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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 14: The Revolutions of Nicolaus Copernicus

"If the Lord Almighty had consulted me before embarking upon the Creation, I should have recommended something simpler."
Alfonso X El Sabio [The Wise], of Castile (1221-1284)
upon being instructed in the Ptolemaic system.

Key Ideas:

The Medieval Interruption

Copernicus' Heliocentric System:

Scientific Objections:

The Great Interruption

Most classical knowledge was lost, or only dimly remembered, in Europe after the fall of Rome c. 400AD.

However, the rise of a brilliant Islamic civilization in the Middle East c.750AD led to survival of classical learning in Arabic translation:

In this way, the ancient classical learning was to be nurtured and extended by Islamic society while most of contemporary Europe fell into a kind of intellectual slumber.

European Rediscovery

Spain became the center for translation of Arabic texts into Latin in the 11th century. As these works made their way out of Spain, Europeans slowly rediscovered their intellectual heritage: In particular, the astronomy of Ptolemy, in the form of his Almagest (the Arabic name of the Syntaxis), was to be elevated to the almost to the level of religious dogma.

From Rediscovery to Rebirth

The period between the Middle Ages and the Renaissance were times of great social and intellectual change:

Nicolaus Copernicus (1473-1543)

Nicolaus Copernicus

Born in Torun, Poland. Educated at Krakow, Bologna, and Padua in mathematics, medicine, law, astronomy, & philosophy. Spent most of his life as a Canon at Frauenberg Cathedral in Poland.

Copernicus was a very traditional thinker:

Copernicus' Discontents

Copernicus sought to purge Ptolemy's system of the messy expedient of the Equant: Note:
Copernicus was not alone in his discontent with the Equant. Arabic astronomers in the 13th and 14th centuries undertook reforms to the Ptolemaic model (specifically the astronomer/mathematicians al-Tusi, al-`Urdi, and ash-Shatir), while preserving its overall geocentric character.

The Heliocentric System Revived

Copernicus revived the Heliocentric system of Aristarchus, which he did not know in detail, but rather through the surviving description in Archimedes' The Sand Reckoner. These were radical ideas for their time, but Copernicus was no radical.

De Revolutionibus Orbi Coelstium

On the Revolution of the Heavenly Orbs (1543) It was not, however, a bestseller: Despite this, his ideas got serious attention, both for and against. Far from being "the book nobody read" as it is often incorrectly portrayed, De Revolutionibus was widely read and commented upon by the scholars of the day, both for and against his ideas.

When is a revolution not a revolution?

Copernicus still clung to strict Aristotelian ideas in his heliocentric system: This made his system complex: Still only described the motions of the planets without explaining them physically.

One point, however, needs to be emphasized. The geocentric models of Ptolemy and others described in Lecture 13 introduced epicycles primarily in order to produce the observed retrograde motion of the planets. In the Copernican system, retrograde motion comes naturally as a consequence of viewing moving planets from the perspective of a moving Earth, without resort to epicycles. While Copernicus' system does not need epicycles to produce retrograde motion, because of his insistance on uniform circular motion he still had to use them in order to get his model to make accurate predictions (i.e., to "preserve appearances"), particularly to reproduce the non-uniform speeds of the planets.

Simple in principle

While Copernicus' Heliocentric system was more complex in detail, it does provide a considerable measure of conceptual simplification.

Herein lies its virtue. It explained naturally what was more difficult to explain in the Ptolemaic system without things being "just so" or highly contrived. Specifically...

Inferior and Superior Planets

Diagram of the Ptolemaic System (10Kb GIF)
Diagram of the Copernican Solar System (13Kb GIF)
In each, much of the detail of both systems has been stripped away to show the basic arrangement of planets and orbits. The Copernican diagram entirely omits the epicycles that Copernicus employed, opting instead for a view of the true Solar System.

Retrograde Motion

(Click on the image to view at full scale [Size: 11Kb])

Geometric Distances to Planets

The Heliocentric System's geometry provides a natural way to measure the distances of the planets from the Sun.

Inner Planets:

(Click on the image to view at full scale [Size: 9Kb])

Outer Planets:

By contrast, you cannot estimate planetary distances with the Ptolemaic system by any simple means. You have to find them some other, arbitrary way, if at all.



The distances from the Sun are only measured relative to the Earth-Sun distance (the Astronomical Unit).

The problem was, neither Copernicus nor anybody else at the time knew how big an AU (the mean Earth-Sun distance) was in everyday units to any degree of precision. That is a another story for another day...

Opposition to Copernicus

The Copernican Heliocentric System met with almost immediate opposition.

Religious Objections:

More detail about this may be found in A Brief Note on Religious Objections to Copernicus. These were of less immediate consequence and concern to Copernicus than the scientific objections.

Scientific Objections

A rotating and revolving Earth was deemed an absurdity by strict Aristotelians:

Both motions require very large speeds:

There was no observational evidence of Earth's orbital motion: There was no observational evidence of Earth's rotation:

Copernicus was aware of these problems, but lacked the observational tools to address them definitively.

The most important was the non-observation of Stellar Parallaxes.

Stellar Parallaxes

Parallax is the apparent back-and-forth motion of nearby stars with respect to more distant stars caused by the changing perspective of the Earth at is orbits around the Sun.
(Click on the image to view at full scale [Size: 7Kb])
The reason stellar parallaxes were not observed is that the stars are much farther than people thought at the time. However, this explanation, while true, sounds like special pleading (it's there, but it is too small to measure), so until it could be measured this remained the principal objection to the Heliocentric system.

The Power of Ideas

In detail, the Copernican System was complex and unwieldy. It was arguably only an incremental improvement over the Ptolemaic system, insofar as it made somewhat better predictions of the positions of the planets than the Ptolemaic calculations of the day, and it was somewhat easier to use mathematically because it eliminated the difficult artiface of the equant.

In practice, however, it made up for this by offering a considerable measure of conceptual simplicity: many of the contrivances of the geocentric system needed to explain retrograde motions and the differences in the motions between inferior and superior planets were eliminated.

In a world of increasing change, the idea behind it was to prove powerful, and truly revolutionary.

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Updated: 2007 October 7
Copyright Richard W. Pogge, All Rights Reserved.