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The Earth; WHY is it round?
As if it wasn't enough just to know that the earth is round, it's an interesting question to ask, WHY is it round?
The roundness issue started a long time ago when folk originally assumed the Earth must be flat, partly for religious reasons, but also because of the commonsense reasoning that things always fall "downwards" which appears to be in the same direction everywhere, then it had to be flat, or else we'd fall off.
Ancient Greek clever person Eratosthenes thousands of years ago did a brilliant experiment where he measured the angle of the sun in different geographical locations and proved that the world must be spherical, and he calculated it must be about 25,000 miles around. Not bad, being only about 1% out, and at a time when the ancient Greek globe of the world would be like a blue football with a postage stamp sized map of Ancient Greece stuck to it.
Later in history voyages by intrepid explorers travelling a complete circumnavigation of the earth generally clinched the issue except for the more fundamentalist dogma believers, and then when astronauts took photos of a round blue marble suspended in space, roundness generally met with global acceptance.
By WHY the earth is round, is a more roundabout kind of an explanation, as it's one of those "why" questions which are more tricky in science, in contrast to experiments which prove whether something IS or IS NOT the case.
So let's start with what goes on in space. In space, things just float about. There's a popularly held belief that there's no gravity in space, but this is not so. OK, there's no gravity in any particular direction, and there's no "down" or "up". But there is gravity to the extent to which objects floating around in space have an attraction to each other. It's a bit like magnets which attract each other.
The earth was formed from chaos, and in that chaos, dust and stuff just floated around, and as the little bits attracted each other they formed into clumps, like those balls of fluff you find under the bed if you're not into the hobby of vacuuming.
Clumps attracted more clumps, and the great mountainous conglomerations of grit and dust were formed. These don't just form anyoldhow but have a natural gravitational state they fall into. If you pile up sand or slag into a big heap, it just won't balance as a towering pinnacle but will fall naturally into a heap-shaped pile with a natural slope to it. This is where the roundness starts to become significant, because the bigger an object is in space, the more keen all the particles of it are to get as close to the centre as possible. At the larger scales the stuff of the earth can be considered as a fluid.
You can see this by putting various sticky toffees into a cup on a hot day and after a while it all forms into its gravitated state which in the case of gummy sticky sweets happens to be the shape of the bottom of the cup.
But in space, as there's no up or down, the most attractive part of a big lump of stuff forming in space is... The Centre! Wherever the centre is, stuff will have a tendency towards that central point, and in the end, after all particles have settled their differences, roundness will be the outcome. It's the most stable state for a collection of stuff. Like globules of liquid metal mercury slide around on a flat surface and have rounded edges, the effect in space with large amounts of stuff, is to carry that to extremes and so to form spheres.
This might explain why the earth is near enough round, but it beggars belief to think it's smoother than a snooker ball. But the way to think about this is to try to imagine what it would be like if Mount Everest were twice as high. Would it stay up? The sheer weight of it would probably make it sooner or later crumble and end up with it being less high. So, in the end, the earth would end up being about as round as it currently is.
Incidentally it's not a perfect sphere, but it is remarkably smooth. The actual shape of the earth is a bit bulgy round the equator. Again, thinking about it being like a fluid, as it's rotating the middle is like the outside of a roundabout and had a tendency to swing out a bit. Not much though. 28 miles difference pole to pole is only about one part in 300.
Other things that are round because of the natural forces include falling raindrops, and cannonballs made by dropping molten lead down a tower. (In these things, it's surface tension rather than gravity which does the trick)
Also see Earth,Moon,Sun - which goes around which?
Other strange truths and misconceptions at this site can be seen and explored.