{"id":535,"date":"2008-09-09T09:39:07","date_gmt":"2008-09-09T16:39:07","guid":{"rendered":"http:\/\/www.bspcn.com\/2008\/09\/09\/10-things-you-don%e2%80%99t-know-about-the-earth\/"},"modified":"2008-09-09T09:39:07","modified_gmt":"2008-09-09T16:39:07","slug":"10-things-you-don%e2%80%99t-know-about-the-earth","status":"publish","type":"post","link":"http:\/\/localhost\/wordpress\/2008\/09\/09\/10-things-you-don%e2%80%99t-know-about-the-earth\/","title":{"rendered":"10 things you don\u2019t know about the Earth"},"content":{"rendered":"\n
Written by Phil Plait<\/a><\/p>\n Look up, look down, look out, look around.<\/em><\/p>\n – Yes, “It Can Happen”<\/p>\n<\/blockquote>\n Good advice from the 60s acid band. Look around you. Unless you’re one of the Apollo astronauts, you’ve lived your entire life within a few hundred kilometers of the surface of the Earth. There’s a whole planet beneath your feet, 6.6 sextillion tons of it, one trillion<\/em> cubic kilometers of it. But how well do you know it?<\/p>\n Below are ten facts about the Earth – the second in my series of Ten Things You Don’t Know (the first was on the Milky Way<\/a>). Some things I already knew (and probably you do, too), some I had ideas about and had to do some research to check, and others I totally made up. Wait! No! Kidding. They’re all real. But how many of them do you<\/em> know? Be honest.<\/p>\n 1) The Earth is smoother than a billiard ball.<\/strong><\/p>\n Maybe you’ve heard this statement: if the Earth were shrunk down to the size of a billiard ball, it would actually be smoother than one. When I was in third grade, my teacher said basketball, but it’s the same concept. But is it true? Let’s see. Strap in, there’s a wee bit of math (like, a really wee bit).<\/p>\n The Earth has a diameter of about 12,735 kilometers (on average, see below for more on this). Using the smoothness ratio from above, the Earth would be an acceptable pool ball if it had no bumps (mountains) or pits (trenches) more than 12,735 km x 0.00222 = about 28 km in size.<\/p>\n The highest point on Earth is the top of Mt. Everest, at 8.85 km. The deepest point on Earth is the Marianas Trench, at about 11 km deep.<\/p>\n Hey, those are within the tolerances! So for once, an urban legend is correct. If you shrank the Earth down to the size of a billiard ball, it would<\/em> be smoother.<\/p>\n But would it be round<\/em> enough to qualify?<\/p>\n 2) The Earth is an oblate spheroid<\/strong><\/p>\n The Earth is round<\/a>! Despite common knowledge, people knew that the Earth was spherical thousands of years ago. Eratosthenes<\/a> even calculated the circumference to very good accuracy!<\/p>\n But it’s not a perfect sphere. It spins, and because it spins, it bulges due to centrifugal force (yes, dagnappit, I said centrifugal<\/a>). That is an outwards-directed force, the same thing that makes you lean to the right when turning left in a car. Since the Earth spins, there is a force outward that is a maximum at the Earth’s equator, making our Blue Marble bulge out, like a basketball with a guy sitting on it. This type of shape is called an oblate spheroid<\/em>.<\/p>\n If you measure between the north and south poles, the Earth’s diameter is 12,713.6 km. If you measure across the Equator it’s 12,756.2 km, a difference of about 42.6 kilometers. Uh-oh! That’s more than our tolerance for a billiard ball. So the Earth is smooth<\/em> enough, but not round<\/em> enough, to qualify as a billiard ball.<\/p>\n Bummer. Of course, that’s assuming the tolerance for being out-of-round for a billiard ball is the same as it is for pits and bumps. The WPA site doesn’t say. I guess some things remain a mystery.<\/p>\n 3) The Earth isn’t<\/em> an oblate spheroid.<\/strong><\/p>\n But we’re not done. The Earth is more complicated than an oblate spheroid. The Moon is out there too, and the Sun. They have gravity, and pull on us. The details are complicated (sate yourself here<\/a>), but gravity (in the form of tides) raises bulges in the Earth’s surface as well. The tides from the Moon have an amplitude (height) of roughly a meter in the water, and maybe 30 cm in the solid Earth. The Sun is more massive than the Moon, but much farther away, and so its tides are only about half as high.<\/p>\n This is much smaller than the distortion due to the Earth’s spin, but it’s still there.<\/p>\n Other forces are at work as well, including pressure caused by the weight of the continents, upheaval due to tectonic forces, and so on. The Earth is actually a bit of a lumpy mess, but if you were to say it’s a sphere, you’d be pretty close. If you held the billiard-ball-sized Earth in your hand, I doubt you’d notice it isn’t a perfect sphere.<\/p>\n A professional pool player sure would though. I won’t tell Allison Fisher if you won’t.<\/p>\n 4) OK, one more surfacey thing: the Earth is not exactly aligned with its geoid<\/strong><\/p>\n If the Earth were infinitely elastic, then it would respond freely to all these different forces, and take on a weird, distorted shape called a geoid<\/em>. For example, if the Earth’s surface were completely deluged with water (give it a few decades) then the surface shape would be a geoid. But the continents are not infinitely ductile, so the Earth’s surface is only approximately a geoid. It’s pretty close, though.<\/p>\n Precise measurements of the Earth’s surface are calibrated against this geoid, but the geoid itself is hard to measure. The best we can do right now is to model it using complicated mathematical functions. That’s why ESA is launching a satellite called GOCE<\/a> (Gravity field and steady-state Ocean Circulation Explorer) in the next few months, to directly determine the geoid’s shape.<\/p>\n Who knew just getting the shape of the Earth would be such a pain?<\/p>\n 5) Jumping into hole through the Earth is like orbiting it.<\/strong><\/p>\n I grew up thinking that if you dug a hole through the Earth (for those in the US) you’d wind up in China. Turns out that’s not true; in fact note that the US and China are both entirely in the northern hemisphere which makes it impossible, so as a kid I guess I was pretty stupid.<\/p>\n You can prove it to yourself with this cool but otherwise worthless mapping tool<\/a>.<\/p>\n But what if you did dig a hole through the Earth and jump in? What would happen?<\/p>\n\n
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<\/p>\n<\/a>OK, first, how smooth is a billiard ball? According to the World Pool-Billiard Association<\/a>, a pool ball is 2.25 inches in diameter, and has a tolerance of +\/- 0.005 inches. In other words, it must have no pits or bumps more than 0.005 inches in height. That’s pretty smooth. The ratio of the size of an allowable bump to the size of the ball is 0.005\/2.25 = about 0.002.<\/p>\n
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