Death by Black Hole: And Other Cosmic Quandaries Read Online Free PDF Page B

it dilutes from the growing spherical shell of space through which it moves. The surface area of this sphere increases in proportion to the square of its radius (you may remember the formula: Area = 4π r 2 ), forcing the light’s intensity to diminish by the same proportion.
     
     
    ALL RIGHT : the stars don’t all lie the same distance from us; they aren’t all equally luminous; the ones we see are highly unrepresentative. But surely they are stationary in space. For millennia, people understandably thought of stars as “fixed,” a concept evident in such influential sources as the Bible (“And God set them in the firmament of the heaven,” Genesis 1:17) and Claudius Ptolemy’s Almagest , published circa A.D . 150, wherein he argues strongly and persuasively for no motion.
    To sum up, if you allow the heavenly bodies to move individually, then their distances, measured from Earth upward, must vary. This will force the sizes, brightnesses, and relative separations among the stars to vary too from year to year. But no such variation is apparent. Why? You just didn’t wait long enough. Edmond Halley (of comet fame) was the first to figure out that stars moved. In 1718 he compared “modern” star positions with the ones mapped by the second-century B.C . Greek astronomer Hipparchus. Halley trusted the accuracy of Hipparchus’s maps, but he also benefited from a baseline of more than eighteen centuries from which to compare the ancient and modern star positions. He promptly noticed that the star Arcturus was not where it once was. The star had indeed moved, but not enough within a single human lifetime to be noticed without the aid of a telescope.
    Among all objects in the sky, seven made no pretense of being fixed; they appeared to wander against the starry sky and so were called planetes , or “wanderers,” by the Greeks. You know all seven (our names for the days of the week can be traced to them): Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon. Since ancient times, these wanderers were correctly thought to be closer to Earth than were the stars, but each revolving around Earth in the center of it all.
    Aristarchus of Samos first proposed a Sun-centered universe in the third century B.C . But back then it was obvious to anybody who paid attention that irrespective of the planets’ complicated motions, they and all the background stars revolved around Earth. If Earth moved we would surely feel it. Common arguments of the day included:
     
If Earth rotated on an axis or moved through space, wouldn’t clouds in the sky and birds in flight get left far behind? (They aren’t.)
If you jumped vertically, wouldn’t you land in a very different spot as Earth traveled swiftly beneath your feet? (You don’t.)
And if Earth moved around the Sun, wouldn’t the angle at which we view the stars change continuously, creating a visible shift in the stars’ positions on the sky? (It doesn’t. At least not visibly.)
     
    The naysayers’ evidence was compelling. For the first two cases, the work of Galileo Galilei would later demonstrate that while you are airborne, you, the atmosphere, and everything else around you get carried forward with the rotating, orbiting Earth. For the same reason, if you stand in the aisle of a cruising airplane and jump, you do not catapult backward past the rear seats and get pinned against the lavatory doors. In the third case, there’s nothing wrong with the reasoning—except that the stars are so far away you need a powerful telescope to see the seasonal shifts. That effect would not be measured until 1838, by the German astronomer Friedrich Wilhelm Bessel.
    The geocentric universe became a pillar of Ptolemy’s Almagest , and the idea preoccupied scientific, cultural, and religious consciousness until the 1543 publication of De Revolutionibus , when Nicolaus Copernicus placed the Sun instead of Earth at the center of the known universe. Fearful that this heretical work would