Death by Black Hole: And Other Cosmic Quandaries by Neil deGrasse Tyson Read Free Book Online Page B

massive planet in the solar system, lasts 10 Earth-hours; Jupiter is 7 percent wider at its equator than at its poles. Our much smaller Earth, with its 24-hour day, is just 0.3 percent wider at the equator—27 miles on a diameter of just under 8,000 miles. That’s hardly anything.
    One fascinating consequence of this mild oblateness is that if you stand at sea level anywhere on the equator, you’ll be farther from Earth’s center than you’d be nearly anywhere else on Earth. And if you really want to do things right, climb Mount Chimborazo in central Ecuador, close to the equator. Chimborazo’s summit is four miles above sea level, but more important, it sits 1.33 miles farther from Earth’s center than does the summit of Mount Everest.
     
     
    SATELLITES HAVE MANAGED to complicate matters further. In 1958 the small Earth orbiter Vanguard 1 sent back the news that the equatorial bulge south of the equator was slightly bulgier than the bulge north of the equator. Not only that, sea level at the South Pole turned out to be a tad closer to the center of Earth than sea level at the North Pole. In other words, the planet’s a pear.
    Next up is the disconcerting fact that Earth is not rigid. Its surface rises and falls daily as the oceans slosh in and out of the continental shelves, pulled by the Moon and, to a lesser extent, by the Sun. Tidal forces distort the waters of the world, making their surface oval. A well-known phenomenon. But tidal forces stretch the solid earth as well, and so the equatorial radius fluctuates daily and monthly, in tandem with the oceanic tides and the phases of the Moon.
    So Earth’s a pearlike, oblate-spheroidal hula hoop.
    Will the refinements never end? Perhaps not. Fast forward to 2002. A U.S.-German space mission named GRACE (Gravity Recovery and Climate Experiment) sent up a pair of satellites to map Earth’s geoid, which is the shape Earth would have if sea level were unaffected by ocean currents, tides, or weather—in other words, a hypothetical surface where the force of gravity is perpendicular to every mapped point. Thus, the geoid embodies the truly horizontal, fully accounting for all the variations in Earth shape and subsurface density of matter. Carpenters, land surveyors, and aqueduct engineers will have no choice but to obey.
     
     
    ORBITS ARE ANOTHER category of problematic shape. They’re not one-dimensional, nor merely two-or three-dimensional. Orbits are multidimensional, unfolding in both space and time. Aristotle advanced the idea that Earth, the Sun, and the stars were locked in place, attached to crystalline spheres. It was the spheres that rotated, and their orbits traced—what else?—perfect circles. To Aristotle and nearly all the ancients, Earth lay at the center of all this activity.
    Nicolaus Copernicus disagreed. In his 1543 magnum opus, De Revolutionibus, he placed the Sun in the middle of the cosmos. Copernicus nonetheless maintained perfect circular orbits, unaware of their mismatch with reality. Half a century later, Johannes Kepler put matters right with his three laws of planetary motion—the first predictive equations in the history of science—one of which showed that the orbits are not circles but ovals of varying elongation.
    We have only just begun.
    Consider the Earth-Moon system. The two bodies orbit their common center of mass, their barycenter, which lies roughly 1,000 miles below the spot on Earth’s surface closest to the Moon at any given moment. So instead of the planets themselves, it’s actually their planet-moon barycenters that trace the Keplerian elliptical orbits around the Sun. So now what’s Earth’s trajectory? A series of loop-the-loops—thirteen of them in a year, one for each cycle of lunar phases—rolled together with an ellipse.
    Meanwhile, not only do the Moon and Earth tug on each other, but all the other planets (and their moons) tug on them too. Everybody’s tugging on everybody else. As you might suspect,