Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: John Chapman on 22/02/2009 17:50:02

Title: Why is The Earth not shaped like an apple with a bite taken out of it?
Post by: John Chapman on 22/02/2009 17:50:02
According to my second most favourite non-porn site, wikipedia, our Moon was formed as the result of a large impact between between the Earth and another planet/asteroid/chunky thing. If that is so why has the planet Earth not got a moon-sized chunk knocked out of it? Come to that, why isn't the moon a corresponding irregular shape?

In other words why are the Earth and Moon both round?
Title: Re: Why is The Earth not shaped like an apple with a bite taken out of it?
Post by: RD on 22/02/2009 17:56:26
'cos the Earth was (and still is) mostly liquid, (outer core and mantle)...


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http://en.wikipedia.org/wiki/Structure_of_the_Earth
Title: Re: Why is The Earth not shaped like an apple with a bite taken out of it?
Post by: LeeE on 22/02/2009 17:57:30
Once a body becomes large enough gravity pulls the body in to a spherical shape.  It's also probable that both the Earth and the planetesimal that collided with it, resulting in the Moon, were largely still molten when the collision occurred.  In any case, the collision would have produced a lot of heat and probably melted anything that had already solidified.

Even today, the Earth is still mostly molten, with just a relatively thin crust of solid material on it's surface (Much of the magma isn't really fluid, but it's not solid either - perhaps one of our geologists will step in here for a fuller explanation).
Title: Re: Why is The Earth not shaped like an apple with a bite taken out of it?
Post by: John Chapman on 22/02/2009 17:59:44
Thanks Lee

I was just about to ask RD what he meant when he said the Earth is still liquid but I think you've just covered that point.
Title: Re: Why is The Earth not shaped like an apple with a bite taken out of it?
Post by: RD on 22/02/2009 18:06:13
If you want a fruity comparison, the Earth is a bit like a tangerine: it is an oblate spheroid (http://en.wikipedia.org/wiki/Oblate).
Rotation makes it bulge a little at the equator  (http://en.wikipedia.org/wiki/Equatorial_bulge) and a little flat at the poles.

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