Finding our Place in the Solar System: The Scientific Story of the Copernican Revolution by Todd Timberlake

Author:Todd Timberlake [Timberlake, Todd]
Language: eng
Format: epub
ISBN: 9781107182295
Amazon: 1107182298
Barnesnoble: 1107182298
Goodreads: 41883967
Published: 0101-01-01T00:00:00+00:00

7.2 A new astronomy from physical causes

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Figure 7.5 Diagram of the path of Mars in the Ptolemaic and Tychonic systems, from Kepler’s Astronomia Nova (1609). Image courtesy History of Science Collections, University of Oklahoma Libraries.

Kepler also defended the idea of Earth’s rotation against common criticisms.

People were not knocked down by the Earth’s motion, nor did they feel a great east wind, because things on and around the Earth shared in its rotation. Nor was the motion of Earth contrary to Scripture, because the Bible was revealed in terms understandable to common people using everyday language, not in the language of natural philosophy. He noted that the Tychonic system provided a nice compromise, but pointed out how Tycho’s model disrupted the beautiful relation between the period and size of a planet’s orbit.

In Part 1 of the Astronomia Nova, Kepler showed that the Ptolemaic, Copernican, and Tychonic theories for planetary motion are geometrically equivalent.53 All three theories were equally capable of reproducing the observed motions of the planets. This geometrical equivalency showed that it was necessary to use physical, rather than geometrical, reasoning to decide between these theories. Kepler also displayed a picture of the Ptolemaic/Tychonic path of Mars through space (see Figure 7.5). According to the Ptolemaic and Tychonic theories, Mars, like the other planets, wound around like a pretzel. The Copernican paths, on the other hand, were nearly circular and simply repeated the same orbit over and over. The implication was clear: after Tycho’s demolition of the hard celestial orbs, only the Copernican orbits made physical sense.

As he had already proposed in the Mysterium, Kepler suggested that the Sun emitted a force that drove the planets around in their orbits as the Sun spun on its axis. Since the Sun was the true source of a planet’s motion, it was important

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Physical causes

to use the actual body of the Sun (the “true Sun”) as the reference point for determining a planet’s orbit. To determine Mars’ location as seen from the Sun it was necessary to measure the planet’s apparent location when the Earth was directly between the Sun and Mars. In other words, the position of Mars must be measured when it is in opposition to the true Sun. All previous astronomers had used oppositions to the mean Sun in constructing their theories of planetary motion, but Kepler, guided by his physical insight, chose to use oppositions to the true Sun instead.54 With that critical point established, Kepler was ready to begin his attack on Mars.

7.3

The war on Mars

In Part 2 of the Astronomia Nova Kepler considered Tycho’s attempts to measure Martian parallax. He pointed out that Tycho’s data showed no evidence of any detectable parallax, and he questioned the ancient value of 3 for the diurnal parallax of the Sun. He then demonstrated that if the plane of Mars’

orbit is assumed to pass through the body of the Sun, then that orbital plane will have a fixed inclination of just under 2◦ relative to the ecliptic. In a single move

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