The Believing Brain - Michael Shermer [163]
Some astronomers speculated that a force made the stars orient themselves in a band across the sky, and that this structure rotated around the sun just like the planets. In 1750, an English watchmaker and teacher named Thomas Wright published his theory of the Milky Way in a book entitled An Original Theory; or, New Hypothesis of the Universe, in which he presciently conjectured that an observer’s orientation in space determines the perception of what is observed. He concluded that the Milky Way was a shell of stars on which our solar system resided, such that looking flat across the shell one sees lots of stars, but in looking up or down away from the shell one sees mostly empty space.1 That’s a close approximation to what we observe, only we now know that the Milky Way is a flat disc, like a Frisbee, and our solar system sits about three-quarters of the way out from its center. If you look “through” the disc—along the thick plane, that is—you see lots of stars, and these then appear as a band across the night sky. When you look away from the band you are looking either up or down from the disc.
Islands in the Sky
Such conjectures, however prescient in hindsight, gained little footing on the intellectual landscape until the great Prussian philosopher Immanuel Kant turned his perceptual powers skyward—if only in his mind’s eye—when he suggested that the elliptical-shaped “nebulous stars” believed by many astronomers to be nearby were actually discs of countless stars very far away: “I easily persuaded myself that these stars can be nothing else than a mass of many fixed stars. On account of their feeble light, they are removed to an inconceivable distance from us.” But why do some nebulae appear round, others elliptical shaped, and still others as a flat plane? Were these different objects entirely, or are they the same species of objects viewed at different angles? Kant reasoned his way to a nearly correct answer: “[I]f such a world of fixed stars is beheld at such an immense distance from the eye of the spectator situated outside of it, then this world will appear under a small angle as a patch of space whose figure will be circular if its plane is presented directly to the eye, and elliptical if it is seen from the side or obliquely.”
These nebulae became known as Kant’s “island universes,” and he waxed poetic about them in his 1755 book Universal Natural History and Theory of the Heavens: “The infinitude of the creation is great enough to make a world, or a Milky Way of worlds, look in comparison with it what a flower or an insect does in comparison with the earth.” As for the Milky Way itself, Kant outlined his theory in his usual insightful manner:
Just as the planets in their system are found very nearly in a common plane, the fixed stars are also related to their positions, as nearly as possible, to a certain plane which must be conceived as drawn through the whole heavens, and by their being very closely massed in it they present that streak of light which is called the Milky Way. I have become persuaded that because this zone, illuminated by innumerable suns, has almost exactly the form of a great circle, our sun must be situated quite near this great plane. In exploring the causes of this arrangement, I have found the view to be very probable that the so-called fixed stars may really be slow moving, wandering stars of a higher order.2
The Great Debate
Kant’s theory of the heavens