d； aberration is similar to how raindrop falling directly down on a speeding auto seem to the passenger； to be falling at a slant； the faster the car goes； the steeper the slant。 If the Earth were stationary at the center of the Universe。 and not speeding in its orbit around the Sun； Bradley would nor have found the aberration of light。 It was a pelling demonstration that the Earth revolved about the Sun。 It convinced most astronomers and some others but not； Bradley thought； the 〃Anti…Copernicans。〃
But not until 1837 did direct observations of the stars prove in the clearest way that the Earth is indeed circling the Sun。 The long…debated annual parallax was at last discovered—not by better arguments； but by better instruments。 Because explaining what it means is much more straightforward than explaining the aberration of light； its discovery was very important。 It pounded the final nail into the coffin of geocentrism。 You need only look at your finger with your left eye and then with your right and see it seem to move。 Everyone can understand parallax。
By the nineteenth century； all scientific geocentrists had been converted or rendered extinct。 Once most scientists had been convinced； informed public opinion had swiftly changed； in some countries in a mere three or four generations。 Of course； in the time of Galileo and Newton and even much later； there were still some who objected； who tried to prevent the new Sun…centered Universe from being accepted； or even known。 And there were many who at least harbored secret reservations。
By the late twentieth century； just in case there were any holdouts； we have been able to settle the matter directly。 We've been able to test whether we live in an Earth…centered system with planets affixed to transparent crystal spheres； or in a Sun…centered system with planets controlled at a distance by the gravity of the Sun。 We have； for example； probed the planets with radar。 When we bounce a signal off a moon of Saturn； we receive no radio echo from a nearer crystal sphere attached to Jupiter。 Our spacecraft arrive at their appointed destinations with astonishing precision； exactly as predicted by Newtonian gravitation。 When our ships fly to Mars； say； their instruments do not hear a tinkling sound or detect shards of broken crystal as they crash through the 〃spheres〃 that—according to the authoritative opinions that prevailed for millennia—propel Venus or the Sun ill their dutiful motions about the central Earth。
When Voyager 1 scanned the Solar System from beyond the outermost planet； it saw； just as Copernicus and Galileo had said we would； the Sun in the middle and the planets in concentric orbits about it。 Far from being the center of the Universe； the Earth is just one of the orbiting dots。 No longer confined to a single world； we are now able to reach out to others and determine decisively what kind of planetary system we inhabit。
EVERY OTHER PROPOSAL； and their number is legion； to displace us from cosmic center stage has also been resisted； in part for similar reasons。 We seem to crave privilege； merited not by our work； but by our birth； by the mere fact that； say； we are humans and born on Earth。 We might call it the anthropocentric—the 〃human…centered〃—conceit。 This conceit is brought close to culmination in the notion that we are created in God's image： The Creator and Ruler of the entire Universe looks just like me。 My； what a coincidence How convenient and satisfying！ The sixth…century…B。C。 Green philosopher Xenophanes understood the arrogance of the perspective：
〃The Ethiopians make their gods black and snub…nosed； the Thracians say theirs have blue eyes and red hair 。 。 。 Yes； and if oxen and horses or lions had hands； and could paint with their hands； and produce works of art as men do； horses would paint the forms of the gods like horses； and oxen like oxen 。 。 。〃
Such attitudes were once described as 〃provincial〃—the naive expectation that the political hierarchies and social conventions of an obscure province extend to a vast empire posed of many different traditions and cultures； that the familiar boondocks； our boondocks； are the center of the world。 The country bumpkins know almost nothing about what else is possible。 They fail to grasp the insignificance of their province or the diversity of the Empire。 With ease； they apply their own standards and customs to the rest of the planet。 But plopped down in Vienna； say； or Hamburg； or New York； ruefully they recognize how limited their perspective has been。 They bee 〃deprovincialized。〃
