was meant to defuse criticism or prosecution by explaining that the ideas contained in the book were clearly nonsensical and not to be taken seriously!

Suggested reading: ``The Sleepwalkers'' by Arthur Koestler. In contrast to the portrait of a revolutionary that many authors paint for Copernicus, Koestler portrays him as a coward who was reluctant to publish his work due to a crippling fear of ridicule.

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