Copernicus and Galileo Revisited

Victor J. Stenger

For Reality Check in Skeptical Briefs September, 2008. Draft of Saturday, July 19, 2008 9:30 AM. For comment only. Do not quote, copy, or distribute.

In 1543 Nicolaus Copernicus triggered the modern scientific revolution with the publication of De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Spheres) in which he proposed that Earth orbits about the sun. A myth that has become attached to this story tells how he looked beyond the vanity of human self-centeredness and removed Earth from the center of the universe. As a result, people began to look at themselves and their place in the universe in a more objective and realistic light.

In fact, the belief that Earth is the center of the universe may not have had much to do at all with human self-centeredness. Ancient people did not look at the center of Earth as a desirable place. After all, it was the location of hell itself while heaven was way out beyond the stars. But the Bible is pretty clear that Earth is absolutely fixed in space and that the sun, moon and stars move with respect to Earth.

Aristotle had made the empirical argument that both Earth and the universe seem to have the same center since heavy objects fall toward Earth and fire travels upward, while celestial bodies seem to rotate about Earth. This was still the scientific view at the time of Copernicus, with astronomers using the complicated geocentric system of Ptolemy to compute the positions of planets on the celestial sphere.

Copernicus’s heliocentric system had been in circulation since 1515 and while other astronomers respected his ideas, they did not like the counterintuitive and apparently counter-empirical idea of a moving Earth. If we are moving, why don’t we notice it? Besides, the original version of Copernicus’ model was no more accurate and only marginally less cumbersome than Ptolemy’s, still containing some thirty epicycles—circles within circles. The Copernican picture did have a certain aesthetic appeal, however, offering a natural explanation for the zigzag motion of the planets as observed from Earth.

Eventually the Copernican model proved more successful, thanks to the careful observations of Tycho Brahe, which corrected a number of previous errors in astronomical data, and the realization by Johannes Kepler that the planetary orbits were not circles but ellipses. This got rid of the epicycles and located the sun at the common focus of the planetary orbits. Kepler also discovered that a planet sweeps out equal areas in equal times as it moves about the sun, now called Kepler’s law. Sixty years later when Isaac Newton trivially derived Kepler’s law from his laws of motion and gravity, the Copernican system finally became firmly established.

Copernicus worded his proposal carefully and it did not cause an immediate religious hullabaloo. That would happen fifty years later at the hands of a brasher scientist, Galileo Galilei (d. 1642). Indeed, over the intervening period the Roman Catholic Church had adopted the heliocentric model as a calculational tool that, with the improvements mentioned, led to more accurate predictions of the positions of astronomical bodies. This was important for the accurate dating of Easter and other festivals. The Church has a long history of interest and support for astronomical research going back to the fourteenth century when Pope Gregory XII instituted the calendar we still follow today.

The story of Galileo's trial by the Inquisition in 1633 for teaching the Copernican system is often presented as a classic example of religion and science coming into conflict. But this story is also part myth and part fact. Historians now largely agree that Galileo was not tried for teaching heliocentrism but for disobeying a Church order.

In 1610 Galileo had published the results of his telescopic observations of the heavens, which, in his mind, provided empirical confirmation of the Copernican view. He initially drew some support from powerful Church leaders, but in 1616 was instructed not to discuss heliocentrism as a fact until he had definitive physical proof.

This he claimed to have in 1632 with the publication of Dialogue on the Two Chief World Systems. In fact the proof presented there, which was based on the assumption that Earth's motion causes the tides, was dead wrong. The argument was convoluted and incorrectly predicted only one high tide a day. Galileo dismissed as "useless fiction" the proposal by Kepler that the moon caused the tides, which turned out to be correct.

The scientific community today severely and justly criticizes any scientist presenting a poorly formulated proof that disagrees with the data. In 1633 Galileo was tried for disobeying the order of the Church. He agreed to a plea bargain in which he would admit he had gone too far, but for still unknown reasons the Inquisition overruled the agreement and handed down a harsh sentence in which Galileo was forced to recant.

Galileo lived out the final nine years of his life under comfortable house arrest, technically forbidden from writing further on physics. Somehow, however, he did some of best work during that time, publishing the Discourse on Two New Sciences that basically invented kinematics, the description of the motion of bodies. Isaac Newton would take off from there.

In the meantime, the Church promoted research into the Earth's motion and actually ran experiments in the 1650s and 1660s that provided empirical support for the Copernican system. In the Catholic Church, the pope and not the Bible has the ultimate authority, so it did not matter so much that a conflict with scriptures existed.

When the Reformation rejected the authority of the pope it had no place else to go but the Bible for a replacement authority. And Copernicus clearly conflicted with the Bible. Martin Luther called Copernicus a “fool who wished to reverse the entire history of astronomy. John Calvin denounced those who “pervert the course of nature” by saying that "the sun does not move and that it is the earth that revolves and that it turns." Feeling the pressure from Luther and Calvin, the Catholic Church after about 1650 began to regard the Copernican model as devaluing humanity and banned its teaching. That may have been the reason for the Inquisition earlier coming down hard on Galileo.

The Catholic Church did not remove its ban on Copernicus until 1822. His book remained on the forbidden list until 1835. In 1992 Pope John Paul II lifted the edict of inquisition against Galileo, 359 years after the trial.

Copernicus and Galileo Revisited

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