earth is a pearlike, oblate-spheroidal hula hoop -- 8/14/15

Today's selection -- from Death By Black Hole by Neil DeGrasse Tyson. Since 2,600 BCE civilizations have predicted and measured the shape of the Earth. Observers as early as Aristotle himself determined that the Earth is round, but the story is decidedly more complex than that. And the Earth's equatorial bulge is so pronounced that Ecuador's Mount Chimborazo sits 1.33 miles farther from Earth's center than does the summit of Mount Everest. So what shape is the Earth, really?:

"One of the earliest representations of the world, preserved on a 2,600-year-old Babylonian clay tablet, depicts it as a disk encircled by oceans. Fact is, when you stand in the middle of a broad plain (the valley of the Tigris and Euphrates rivers, for instance) and check out the view in every direction, Earth does look like a flat disk.

Babylonian map of the world, ca 500 BC.

"Noticing a few problems with the concept of a flat Earth, the ancient Greeks --including such thinkers as Pythagoras and Herodotus -- pondered the possibility that Earth might be a sphere. In the fourth century B.C., Aristotle, the great systematizer of knowledge, summarized several arguments in support of that view. One of them was based on lunar eclipses. Every now and then, the Moon, as it orbits Earth, intercepts the cone-shaped shadow that Earth casts in space. Across decades of these spectacles, Aristotle noted, Earth's shadow on the Moon was always circular. For that to be true, Earth had to be a sphere, because only spheres cast circular shadows via all light sources, from all angles, and at all times. If Earth were a flat disk, the shadow would sometimes he oval. And some other times, when Earth's edge faced the Sun, the shadow would be a thin line. Only when Earth was face-on to the Sun would its shadow cast a circle.

"Given the strength of that one argument, you might think cartographers would have made a spherical model of Earth within the next few centuries. But no. The earliest known terrestrial globe would wait until 1490-92, on the eve of the European ocean voyages of discovery and colonization.

"So, yes, Earth is a sphere. But the devil, as always, lurks in the details. In Newton's 1687 Principia, he proposed that, because spinning spherical objects thrust their substance outward as they rotate, our planet (and the others as well) will be a bit flattened at the poles and a bit bulgy at the equator -- a shape known as an oblate spheroid. To test Newton's hypothesis, half a century later, the French Academy of Sciences in Paris sent mathematicians on two expeditions -- one to the Arctic Circle and one to the equator -- both assigned to measure the length of one degree of latitude on Earth's surface along the same line of longitude. The degree was slightly longer at the Arctic Circle, which could only be true if Earth were a bit flattened. Newton was right.

"The faster a planet spins, the greater we expect its equatorial bulge to be. A single day on fast-spinning Jupiter, the most 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 tidal and the phases of the Moon.

"So Earth's a pearlike, oblate-spheroidal hula hoop."


Neil deGrasse Tyson


Death by Black Hole: And Other Cosmic Quandaries


W. W. Norton & Company


Copyright 2007 by Neil deGrasse Tyson


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