Galileo's defense of Copernicus and the Sun-centered universe

Galileo Galilei of Forence, Italy passionately promoted the Copernican model of a Sun-centered universe, supported by his own observations of the tides and of the planets using his telescope.  He saw some of the moons of Jupiter, ears on (really the rings of) Saturn, and the phases of Venus.  He constructed credible arguments to explain things like how an arrow, shot upward from a moving Earth, could land back where it started.

Today we think of Galileo as teaching an obvious truth, only to be punished by a backward Church.  In fact, Galileo was outside the main stream of the scientific community.  The vast majority of astronomers of his day favored the more or less Earth-centered models, including a new one recently developed by Tyco Brahe based on a massive store of data. 

There were many problems with the Copernican model.  In particular, people simply didn’t feel the predicted rapid motion of the Earth; the model didn’t predict planet locations very well or explain why their distances from Earth didn’t appear to change much; and the only explanation for the lack of relative motion (parallax) of the stars from a moving Earth would be that the stars were fantastically far away – which no one believed.  Even Galileo thought parallax should be observable if it existed – thus seeming to disprove the theory.  Still, he discounted this evidence that Copernicus was wrong.

Galileo was famously confined to house arrest by the Catholic Church after he published his book, “Dialogs” making his case, even though the Pope (an old friend of Galileo’s) had seemingly given him the green light to proceed.  In his book, Galileo made some serious political and scientific mistakes:

First, although Kepler had recently published his theories on the orbits of planets, Galileo didn’t make use of them.  Had he, Galileo could have answered many of his critics’ objections.  But his conception, like Copernicus’s, remained rooted in Aristotle’s spheres. 

Second, his book was supposed to be a balanced comparison of the alternative models when in fact it was a brilliant but thinly veiled appeal for Copernicus.  And Galileo chose to express the Ptolemaic position of his friend, The Pope, through a character called “Simplicio”, a name which sounded just as derisive in Galileo’s time as it does today.  The Pope felt betrayed and, hounded by offices within the Church, he punished Galileo in a way that seems harsh to us but may have seemed lenient to the Pope.  Today we would strip his government funding and force him into a new line of work.  By comparison, Galileo continued to benefit from Medici patronage even after his indictment and house arrest.

Galileo can be thought of as an early victim of the concept of “settled science”.  He gave greater weight to the ever more glaring flaws in the accepted theories (flaws the establishment had become comfortable with) than to the problems with a theory that his observations and his scientific intuition told him was at least on the right track.  It would be hundreds of years before all of the apparent contradictions in the Copernican model – even as repaired by Kepler – could be explained scientifically.