Tycho and Kepler: solid myth versus subtle truth

Social Research, Spring, 2005 by Owen Gingerich, James R. Voelkel

For the theologians, the Copernican system contradicted the most straightforward interpretation of the Bible. After all, at the Battle of Gibeon Joshua commanded the sun, and not the earth, to stand still. And Psalm 104 said that the Lord God laid the foundations of the earth that it not be moved forever. In private correspondence Tycho discussed such issues, but in his public writings he never played the religious card. It was always the physics that exercised him, and he saw no way to save the physics apart from a cosmology with a stationary earth. In the absence of a physics in which the earth could be moved without our sensing it, scriptural issues never came to the fore.

Precisely when Tycho began to think about a system that would keep the earth fixed yet allow for the other planets to revolve about the sun is hard to establish. But Tycho soon appreciated that such ideas were "in the air," so to say. Eventually, to establish his priority in the matter, he wished to credential himself as conceiving such thoughts as early as possible, so his own claims must be considered rather judiciously. In a brief German treatise on the comet of 1577 he mentioned that some people believe that Mercury and Venus encircle the sun, a statement that he could have picked up from several different sources, including Copernicus's De revolutionibus. But Tycho himself does not claim to have discovered a geo-heliocentric system until around 1583-84.

In the summer of 1580 an enthusiastic visitor to Hven, an itinerant mathematician named Paul Wittich, energized Tycho's cosmological thinking. Wittich, in the back of his copy of De revolutionibus, had made a series of diagrams showing how the technical details of Copernicus's planetary mechanisms could be rendered in a geocentric form, and he included one that showed the sun in orbit about the earth but carrying Mercury and Venus in orbit about the moving sun. However, at this point he was stuck. If he also put Mars in orbit around the sun, its distance relative to the earth would require that its path intersect with the circle of the sun's orbit, thereby violating the traditional view of transparent crystalline celestial spheres.

Tycho, seeing Wittich's diagram, may well have asked himself if Mars did indeed come closer than the sun, as it must do in the Copernican system, or as it would do in a geo-heliocentric system where it would cross over the circle for the sun (see diagram). Thus, Wittich's visit might have been the serendipitous event that triggered Tycho's search for the parallax (that is, the distance) of Mars. While Tycho in his correspondence presented his observational search in terms of the Copernican versus the Ptolemaic systems, he could also have realized that a successful discovery that Mars came closer than the sun would prepare the way for a physically real geo-heliocentric cosmology just as well as for a Copernican cosmology. Thus, when in 1587 he thought that he had achieved exactly such a result, he hastily revised the proposed contents of a work in progress, his De mundi aetherei phenomenis recentioribus, and in 1588 he published what has become known as the Tychonic system. In it the sun and moon circle a fixed, central earth, while the sun in turn carries an entourage of planets circling around it. Though Tycho presented his cosmology as observationally driven, it may well be that the cosmological idea drove the bold observational quest in the first place. As to his claims to have successfully measured Mars's parallax: those faded as Tycho began to appreciate the complex and contradictory effects of parallax and refraction. Like the Cheshire Cat, they vanished, leaving only the Tychonic system behind.