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to be a celestial object like any other.
    The art historian Joseph Koerner explained to me that Galileo could use light and shadows to identify craters in part because of his artistic background. Galileo’s perspectival training helped him understand the projections he saw. He immediately recognized the implications of these images, even though they weren’t fully three-dimensional. He wasn’t interested in mapping the Moon, but in understanding its texture. And he understood right away what he saw.
    The third significant set of observations that validated the Copernican point of view related to the phases of Venus—illustrated in Figure 9. These observations were particularly significant in establishing that celestial bodies orbited around the Sun. The Earth clearly was not unique in any obvious way, and Venus clearly didn’t rotate around it.

    [ FIGURE 9 ] Galileo’s observation of the phases of Venus demonstrated that it too must orbit the Sun, invalidating the Ptolemaic system.
    From an astronomical perspective, the Earth was not so special. The other planets behaved like ours, orbiting the Sun with satellites orbiting them. Furthermore, even beyond the Earth—evidently sullied by human beings—not everything was unblemished perfection. Even the Sun was besmirched by sunspots that Galileo had also identified.
    Armed with these observations, Galileo famously concluded that we are not the center of the universe and that the Earth revolves around the Sun. The Earth is not the focal point. Galileo wrote up these radical conclusions. In doing so, he defied the church—although he later professed to reject Copernicanism in order to reduce his punishment to house arrest.
    As if his observations and theorizing about the large scales of the cosmos were not enough, Galileo also radically altered our ability to perceive small scales. He recognized that intermediate devices could reveal phenomena at small scales, just as they did at large ones, and he advanced scientific knowledge at both frontiers. In addition to his (in)famous astronomical investigations, he turned technology inward—to investigate the microscopic world.

    I was a little surprised when a young Italian physicist, Michele Doro, who was my guide to the San Gaetano exhibit in Padua, said without hesitation that Galileo had invented the microscope. I’d say that outside Italy at least the consensus is that it was invented in the Netherlands, but whether it was Hans Lippershey or Zacharias Janssen (or his father) is anyone’s guess. Whether or not Galileo invented the telescope (and he almost certainly didn’t), the fact is that he built a microscope and used it to observe smaller scales. It could be used to observe insects with accuracies never before possible. In his letter to friends and other scientists, Galileo was the first we know of to write about the microscope and its potential. The exhibition displayed the first publication to display the systematic observations that could be made with a Galilean microscope: dating from 1630, it illustrated Francesco Stelluti’s detailed studies of bees.
    The exhibit also showed how Galileo had studied bones—exploring how their structural properties would need to change with size. Apparently, in addition to his many other insights, Galileo was acutely aware of the significance of scale.
    The exhibit left no doubt that Galileo fully understood the methods and goals of science—the quantitative, predictive, and conceptual framework that tries to describe definite objects, which act according to the dictates of precise rules. Once these rules have provided well-tested predictions about the world, they can be used to anticipate future phenomena. Science searches for the most economical interpretation that can explain and predict all observations.
    The story of the Copernican revolution nicely illustrates this point too. In Galileo’s era, Tycho Brahe, the great observational astronomer, came to a different—and