Geocentric vs heliocentric model: from Ptolemy to Galileo

Ptolemy’s geocentric model of the solar system

In the geocentric model the Earth sits at the center of the cosmos. To account for the paths of the planets, Ptolemy used three geometric devices, the eccentric, the epicycle and the equant, built to reduce a tangled path to a sum of perfect circular motions. With the eccentric and the epicycle, the planets’ motions were not centered exactly on the Earth. The equant caused a separate trouble: it contradicted the very principle of perfect circular motion, and for that reason it bothered many thinkers.

Practical example: faced with a planet on a tangled path, the astronomer looked for circles that could add up to it. The eccentric and the epicycle belonged to that work, while the equant broke the rule of the perfect circle.

The Copernicus heliocentric model

In Book I of De revolutionibus, Copernicus holds that the motions of the heavenly bodies must be circular and uniform. In his heliocentric model of the universe the Earth moves around the Sun. Rheticus oversaw most of the printing of the work.

Moving the Earth was more than a geometric tweak. In the physics of Aristotle, the physical elements tended toward the Earth as their center, and putting it anywhere else challenged that whole picture.

Practical example: anyone who accepted Copernicus had one more question to answer. If the Earth is not the center, what do the physical elements tend toward?

Ptolemaic vs Copernican model, with Tycho’s compromise

The Tychonic system is a compromise. Tycho could not adopt the Copernican system, so he kept the Earth still at the center.

Tycho and the missing parallax

Tycho Brahe (1546-1601) was the greatest astronomical observer before the telescope was invented. In the Copernican model a parallax shift should be observable, and Tycho, though he tried, detected none. In his own system the other planets circle the Sun, while the Earth, with the Moon in orbit, stays still at the center of the Universe. In 1838 Bessel announced the distance to 61 Cygni, worked out from parallax.

Practical example: the Copernican model implies a parallax. Tycho looks for it with his instruments, and the result is zero.

Kepler and Galileo: ellipses and the telescope

In 1609 Kepler’s Astronomia nova appeared, with the first two of his laws. The first says that a planet’s orbit is an ellipse with the Sun at one focus, whereas Ptolemy and Copernicus had built their motions from circles. In that book Kepler relies heavily on Tycho’s observations. In the Sidereus Nuncius of 1610, Galileo announced that Jupiter has four moons, known as the Galilean moons. Later that year he saw that Venus shows phases, a sign that it circles the Sun. Following the sunspots, he found that the Sun rotates on an axis, as the Earth does in the Copernican view.

Practical example: Galileo observed Venus and saw its phases, a sign that the planet circles the Sun.

1616 and 1633

In 1616 the proceedings against Galileo ended with Cardinal Bellarmine admonishing him neither to defend nor to hold the Copernican doctrine.

At the 1633 trial Galileo was found guilty of vehement suspicion of heresy and had to recite and sign an abjuration. The sentence of formal imprisonment was commuted to house arrest.