Galileo and the Telescope

Michael Fowler, UVa  Physics

Copernicus Challenges Ptolemy’s Scheme

Ptolemy’s picture of the solar system was almost fully accepted for the next fourteen hundred years, to be challenged by Copernicus (real name: Nicolaus Koppernigk) a mathematician and astronomer with a Polish father and a German mother, in 1530. 

Copernicus’ picture of the solar system had the sun at the center, and the earth went around it, as did the other planets. 

We show here the picture from his original publication.  Notice that the only exception to the rule that everything goes around the sun is the moon, which continues to go around the earth.  One objection to the picture was that if the earth was indeed just another planet, how come it was the only one with a moon?

Other objections were based on the Aristotelian point of view—it was difficult to believe that all the other planets were composed of aither, and the earth of the other four elements, if they were all behaving in so similar a fashion.  A further objection, which had long ago been raised by Aristotle to the idea of a rotating earth, was that the stresses would cause it to fly apart, and furthermore, anything thrown in the air would land far to the west. 

Despite these problems, Pope Clement VII approved of a summary of Copernicus’ work in 1530, and asked for a copy of the full work when it was available.  This was not until 1543, the year Copernicus died. 

As Copernicus’ new picture of the universe became more widely known, misgivings arose.  The universe had after all been created for mankind, so why wasn’t mankind at the center?  An intellectual revolutionary called Giordano Bruno accepted Copernicus’ view, and went further, claiming that the stars were spread through an infinite space, not just on an outer sphere, and there were infinitely many inhabited worlds.  Bruno was burned at the stake in 1600. 

The real breakthrough that ultimately led to the acceptance of Copernicus’ theory was due to Galileo, but was actually a technological rather than a conceptual breakthrough.  It was Galileo’s refinement and clever use of the telescope that persuaded people that the moon was a lot like the earth, and in some ways, so were the planets. 

In fact, the Copernican heliocentric model had been suggested much earlier, by Aristarchus, as discussed in an earlier lecture. But it was officially frowned upon, and not widely disseminated. Copernicus actually mentioned Aristarchus in the first edition of his work, but dropped the reference in later versions.

Another quite likely connection is to earlier Islamic work. In accounting for differing planetary speeds at different orbital points, Copernicus replaced Ptolemy's equant explanation with epicycles that are essentially identical to al Tusi's couple, even in notation. It's not clear how that idea might have been transmitted, but Copernicus was familiar with some Islamic work, and there were Jewish scholars traveling widely in Europe and the east at that time. More details are available here.