Modern science has been a voyage into the unknown； with a lesson in humility waiting at every stop。 Many passengers would rather have stayed home。
CHAPTER 3 THE GREAT DEMOTIONS
＇One philosopher＇ asserted that he knew the whole secret 。 。 。 ＇H＇e surveyed the two celestial strangers from top to toe； and maintained to their faces that their persons； their worlds； their suns； and their stars； were created solely for the use of man。 At this assertion our two travelers let themselves fall against each other； seized with a fit of 。 。 。 inextinguishable laughter。
VOLTAIRE； MICROMEGAS。 A PHILOSOPHICAL HISTORY （1752）
In the seventeenth century there was still some hope that； even if the Earth was not the center of the Universe； it might be the only 〃world。〃 But Galileo's telescope revealed that 〃the Moon certainly does not possess a smooth and polished surface〃 and that other worlds might look 〃just like the face of the Earth itself。〃 The Moon and the planets showed unmistakably that they had as much claim to being worlds as the Earth does—with mountains； craters； atmospheres； polar ice caps； clouds； and； in the case of Saturn； a dazzling； unheard…of set of circumferential rings。 After millennia of philosophical debate； the issue was settled decisively in favor of 〃the plurality of worlds。〃 They might be profoundly different from our planet。 None of them might be as congenial for life。 But the Earth was hardly the only one。
This was the next in the series of Great Demotions； downlifting experiences； demonstrations of our apparent insignificance； wounds that science has； in its search for Galileo's facts； delivered to human pride。
WELL； SOME HOPED； even if the Earth isn't at the center of the Universe； the Sun is。 The Sun is our Sun。 So the Earth is approximately at the center of the Universe。 Perhaps some of our pride could in this way be salvaged。 But by the nineteenth century； observational astronomy had made it clear that the Sun is but one lonely star in a great self…gravitating assemblage of suns called the Milky Way Galaxy。 Far from being at the center of the Galaxy； our Sun with its entourage of dim and tiny planets lies in an undistinguished sector of an obscure spiral arm。 We are thirty thousand light years from the Center。
Well； our Milky Way is the only galaxy。 The Milky Way Galaxy is one of billions； perhaps hundreds of billions of galaxies notable neither in mass nor in brightness nor in how its stars are configured and arrayed。 Some modern deep sky photographs show more galaxies beyond the Milky Way than stars within the Milky Way。 Every one of them is an island universe containing perhaps a hundred billion suns。 Such an image is a profound sermon on humility。
Well； then； at least our Galaxy is at the center of the Universe。 No； this is wrong too。 When the expansion of the Universe was first discovered； many people naturally gravitated to the notion that the Milky Way was at the center of the expansion； and all the other galaxies running away from us。 We now recognize that astronomers on any galaxy would see all the others running away 。 from them； unless they were very careful； they would all conclude that they were at the center of the Universe。 There is； in fact； no center to the expansion； no point of origin of the Big Bang； at least not in ordinary three…dimensional space。
Well； even if there are hundreds of billions of galaxies； each with hundreds of billions of stars； no other star has planets。 If there are n other planets beyond our Solar System； perhaps there's no other life in the Universe。 Our uniqueness might then be saved。 Sing planets are small and feebly shine by reflected sunlight； they're hard to find。 Although applicable technology is improving wit breathtaking speed； even a giant world like Jupiter； orbiting the nearest star； Alpha Centauri； would still be difficult to detect。 Iii our ignorance； the geocentrists find hope。
There was once a scientific hypothesis—not just well received but prevailing—that supposed our solar system to have formed through the near collision of the ancient Sun with another star； the gravitational tidal interaction pulled out tendrils of sunstuff that quickly condensed into planets。 Since space is mainly empty and near stellar collisions most rare； it was concluded that few other planetary systems exist—perhaps only one； around that other star that long ago co…parented the worlds of our solar system。 Early in my studies； I was amazed and disappointed that such a view had ever been taken seriously； that for planets of other stars； absence of evidence had been considered evidence of absence。
Today we have firm