Our lovely and fragile home. (ESA Â©2009 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA)

It’s easy to take Earth for granted, since we see it every day. It becomes — it is — part of life’s background.

But when you see the world through the eyes of science, nothing is mundane. We live on the surface of this great giant space-borne water-laden spinning rock, separated from the rest of the Universe beneath a thin veil of nitrogen and oxygen molecules. Even though you’re immersed in its influence, what do you really know about the Earth?

Here are some facts about our planet for you to ponder.

1) There are a lot of different ways to measure how long it takes the Earth to go around the Sun, but if you say it takes pi x 10 million seconds, you’ll only be off by a half a percent.

2) The Earth has a volume of about one trillion cubic kilometers. Can you picture a cube 1000 meters high, 1000 meters deep, 1000 meters across? Now picture a trillion of them. That’s the Earth.


3) The Earth has a mass of 6,000,000,000,000,000,000,000,000 kilograms, or, if you prefer, 6 sextillion tons. In pounds, that’s actually . . . 0. Nothing. Mass is a measure of how much stuff an object contains, but weight is how hard gravity pulls on that mass. The Earth is in space, orbiting the Sun, so it’s in freefall. It has mass, but no weight at all.

4) The Earth isn’t a perfect sphere. It spins, so it’s a flattened at the poles a little bit. The diameter through the poles is 12,713.6 kilometers (7882.4 miles), but it’s 12,756.2 kilometers (7908.8 miles) through the equator. That difference of 43 kilometers is only about 0.3 percent, though, so really we’re pretty close to a perfect sphere.


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 KÃ¡rmÃ¡n 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.


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.



15) The Earth’s atmosphere weighs 5 quintillion kilograms, or 5000 trillion tons! You can do this math yourself: Weight is equal to pressure times area. Atmospheric pressure on the Earth’s surface is about 1 kilogram per square centimeter. Multiply that by the number of square centimeters on the Earth’s surface and you get the weight of all that air. Hint: The area of a sphere is 4 x pie x radius squared. [Note: Yes, I know kilograms are a mass and not a weight.]

And a bonus, because it’s important:

16) The Earth is the only place in the entire Universe where we know that life exists. But that won’t be true forever.


Is any like Earth? Almost certainly, and in fact there may be billions of planets like ours orbiting alien stars. But while they are like ours, they aren’t ours. As with any individual, our world is unique, and precious, and wonderful. Let’s keep it that way.

Phil Plait, the creator of Bad Astronomy, is author of “Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing ‘Hoax’ ” and “Death from the Skies! These Are the Ways the Universe Will End.”

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