Copernicus Recognizes a Need for Change

In the domain of the educated, Ptolemy's system, while largely unquestioned, had nevertheless always been regarded with a bit of quiet uncertainty. The universe, being that of the divine God, was considered to be perfect. A perfect God could create nothing less than a perfect universe. Thus, if the universe was perfect, why, then, did Ptolemy's system place the Earth off center? This was not perfection. It was cause for growing scandal among the scholars concerning the Earth-centered system, and it was inevitable that someone would eventually take genuine interest in setting things right. Copernicus was that person.

Over the past generations, observational techniques had slowly improved, and by this time it was known that when it came to predicting the seasons, Ptolemy's calendar was sometimes off by as much as a month. It disturbed Copernicus to know that if one were to employ Ptolemy's math, the Moon would have to change its size as it made its monthly revolutions in order to stay in keeping with Ptolemy's model. Such an inconsistency was unacceptable to him. This, together with a need to revise the current calendar, was cause for Copernicus to seek out a different, more perfect model for the construction of the universe. It was obvious that the true motions of

A Brief History of Accepted Astronomy

Th eories of the early philosophers such as Aristotle (384—322 B.C.E.) and Ptolemy (ca. 87-150 C.E.) of an Earth-centered solar system (Ptolemaic system) were the accepted truths up until and during Copernicus's time. There were others before Copernicus, such as the

PTOLEMY'S SIMPLEST MODEL

Sphere of fixed stars

PTOLEMY'S SIMPLEST MODEL

Sphere of fixed stars

Ptolemaios Earth Centered

For the first, simplest model, Ptolemy described each planet orbiting the Earth with uniform circular motion contained within a sphere of fixed stars.

PTOLEMY'S ECCENTRIC CYCLE

Sphere of fixed stars

Sphere of fixed stars

Ptolemy Geocentric
Ptolemy attempted to correct observed errors in the motions of the planets by modifying his simple Earth-centered system. He did this by placing the (fixed) Earth off-center while the planets remained orbiting the center with uniform circular motion. This system was not satisfactory either.

Greek philosopher Aristarchus of Samos (ca. 310-230 B.C.E.), who suggested the opposite: that the Earth revolved around the Sun. Lacking any real proof of this, however, Aristarchus's theory was rejected and Ptolemy's complex system stood for nearly 1,500 years. European civilization rested comfortably on the assumption that the

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PTOLEMY'S EPICYCLE AND DEFERENT

PTOLEMY'S EPICYCLE AND DEFERENT

Ptolemy Equant

Ptolemy added an epicycle, or small circle, to explain planetary retrograde motion. In the epicycle model, the planets orbit a center point (p) while the center travels around a larger circle, called a deferent, with the center offset from Earth. The equant point (e) is a location in space where calculations show that the planet would appear to be traveling uniformly.

Earth, not the Sun, was the fixed center of the universe and all celestial objects—the stars, planets, and the Sun—revolved around our stationary planet. Today this idea seems absurd, but during Copernicus's time it was considered the only truth. The Catholic Church had long ago accepted the Ptolemaic system as divine. It was the church's continued widespread influence along with human superstition that prevented the formal adoption of a heliocentric (Sun-centered) theory until the early 17th century.

When Ptolemy first began calculating the known motions of the planets and devising a model of the solar system that could be used to predict their positions in the future (and in that way predict the seasons), common sense placed the Earth in the center with all the stars, planets, and the Sun revolving around it. After all, was it not obvious that the Earth was standing still as everything in the heavens sailed past in circles? Thus, Ptolemy devised his first, perfectly balanced, Earth-centered system: the simplest of his systems.

Th is first straightforward system, however, did not account for the observed paths of the planets, which were known to make strange loops in the heavens called retrograde motion. Ptolemy could not force the math to work with the Earth at the center and still account for the planets' motions. He then tried putting the Earth off to the side of center, with the planets in an eccentric circle. This worked better, but still did not explain the quirky retrograde motions made by the planets.

Finally, in a brilliantly complex model, Ptolemy forced the Earth-centered system to work by employing his eccentric system and then applying individual motion to the planets, causing them to revolve around their own fixed center. Th is final system worked well enough to predict the planets' positions and gained acceptance as the working system of the universe. Ptolemy was not single-handedly responsible for devising the Earth-centered system, but he is usually given credit since it was he who made actual written recordings of the Earth-centered system in his book called the Almagest (Collection, ca. 140 C.E.).

the heavenly bodies were still not fully understood. When he started, he did not really intend to challenge Ptolemy's system, but rather rework the math until the system was perfect. Radical change, however, is exactly what Copernicus discovered was needed.

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