30May2017

15 Amazing Facts About Our Earth

To us, our earth seems huge, solid, tailor-made for us, and permanent. Photo: NASA

It’s easy to take our 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 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. Actually, if you were that big, it would be easy.

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.

To us, our earth seems huge, solid, tailor-made for us, and permanent. But that is just one perspective, born of living on its surface. From a different perspective, none of those things is true. Seen from space, it looks much less unbreakable. Seen from deep space it shrinks to nothing more than a dot, barely visible in the reflected light of the sun.

From another star, even seeing our planet at all would be a colossal task. We are, after a monumental effort spanning decades, only just now finding other planets orbiting other stars.

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. Source: Sydney Morning Herald

Why Doesn’t the Moon Spin?

I’ve been asked on a number of occasions recently why the moon always seems to show us the same face — the lunar nearside. I’m not sure why there’s the sudden interest, but it’s a very good and valid question, especially as tonight (March 27) is a full moon.

Look at the moon at any time and — aside from the constantly changing phases that are caused by changing relative positions of the Earth, the moon and sun — it does indeed show us the same face, constantly. Perhaps surprisingly, it’s ‘non-rotation’ (from our perspective) comes from its interaction with the Earth.

Both the Earth and moon are big lumps of rock with the moon in orbit around the Earth or, more precisely, both objects in orbit around their common center of gravity — known as the ‘barycenter.’ It just happens that this point lies very close to the center of our planet, so to all intents the moon orbits us.

The moon’s orbit is elliptical, taking it furthest away from Earth at a point called “apogee” (406,720 kilometers) and a closest point called “perigee” (356,375 kilometers) and as it orbits both objects gravitationally tug a little at each other. This tugging creates a bulge on the moon, and to a lesser extent on the Earth, which we call tides.

Common sense suggests that these two tidal bulges would be perfectly aligned with each other, but in reality, the rotation of the two objects tend to drag the bulge along a little. The end result is the extra pull from the slightly off-set bulge acts like a bicycle brake on the moon slowing its rotation to such a degree that it now takes the same time for it to orbit the Earth once as it does to rotate on its axis once.

Therefore, the moon does rotate, just once per orbit. Its a phenomenon known as ‘synchronous rotation’ and is commonly seen on other ‘tidally-locked’ moon systems in our solar system.

So, it’s the tidal forces exerted on both bodies that creates the bulges resulting in the same hemisphere of the moon facing Earth. Another effect from the tides is that the moon is slowly accelerating in its orbit, causing it to move very slightly further away from us at a rate of around 4 centimeters per year. Source: Zen Haven

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