Galileo, the Impact of the Telescope, and the Birth of Modern Astronomy

By Owen Gingerich

Proceedings of the American Philosophical Society, Vol. 155 Issue 2 (2011)

Abstract: The article examines astronomer Galileo Galilei’s contributions to the development of modern astronomy, with emphasis given to his use of the telescope when conducting astronomic observations. The author begins by explaining Galileo’s belief in the heliocentric model of the Solar System. He goes on to detail how Galileo’s telescopic observations of the Moon, the orbits of Jupiter’s satellites, and the phases of Venus helped convince him of the validity of Copernican cosmology.

Introduction: One of the leading misconceptions from the 2009 International Year of Astronomy was the idea that Galileo’s brilliant telescopic observations proved the motion of the Earth. Allied with this myth is the parallel notion that the Catholic Church stubbornly clung to the past, refusing to accede to the obvious march of astronomical science. It is true, of course, that the Inquisition forced Galileo under the threat of torture to recant his belief in Copernicus’s heliocentric system, but the efficacy of the Copernican system actually played a very small role in the trial. Galileo would have dearly loved to explain to his examiners how his observations made belief in the Copernican system more intellectually respectable even though he had no irrefutable proof of the Earth’s motion, but this was an opportunity he never got.


So let us examine the status of the Sun-centered system from the pre-telescopic world of Copernicus up to the age of Galileo and beyond.

Ancient astronomers had long observed that, while the planets generally move eastward against the background of stars, occasionally they stop and then move westward for a few weeks. Ptolemy modeled this so-called retrograde motion by assuming that the planets rode on secondary circles, or epicycles, which in turn traveled eastward against the background of stars, occasionally they stop and then move westward for a few weeks. Ptolemy modeled this so-called retrograde motion by assuming that the planets rode on secondary circles, or epicycles, which in turn traveled eastward on large carrying-circles, called deferents. The relative size of the epicycles with respect to their deferents was established by observations of the duration and arc length of the retrograde motion. Thus for Jupiter the epicycle was about a tenth as large as its deferent, Saturn’s was comparatively smaller, and Mars’s was huge, about two-thirds as large as its deferent. In Ptolemy’s system, the mechanism for each planet was independent or, as Copernicus put it, more monster than man, with an arm from one creature, a leg from another, and a head from yet another.

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