15 facts about planet Earth

It's easy to take earth for granted, since we see it every day. It becomes - it is - part of life's background.


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 per cent.

2) The earth has a volume of about one trillion cubic kilometres. Can you picture a cube 1000 metres high, 1000 metres deep, 1000 metres 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 tonnes. 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 kilometres, but it's 12,756.2 kilometres through the equator. That difference of 43 kilometres is only about 0.3 per cent, though, so really we're pretty close to a perfect sphere.

5) Not only is it flattened, but the gravitational forces of the sun and moon (what we call tides) distort its shape even more, pulling bulges out from it. The earth is lumpy! Out in the deep ocean, the bulge of water due to the sun and moon can have an amplitude (change in height from minimum to maximum) of about a metre. The solid earth deforms due to the tides, too, with an amplitude of roughly 50 centimetres. Even the air is affected by tides; though there are several factors that greatly complicate it (like expansion due to heating from the sun during the day, and, simply, weather).

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 Krmn 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 metres 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 flavours 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 catalogued 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 metres across, and never gets close enough to us to be a danger.

13) The earth orbits the sun on an ellipse. The shape changes slightly over time due to the influence of the other planets, but on average the closest we get to the sun (perihelion) is about 147.1 million kilometres and the farthest (aphelion) about 152.1 million kilometres. That difference is only about 3 per cent, which by eye is very nearly a perfect circle.

14) If you took all the water on earth and collected it into a single drop, it would be just less than 1400 kilometres across.

15) The earth's atmosphere weighs 5 quintillion kilograms, or 5000 trillion tonnes! You can do this calculation yourself: Weight is equal to pressure times area. Atmospheric pressure on the earth's surface is about 1 kilogram per square centimetre. Multiply that by the number of square centimetres 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.




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.