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wrong. Astronomers, for example, would later reduce the Milky Way's girth from 300,000 to 100,000 light-years, once they better understood the difference between the fast and slow variable stars and affirmed the presence of interstellar dust, which made celestial objects appear dimmer and hence more distant than they actually were. This made Shapley mistakenly believe that the Milky Way was more extensive than it actually was. Yet even when the galaxy's width was reduced to 100,000 light-years, it was still bigger than Kapteyn and his supporters had been hawking. Shapley's discovery held up over time on the essentials: first of all, that the Milky Way was a far larger metropolis of stars than previously suspected and, second, that the Sun was situated in its suburbs.

Shapley's shift of the Sun's position was fully confirmed in the mid-1920s when Bertil Lindblad, a Swedish expert on stellar dynamics, and Jan Oort, at the Leiden Observatory, in the Netherlands, demonstrated that stars were circulating within the Milky Way around a point situated in Sagittarius, exactly where Shapley had pegged the galactic center. Once Lindblad worked out the theory, Oort rounded up the evidence to prove it. If anyone was still questioning Shapley's pushing the Sun off into the galactic boondocks, Lindblad and Oort swept away all doubts. Like the horse on a carousel, the solar system travels in a continual loop, completing one full circuit around the galactic disk roughly every 250 million years. The last time we were in this neck of the celestial woods, the Appalachian and Ural mountains were just being formed and the dinosaurs were getting ready to rule the Earth.

Shapley's new model of the Milky Way had broad repercussions, especially regarding the spiral nebulae. The idea of island universes, then on the verge of acceptance, was back on shaky ground. “With the plan of the sidereal system here outlined,” reported Shapley, “it appears unlikely that the spiral nebulae can be considered separate galaxies of stars.” There was still the problem of the exceptionally bright novae seen earlier in the spiral nebulae. How do you explain that? asked Shapley. And then there were van Maanen's rotations to take into account. Not everyone was swayed by Shapley's worries; the most ardent believers in external galaxies still held fast to their convictions—not only Curtis but also such major players as Arthur Eddington, W. W. Campbell, and V. M. Slipher. It was the undecideds who were most affected by Shapley's arguments and so remained huddled on the fence. What resulted were two completely different views of the universe, which were difficult to reconcile. The writer MacPherson poetically put it this way: “We may compare our galactic system to a continent surrounded on all sides by the ocean of space, and the globular clusters to small islands lying at varying distances from its shores; while the spiral nebulae would appear to be either smaller islands, or else independent ‘continents’ shining dimly out of Immensity.” As the Roaring Twenties was about to make its appearance, Shapley voted for the “smaller islands,” Curtis for the “continents.”

He Surely Looks Like the Fourth Dimension!

Astronomy was not the only field in a tumult as the nineteenth century turned into the twentieth. Physics, too, was in upheaval.

Doctors across the globe were still reeling over their newfound ability to use X rays, discovered in 1895 by the German physicist Wilhelm Röntgen, to peer inside the human body. Soon after, in Paris, Henri Becquerel accidentally stumbled upon a phenomenon, what came to be called radioactivity, when he was investigating the properties of uranium salt crystals. And in England J. J. Thomson identified the first particle smaller than an atom—the electron. In the new, topsyturvy world of quantum physics, light itself was soon imagined as either a wave or a particle, and physicists were realizing that their trusted laws of motion, dating back more than two hundred years to Isaac Newton, could not reliably gauge how light, whatever its form,