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Asked by mitchellharrington to Dilwar, Lou, Rachel, Simon, Susan on 13 Nov 2013.
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Simon Langley-Evans answered on 13 Nov 2013:
Hello Mitchell
We have had quite a lot of questions where the answer is the same as this one. It is all about gravity.
Like all of the planets the Earth is a very heavy object that has a lot of mass. That mass has gravity which attracts other objects by pulling on them. All objects with mass will do this- even small things like your computer, your cup, or even you will do this. For small things this force of gravity is tiny, but for a big object like the Earth all of the mass of the planet pulls on the things that make up the planet. When this mass pulls together it forms the most efficient shape which is a sphere. Spheres take less energy to form and maintain, which is why so many other things are spherical, like a raindrop or a bubble, or a balloon.
There is a catch to this though. If you are ever lucky enough to look at Jupiter through a telescope you will notice that it isn’t really a sphere. It looks as though somebody has sat on it, so it bulges in the middle. This is because Jupiter has such super-massive gravity and is mostly made of gas. The gases at the south and north poles are pulled down by the central core, more than they are along the equator. Earth is also slightly squashed like this (it is called being ‘oblate’) but because it is smaller, this oblateness is not noticeable.
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Susan Skelton answered on 14 Nov 2013:
Hi Mitchell!
Yes, it’s all down to gravity. Gravity is very important because it is the glue that holds together entire galaxies and planets, including the Earth!!Anything that has mass has some gravity, so every object in the universe (no matter how big or small) exerts a gravitational force on everything else. Even you and I are exerting a small gravitational force on each other! Basically, everything on Earth – the atoms, the soil, the grass, the cars, the mountains – all exert a gravitational force on everything else. This force pulls everything together and makes it stick!
For small objects, like your computer, your car, and even a building, the force of gravity is tiny. But when you have millions, and even trillions of tonnes of mass, the effect of the gravity really builds up. All of the mass pulls on all the other mass, and it tries to create the most efficient shape… a sphere.
For smaller objects, like asteroids, the force of gravity trying to pull the object into a sphere isn’t enough to overcome the strength of the rock keeping it in shape. But once you get above a certain mass and size, the strength of the object can’t stop the force of gravity from pulling it into a sphere. Objects larger than about 1,000 km in size are able to pull themselves into a sphere, and this is one of the requirements for an object to be called a planet.
Of course, the Earth isn’t perfectly round. Because it’s turning on its axis approximately once every 24 hours, the Earth’s equator bulges outwards. And the surface of the Earth is rough – there are valleys and mountains and large waves on the oceans. The Earth has enough mass to create a rough sphere, but not enough to make it completely round and smooth.
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