Mechanics Notes
Copernicus' Conundrum


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The Solar System Before Copernicus:

The Greeks had many and varied ideas about the structure of the universe - after all, they were critical and original thinkers.

The ideas that came to be held by most medieval thinkers descended from Aristotle. Aristotle's wrote that the heavenly bodies were fundamentally different from earthly bodies, both in behavior and composition. In Aristotle's view, the heavens were perfect and unchangeable.

The Alexandrian Greek scientist Ptolemy, following Aristotle, wrote that (naturally) the earth was at rest in the center of the universe, and the Sun, Moon, planets, and stars moved about the Earth in circular orbits. (A solar system with the Earth at the center is called a geocentric solar system - "geo" = Earth, etc.)

Historically, Ptolemy's work survived mainly due to the Arabs, who had the highest regard for Ptolemy's ideas. Ptolemy's main work became known in the West during the Middle Ages as the Almagest, which is from the Arabic for "The Greatest". The monastic scholars of the Middle Ages also had great respect for the ideas of the Greeks, and gradually many ideas of Aristotle - and Ptolemy - became linked to Church doctrine.

Scientifically, the big problem with Ptolemy's ideas were the orbits of the planets. Viewed from the Earth over the course of several months, planets have a strange motion - sometimes they move forward, sometimes they stop, and move backward (retrograde motion). Actually, Greek astronomers had made accurate-enough observations by Ptolemy's time to know that circular orbits for the planets just didn't fit - if you predict the position of a planet using the idea of circular orbits, then look for the planet in that position - it isn't there.

Ptolemy's solution to matching the model with reality was to create a rather complex system of adjustments to the circular orbits. For some planets, the Earth's position was offset from the exact center of the planet's orbit by a distance called the eccentric. It might be necessary for the planet to be attached to a smaller circle called an epicycle, whose center followed the main orbit of the planet. This might not suffice to accurately portray the orbit of a planet - the center of the epicycle may need to be offset by an amount called the equant.

This system seemed to work pretty well for predicting the motions of the planets, except:

Other references:

The Ptolemaic System

The Universe of Aristotle and Ptolemy

Copernicus's Ideas:

Copernicus Portrait

Nicholas Copernicus (1473 - 1543)

The Polish cleric Copernicus suggested, in the late 16th century, that the Sun was actually the center of the solar system, and that the Earth was a planet that revolved about the sun, just like any other planet. (This is a heliocentric solar system - "Helios" = sun...)

Religious and Political Objections to Copernicus:

To understand the religious and political furor that Copernicus' ideas set off, you have to understand some of the history of the time.

First, the Catholic Church was not only a religious, but also a political power. However, the Church was not without its troubles - this was the time of the Reformation. A king or even local lord would convert to Protestantism (sometimes for political or financial reasons). This would trigger a war with the neighboring Catholic king or lord. These wars were always bloody and brutal, and often long (the Thirty Years War, for instance).

In this atmosphere, no one was ready for any dissent whatever - and certainly not revolutionary ideas like Copernicus'. Copernicus new this - he delayed publication of his book so that he saw the first copy as he lay on his deathbed. He wasn't going to be around for the fallout from this! People had been (and continued to be) burned at the stake (or worse) for far less that this!

Philosophically and religiously, Copernicus had posed serious problems, even in the most serene of times. What Copernicus had done was no less than to change people's place in the universe. In the universe of the time (Ptolemy's, essentially), people were the center of God's Universe. Everything revolved around the Earth - and the people on it. Copernicus reduced the Earth to one of several planets revolving one of billions of stars, and reduced the status of the people on the Earth - to what?

Scientific Objections to Copernicus:

There were serious scientific objections to Copernicus' ideas also. Copernicus knew that he couldn't answer these objections any more than he could answer the religious objections. For instance:

The heliocentric solar system requires the Earth to rotate on its axis once per day which means that you, at this moment have a speed of about 1 000 mi/hr (= 25 000 mi/24 hr). (If you calculate the speed that the Earth must have to orbit the Sun once per year, it makes 1 000 mi/hr seem pokey...)

Other sources:


BHS -> Mr. Stanbrough -> Physics -> Mechanics -> Newton's Laws -> Newton's First Law -> this page

last update November 19, 2007 by JL Stanbrough