• Question: Why does the earth revolve around the sun and not the sun revolve around the earth?

    Asked by afif to Austin, Kirsty, Nike on 21 Jun 2012.
    • Photo: Nike Dattani

      Nike Dattani answered on 21 Jun 2012:


      The sun is bigger, and gravity makes smaller things orbit bigger things.

      We have sent spaceships into space and if you were in one of them and far enough away, and watched for long enough, you’d be able to see that Mercury, Venus, Earth, Mars, Jupiter, etc… all orbit the sun !

    • Photo: Kirsty Ross

      Kirsty Ross answered on 22 Jun 2012:


      Hi Afif

      I have an answer but it may require some imagination from you!

      Imagine Superman is standing on Mt. Everest holding a football. He throws it as hard as he can, which is incredibly hard because he’s Superman. Just like if you threw a football, eventually it will fall back down and hit the ground. But because he threw it so hard, it goes past the horizon before it can fall. And because the Earth is curved, it just keeps on going, constantly “falling,” but not hitting the ground because the ground curves away before it can. Eventually the football will come around and smack Superman in the back of the head, which of course won’t hurt him at all because he’s Superman. That is how orbits work, but objects like spaceships and moons are much farther from the Earth than the football that Superman threw. This same situation can be applied to the Earth orbiting the Sun – except now Superman is standing on the Sun (which he can do because he’s Superman) and he throws the Earth.

      The basic reason why the planets revolve around, or orbit the sun (rotate actually is used to describe their spin, for example, the Earth completes one rotation about its axis every 24 hours, but it completes one revolution around the Sun every 365 days), is that the gravity of the Sun keeps them in their orbits. Just as the Moon orbits the Earth because of the pull of Earth’s gravity, the Earth orbits the Sun because of the pull of the Sun’s gravity.

      Why, then, does it travel in an elliptical orbit around the Sun, rather than just getting pulled in all the way? This happens because the Earth has a velocity in the direction perpendicular to the force of the Sun’s pull. If the Sun weren’t there, the Earth would travel in a straight line. But the gravity of the Sun alters its course, causing it to travel around the Sun, in a shape very near to a circle. This is where the Superman analogy comes in!

Comments