• Question: why is the earth round?

    Asked by will k to Heather, Helen, Hugh, Jane, Julian on 11 Nov 2014. This question was also asked by amazing person called flo.
    • Photo: Jane MacArthur

      Jane MacArthur answered on 11 Nov 2014:

      Gravity pulls everything towards the centre, so naturally favours the formation of round spheres, providing there is sufficient mass. It ‘bulges’ at the equator due to forces from it rotating on its axis once every 24 hours. However, plate tectonics, mountain ranges etc means it is not a ‘perfect’ sphere. Some asteroids have insufficient mass for gravity to make them round and we see more irregular shapes.

    • Photo: Julian Onions

      Julian Onions answered on 11 Nov 2014:

      The Earth is round because it is so big. It makes a large amount of gravity all that mass, so all bits of the Earth are pulling on each other. The smallest shape you can have that takes the least volume is a sphere, so thats what all big planets form into (and stars), and in fact its one of the definitions of a planet, that it is big enough for the gravity to over come the strength of the rocks to pull it into a sphere.

      Of course there are mountains that poke above the surface, but there is only a certain height they can rise to before gravity wins, and in the end they tend to get eroded away, so without plates moving beneath the earth to form new ones we’d eventually get an almost perfect sphere!

    • Photo: Hugh Osborn

      Hugh Osborn answered on 11 Nov 2014:

      Great question! In fact, all of the large bodies in the solar system are round. However, lots of smaller things like asteroids or comets look like potatoes. Why would this be? Well, large planets like the Earth have stronger gravity. Anything slightly further from the centre of the Earth (imagine a really tall mountain) is being pulled downwards towards the lower regions. This acts to smooth everything out into a nearly perfect sphere. Those lumpy asteroids just don’t have enough gravity to pull everything together into a ball, so end up lumy. Hope that helps!

    • Photo: Helen Johnson

      Helen Johnson answered on 11 Nov 2014:

      The Earth is round because of gravity. Everything is being pulled towards the centre of the Earth, and the most efficient shape for it all to be pulled into is a sphere. For smaller objects like asteroids, their gravity is weaker than the forces holding the rock together, so they tend to be weird and wonderful shapes (like comet 67P – duck shaped!).
      In 2006 it was decided that if something wasn’t big enough to pull itself into a sphere, then it couldn’t be a planet. The other deciding factor was if it had cleared its orbit of smaller objects / space debris. Sadly this is when Pluto was demoted 🙁

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