Pharos-Tribune

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April 22, 2013

Slate: 15 facts about our planet

(Continued)

6. There is no physical place where Earth's atmosphere stops and space begins; the air just gets thinner and thinner and eventually fades away. But we love definitions, so the official height above the Earth's surface considered to be where space begins — called the Kármán line — is at an altitude of 100 km. Anyone who gets higher than that is considered an astronaut.

7. The Moon's radius is about 1/4 that of the Earth's, making it the largest satellite compared to its parent planet. Charon, Pluto's biggest moon, is about half the diameter of Pluto itself. So if you don't consider Pluto a planet, the Earth and Moon win.

8. The Moon is farther away from Earth than you think. As an analogy, if the Earth were a basketball, the Moon would be the size of a tennis ball 7.4 meters (24 feet) away.

9. The Earth's atmosphere is only transparent to a narrow slice of the electromagnetic spectrum. What we call visible light (mostly!) gets through, but most flavors of infrared, ultraviolet, X-rays and gamma-rays are stopped cold. Those last few are dangerous to life as we know it, so that works out well. But it's not a coincidence: if our air didn't protect us, we'd have evolved differently.

10. The Earth is warming up. It's a fact. Deal with it.

11. Fewer than 200 impact craters have been cataloged on Earth. The Moon has billions. We'd have just as many, but our air and water erode them over time, erasing them. Old craters on the Earth are hundreds of millions of years old; on the Moon those would be considered young.

12. An asteroid, 2010 TK7, shares an orbit with the Earth. It's about 300 meters (1000 feet) across, and never gets close enough to us to be a danger.

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