That made it easy to estimate their distances from the Sun in terms of the Earth-Sun distance. Mars, Jupiter and Saturn moved in bigger circles, and they moved more slowly, so that whenever the Earth overtook them, they seemed to move backwards.

    Many books and plays exist on the life of Galilei, the Italian scholar who laid the foundation to the discipline known for many years as "natural philosophy," now called physics.

    He was the first to observe the planets through a telescope, and what he saw convinced him that Copernicus was right. How his agressive defense of the Copernican theory turned the Catholic church against him and cost him his freedom is a fascinating story, but it goes beyond our scope here. (For a 1636 painting of Galileo, with links to more information, see here.)

    Galileo did not invent the telescope; that was done by lensmakers in Holland and elsewhere (eyeglasses had been in use for centuries). Unlike later astronomical telescopes, which turn the picture upside down, the first version worked the way opera glasses do, combining two lenses of different types. Opera glasses magnify about 2-3 times: Galileo pushed the technology to its limits, magnifying his view 8-fold and in a later instrument 33 times.

    That was the instrument with which, in 1609-10, Galileo made his revolutionary discoveries. He observed the Moon and saw a world with mountains and "seas," and risking blindness (since the Sun should never be looked at through a telescope) he also observed sunspots. When he turned his telescope to the planet Jupiter, he saw four moons orbiting around it, all practically in the same plane, close to the ecliptic (they and the planet all seemed to lie on the same straight line; you can get the same view through good binoculars or any telescope), very much like a miniature version of the kind of solar system proposed by Copernicus.

    And when he looked at Venus, he saw its visible shape changing like that of the moon, becoming a crescent when Venus was between us and the Sun, a time when most of its sunlit half faced away from Earth. Galileo was persecuted for advocating the world view of Copernicus, but his observations, which were soon confirmed by other astronomers, convinced all scholars that this was indeed the way the Sun, Earth, Moon and the planets were related.

Exploring Further:

Very detailed and long site "Orbits and Gravitation" on the history of astronomy.

Arthur Koestler, The Sleepwalkers, Arkana reprint edition 1990

A concise review, by Owen Gingerich, of "De Revolutionibus Orbium Celesium " (On the Revolutions of Heavenly Spheres) by Nicolaus Copernicus, appeared in the journal Nature--vol. 391, p. 140, 8 January 1998. A 1992 translation of the book by Rossen (452 pages) is available from Willmann-Bell ($39.95)--see http://www.willbell.com. That publisher is also offering other writings by Copernicus, "Galileo at Work" by Drake and various books on the history of astronomy.

Laura Fermi and Gilberto Bernadini: Galileo and the Scientific Revolution, Basic Books 1962

The History and Practice of Ancient Astronomy by James Evans, Oxford Univ. Press, 1998; reviewed by J.D. North, "Nature ", vol. 398, p. 385, 1 April 1999