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97 11 SEASONS OF PARADOX The Sun moves along the ecliptic by definition; it is the Sun’s path traced out in the sky. Since the ecliptic is inclined 231 ⁄2⬚ from the celestial equator, over the year the Sun moves no closer than 90⬚ ⫺ 231 ⁄2⬚ ⫽ 661 ⁄2⬚ from either celestial pole. It does so in June when it is nearest the north celestial pole and again in December when closest to the south celestial pole. The Moon and all of the planets except tiny Pluto move in orbits whose planes are inclined 7 degrees or less from the plane of the ecliptic (see Fig. 10.1). What if one planet was not closely aligned with the ecliptic? We speculate here that one of our bright planetary neighbors happens to be in an orbit highly inclined to the rest of the solar system. This is extremely unlikely to have occurred in the process of formation. When the solar system condensed out of the nebula that gave it birth, it began to rotate. This is a result of the inherent turbulence of matter in unbounded space. Condensing as it did only caused the speed of its rotation to increase to the point where the centrifugal force balanced the condensation in the plane of the rotation, but perpendicular to it the collapse would have continued long afterward until it was condensed into a thin disk. It is not by chance that almost all of the mass of the system is nearly coplanar, for the Milky Way and other galaxies did much the same thing on a far grander scale. Disklike formations are much more common than globular ones, although some of the latter do exist. The objects that move in orbits highly inclined to the ecliptic all share one common trait; they are all very small and are all either comets or asteroids. They are the ones that could be tilted into a much different orbital plane by a near miss with a body of larger mass. This is why no massive planets are found well outside the primary plane of the solar system. But what if somehow, from the gravitational influence of a passing star or something else, a bright planet did get dislodged from the plane of choice into an orbit that carries it near the poles, which one would it be? Not Mercury or Venus, because being closer to the Sun than we are, they wouldn’t likely end up very far north or south of the ecliptic. That leaves Mars, Jupiter, or Saturn. Let us suppose that Saturn does indeed move in an orbit inclined almost 90 degrees from our celestial equator. Saturn takes just about 30 years to circle the Sun once and shines between magnitudes ⫺0.5 and ⫹1.0, making it among the ten brightest objects in the sky most of the time. The planet is seen in the full phase since, like the other planets beyond the Earth, we can never get far enough from the Sun in angular terms to see more than a sliver of its dark side. Saturn moves one-thirtieth of its way around each year, or about 12 degrees annually, and if it were in a highly inclined orbit, we would see it up near the north pole region for several years. Then it would proceed southward by nearly 12 degrees a year until it reached the vicinity of the south pole, where it would again linger. For several years around this time Saturn could not be seen at all from our midnortherly latitudes , but half a Saturnian year before and afterward, we would see it all night long every clear night near the Big and Little Dippers. What would earlier societies have made of this anomaly? Would they develop some folk tale about the bad planet-god that was relegated or banished into its own kind of exile? Might Eudoxus just assign its crystalline spherical shell to its regular place between those of Jupiter and the fixed stars, even if it were upended along the way? Eudoxus, along with Aristotle and Plato, considered spherical shells, but later Hipparchus and Ptolemy required rings more or less in the plane of the system with their epicycles and deferents to carry the planets around the Earth. None of these geocentric models sheds light on the entire system until long after the time of Newton, when the dynamics of star formation were first recognized. Only then would this peculiarity...

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