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and, after a few dozen steps, asserting that you’ve reached the center simply because you see the same number of trees in every direction.
    By 1920—but before the light-absorption problem was well understood—Harlow Shapley, who was to become director of the Harvard College Observatory, studied the spatial layout of globular clusters in the Milky Way. Globular clusters are tight concentrations of as many as a million stars and are seen easily in regions above and below the Milky Way, where the least amount of light is absorbed. Shapley reasoned that these titanic clusters should enable him to pinpoint the center of the universe—a spot that, after all, would surely have the highest concentration of mass and the strongest gravity. Shapley’s data showed that the solar system is nowhere close to the center of the globular clusters’ distribution, and so is nowhere close to the center of the known universe. Where was this special place he found? Sixty thousand light-years away, in roughly the same direction as—but far beyond—the stars that trace the constellation Sagittarius.
    Shapley’s distances were too large by more than a factor of 2, but he was right about the center of the system of globular clusters. It coincides with what was later found to be the most powerful source of radio waves in the night sky (radio waves are unattenuated by intervening gas and dust). Astrophysicists eventually identified the site of peak radio emissions as the exact center of the Milky Way, but not until one or two more episodes of seeing-isn’t-believing had taken place.
    Once again the Copernican principle had triumphed. The solar system was not in the center of the known universe but far out in the suburbs. For sensitive egos, that could still be okay. Surely the vast system of stars and nebulae to which we belong comprised the entire universe. Surely we were where the action was.
    Nope.
    Most of the nebulae in the night sky are like island universes, as presciently proposed in the eighteenth century by several people, including the Swedish philosopher Emanuel Swedenborg, the English astronomer Thomas Wright, and the German philosopher Immanuel Kant. In An Original Theory of the Universe (1750), for instance, Wright speculates on the infinity of space, filled with stellar systems akin to our own Milky Way:
We may conclude…that as the visible Creation is supposed to be full of sidereal Systems and planetary Worlds,…the endless Immensity is an unlimited Plenum of Creations not unlike the known Universe…. That this in all Probability may be the real Case, is in some Degree made evident by the many cloudy Spots, just perceivable by us, as far without our starry Regions, in which tho’ visibly luminous Spaces, no one Star or particular constituent Body can possibly be distinguished; those in all likelyhood may be external Creation, bordering upon the known one, too remote for even our Telescopes to reach. (p. 177)
     
    Wright’s “cloudy Spots” are in fact collections of hundreds of billions of stars, situated far away in space and visible primarily above and below the Milky Way. The rest of the nebulae turn out to be relatively small, nearby clouds of gas, found mostly within the Milky Way band.
    That the Milky Way is just one of multitudes of galaxies that comprise the universe was among the most important discoveries in the history of science, even if it made us feel small again. The offending astronomer was Edwin Hubble, after whom the Hubble Space Telescope is named. The offending evidence came in the form of a photographic plate taken on the night of October 5, 1923. The offending instrument was the Mount Wilson Observatory’s 100-inch telescope, at the time the most powerful in the world. The offending cosmic object was the Andromeda nebula, one of the largest on the night sky.
    Hubble discovered a highly luminous kind of star within Andromeda that was already familiar to astronomers from surveys of stars much