For about 1600 years from the time of the ancient Greeks until the 1600’s people believed that the Sun, stars and planets orbited the Earth.

This idea was defended in written form by an Egyptian named Ptolemy.

But then came:

The Copernican Revolution

Ptolemaic model was not seriously challenged until the 15th century during the Renaissance.

Nicholas Copernicus ( ) rediscovered the heliocentric model (Aristarchus).

He found to his dismay that it better fit the observed facts than the geocentric model.

Seven points of the Copernican system:

1. The celestial spheres do not have one common center. The Earth is not at the center of everything.

2. Earth is not the center of the universe, only the center of gravity and the lunar orbit. Only the Moon orbits Earth.

3. All the spheres orbit the Sun. Spheres means the planets.

4. Compared to the distance to the stars, the Earth to Sun distance is almost nonexistent. The stars are very much farther away than the Sun.

5. The motion of the stars is due to the Earth rotating on its axis.

6. The motion of the Sun is the result of the Earth’s motions. (rotation and revolution)

7. The retrograde and forward motions of planets is caused by the Earth’s motion. It is caused by the fact that Earth’s orbit is a different length than the other planets.

The Copernican model was not well accepted by scholars or the public. It violated the religious teaching of the time.

Copernicus’ book De Revolutionibus was published in 1543 (the year Copernicus died).

It included an anonymous preface that stated that the new model was merely an aid to calculation and suggested that Copernicus really did not believe it.

Galileo Galilei ( ) Italian mathematician and philosopher. He performed experiments to test his ideas (a radical idea then). He is regarded as the father of experimental science.

The telescope was invented in Holland early in the 17th century. Galileo heard about it and, although never having seen one, made his own in 1609.

With it he saw the mountains, valleys, and craters of the Moon; spots on the Sun (which eventually blinded Galileo); the phases of Venus; and, perhaps most significant:

he saw 4 moons orbiting Jupiter (known as the Galilean moons).

All these observations suggested that the Ptolemaic model was wrong and the Copernican model was correct.

Galileo published his findings in Siderius Nuncius (The Starry Messenger) in 1610.

This was a very risky action for Galileo. In 1600 Giordano Bruno was burned at the stake in Rome for (among other things) teaching that the Earth orbited the Sun.

In 1616, Copernicus’ works were banned by the Roman Church and Galileo was told to stop researching and stating such nonsense.

Galileo published a new book in 1632: Dialogue Concerning the Two Chief World Systems.

Three people debating the Ptolemaic and Copernican models. The Aristotelian wins, but his arguments are obviously inferior. His name? Simplicio.

Also, this book was written in Italian, not Latin, so the common man could read this book. The church was not amused.

The Inquisition forced Galileo, under threat of torture, to recant his claim that the Earth orbited the Sun.

He was placed under house arrest in 1633 and remained imprisoned until his death in 1642.

Galileo’s crimes were publicly forgiven by the Catholic Church in 1992.

But, by this time the damage was done, and the Copernican model continued to gain acceptance as the years passed.

Earth’s orbit of the Sun couldn’t be proven until the unmanned probes of the 1960’s, 70’s, and 80’s.

The fact that the Earth moves at all was proven by parallax in the 19th century.

From Aristarchus belief until actual proof took over 2000 years.

Copernican principle ­- Earth is not special in a cosmological sense.