why is the earth round?

[jimmer]

I know its not really round coz of mountains etc and its a bit squashed but what's the science behind the shape of the world?

 

jim

[bernardo]

Gravity.

 

In other words, it all squashes down on itself.  If it started as a cube (for example), the corners would be such great big mountains that they would crumble and collapse in on their own weight.

 

The maximum height-difference between the highest and lowest points on Earth (Mount Everest and the Mariana Trench in the Pacific) is about 12 miles.  This is actually the maximum that is allowed by the gravity of the Earth.  The structure of rocks etc is only strong enough to support that weight.  If a mountain 20 miles high were to appear by magic, the bottom layer of rock would not be strong enough to stay intact and the whole thing would crumble and fall.

[Clanad]

Bernardo is essentially correct... however the highest "mountain" in the solar system is Olympus Mons, on Mars which is nearly 17 miles high.  It attained and maintains that heighth due to the lessened garvity on Mars.  If you weighed 70 pounds on earth one would weigh but about 27 pounds on Mars.  One minor point of correction... if a 20 mile high mountain were to suddenly appear on earth, it wouldn't crumble, but would actually flow from the substrata due to the remendous pressures of the weight...

[Clanad]

Did I say "garvity"?  Silly keyboard... it's gravity, of course...

[Gef]

The earth is "round" because it was once molten and a spinning mass of molten material will take up a spherical shape. It has nothing to do with gravity.

[magicdice]

gef? don't be silly, it's NOT nothing to do with gravity - the spinning object has inertia which acts as a centripetal force, whereas with a much larger body gravity would add to the force provided by inertia (it contributes to it for the smaller body as well but gravitational fields are so very weak that it's imperceptible)

also, on a note that is somehow related but i can't be bothered to think through why, a sphere is the most energy-efficient shape to form, which is why bubbles are spherical

[bernardo]

Gef - it's got everything to do with gravity.

[bernardo]

...and it only became a "spinning mass of molten material" because of gravity squashing it together in the first place.

[Gef]

Sorry bernado, it has nothing to do with gravity ALONE. It is a combination of gravitational forces pulling it together and the spinning which make it round,

[bernardo]

It's got nothing to do with spinning either.  If the Earth were not spinning, it would still mould itself into a spherical shape due to gravity.

[Gef]

So Bernado, can you please explain why Pluto, asteroids and all of the galaxies are not spherical?

[bernardo]

Pluto and the asteroids (and various moons around the planets) are not spherical because thay are much smaller than the Earth, and therefore not heavy enough to crumble the higher extremities as much.  The smaller the object is, the greater tolerance it allows between the highest and lowest points.  As Clanad said earlier, the highest mountain on Mars is much bigger than Mt Everest because the Martian gravity is weaker and therefore doesn't crush the rocks at the base of the mountain as much.

 

Galaxies are swirling round, and the individual elements in the galaxies mostly have stable orbits around the centre.  There is a big difference between a vast loosely-packed thing like a galaxy (which is mostly empty space) and a single object like the Earth or the Sun.