Friday, February 22, 2008

“More Pleasing to the Mind”

This uncertainty in the calendar bothered Copernicus deeply. Many astronomers thought that any errors in the Ptolemaic system might be due to the many small “typos” that had crept into the manuscript with centuries of copying by scribes. However, connections between east and west at this time meant that Copernicus was able to have a nearly pristine copy of Ptolemy’s work Almagest. Any errors had to be errors in the model itself.

Ptolemy’s complex geocentric system of epicycles Copernicus questioned the Ptolemaic system on the very basis that a modern scientist might. The model had become too complicated, and scientists tend to seek simplicity (where possible) in their models of the universe. The printing presses that were firing up across Europe at the time made it possible for many more scholars to read good copies of ancient works.

As Copernicus started reading Greek manuscripts that had been long neglected, he rediscovered Aristarchus’s old idea of a heliocentric (sun-centered) universe. He concluded that putting the sun at (or near) the center of a solar system with planets in orbit around it created a model that was “more pleasing to the mind” than what Ptolemy had proposed and medieval Europe accepted for so many centuries.

But he did not rush to publish, only after much hesitation privately circulating a
brief manuscript, De Hypothesibus Motuum Coelestium a Se Constitutis Commentariolus
(A Commentary on the Theories of the Motions of Heavenly Objects from Their Arrangements)

in 1514. He argued that all of the motion we see in the heavens is the result of the earth’s daily rotation on its axis and yearly revolution around the sun, which is motionless at the center of the planetary system.
Sound familiar?

Aristarchus had suggested it almost 2,000 years earlier, but no one had listened. The earth, Copernicus explained, was central only to the orbit of the moon. For almost two more decades he refined his thought before consenting, in 1536, to publish the full theory. But largely because of opposition from Martin Luther and other German religious reformers, De Revolutionibus Orbium Caelestium (On the Revolutions of the Celestial Spheres) wasn’t actually printed until 1543.

As close as Copernicus’s model came to representing the motion of planets in the solar system, it insisted on the perfection of circular orbits, so that it actually had no better predictive ability for planetary motions than the Ptolemaic model it replaced. For all its creakiness, the Ptolemaic model still predicted, for example, where Mercury would be on a particular night about as well as Copernicus’s model did.

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