Early Models of the Universe

Foreword: Today we tend to snicker at ancient models of the universe. "What a bunch of clowns." Geocentric universe - right! NOT!! OK, today we know better. However, while early models of the universe may be wrong, many were scientifically sound given the facts that were known at the time. At least in my own case I would not presume that I am more intelligent than were Aristotle and Ptolemy. We know more astronomical facts. We have better and more varied observational tools. We have the benefit of hindsight (which is always 20-20, or better!). So, why study models that are wrong? There are many answers to that question involving cultural heritage (Western), the evolution of scientific thought, our place in the Universe, etc... Let's do it!

Who Was the First Astronomer?

The first astronomers are lost in pre-history. However, it is clear that ancient civilizations world-wide were very aware of the heavens and the motions of objects in it. We know this from artifacts that still exist all over the world that were built to keep track of the heavens, e.g.

The average ancient was undoubtedly far more aware of the heavens than we are today. They had no light pollution and their lives depended upon knowledge of the changing patterns in the sky. Why?

Although we know that all ancient cultures observed and tracked the heavens, we have no knowledge of ancient Models of the Universe until the early Greeks began making records that survived their passing.

**Aside: What do we mean by Model of the Universe ?**

In fact, what do we mean by the word "Model". This word can have several meanings . . . e.g.

"Scientific Method"

Note:

If any fact is not consistent with the model,

If there is a choice between two models, then

Ancient Model of the Universe . . . A Guess

When prehistoric people studied the heavens, they, like us, had to notice several facts:

The stars appeared in patterns

The patterns of stars never changed

The paths of the Sun and the Moon through the stars

Based on these facts, a plausible ancient model of the Universe was not too different from the Celestial Sphere:

The Problem of the Planets

Very careful observations of the sky showed ancient astronomers that there were five "stars" that did not stay put, but instead wandered through the constellations. Later, the Greeks would name these objects "Planets", the Greek word for wanderer.

What is meant by "Wandering"

Sometimes a planet would do this:

There was no place for these planets in the simple prehistoric model of the Universe . . . the model could not explain their motion; therefore the model must be discarded or changed. The problem of the Planets would remain the central problem of astronomy (and indeed physics) for several thousand years until the time of Newton.

The Astronomy of the Greeks

Although there were many ideas regarding the structure of the Universe that came out of the Greek era (even a heliocentric model by Aristarchus), it was the model of Aristotle, later modified by Ptolemy, that would become accepted and regarded as correct for some 1500 years.

Geocentric Model of Aristotle (c. 350 B.C.)

This model places the Earth at the center of the Universe, and the heavens are composed of eight concentric spheres that independently (at slightly different speeds) rotate around the Earth.
This gives the Prograde Motion of the Planets:

Modifications of Ptolemy (c. 140 A.D.)

To explain Retrograde Motion of the Planets, Ptolemy invented the Epicycle. How does an Epicycle explain Retrograde Motion?

The Ptolemaic Model gave fairly accurate predictions of planetary motion, and it was later accepted by the Church, and thus was THE Model of the Universe for more than a thousand years. In fact, when Harvard University opened in Boston in the seventeenth century, the Ptolemaic Model was taught in the natural philosophy classes!
However, as observations of the locations of the planets got better and better, the Model had to modified (e.g. more epicycles had to be added and the Earth had to be moved slightly from the center); the model got more and more complicated.

Copernican Revolution (c. 1543)

After his death, Nicolas Copernicus' (Polish Cleric) Model of the Universe was published. His motivation was not based on any observations that showed the Ptolemaic Model incorrect; instead, he believed that his Heliocentric Model accounted for the motion of the planets in a much simpler manner.

According to Copernicus:

What about Retrograde Motion?

Even though the Copernican Model is far simpler than the Ptolemaic Model, it gave no better predictions of planetary motion. . . . Why?

Proof of the Copernican Model

In 1609, the Italian Galileo Galilei built a telescope and became the first astronomer to use the it for observations. Among the many observations that he reported, two prove that Copernicus was right:

Phases of Venus

Moons of Jupiter

Yes, Galileo's support of Copernicus got him into BIG trouble with the Church. He was held under house arrest and forced to publicly recant his Heliocentric ideas ("and yet it moves"). In 1992 (yes, nineteen ninety-two), after some review, the Church decided that Galileo was right after all! An apology was issued and Galileo was "rehabilitated."

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