• Question: Why does the earth orbit the sun? Why doesn't the gravitational force cause all the planets to just crash into the sun?

    Asked by Zobo64 to Colin, John, Kevin, Shikha, Triona on 13 Nov 2014. This question was also asked by xX_FaZe_FiNnErS_Xx.
    • Photo: Colin Johnston

      Colin Johnston answered on 13 Nov 2014:

      You’re right that the Sun puts a huge gravitational force on the Earth (and every other planet too). Earth doesn’t fall into the Sun because the Earth has a velocity in the direction at 90 degrees to the force of the Sun’s gravitational pull. If the Sun’s gravitational pull wasn’t there, the Earth would travel on in a straight line. But the gravity of the Sun alters its course, causing it to travel around the Sun in an almost circular orbit.

      The planets (plus asteroids, comets, dwarf planets etc) are lucky. They’re in nice stable orbits. The Sun is made of stuff that wasn’t so lucky and fell into it as the Solar System formed 4.5 billion years ago.

    • Photo: Kevin Motherway

      Kevin Motherway answered on 13 Nov 2014:

      The Earth is not rotating around the sun: but the Earth and Sun are rotating around a common point between the two which is really really close to the Sun. The same for all the other planets, but it gets even more complicated in that all the planets exert gravitational forces on each other too. So the Sun is pulling the earth towards it and Jupiter is pulling the earth in the other direction away from the Sun. This orbital system has taken billions of years to stabilise and settle down and is still evolving over time really really slowly. So there’s checks and balances that keep things stable. The planets are being pulled towards the sun, but the fact they’re moving in a circle means there’s centrifugal force flinging them away from the sun; as well as the force of all the planets acting on each other at the same time.

      Just like the earth and the moon; The moon is slowly slowly drifting further away from the earth (about 3.8 cm a years). It was way closer millions of years ago and tides on earth were huge compared to today, but over time everything slows down as the earth spins ever slower and to preserve angular momentum the moon moves further away into a bigger orbit. Next time you’re on a merry go round (don’t give me that look; I’m 41 and I still do it) if you stand on the outside it’ll spin slow, if you step into the centre it speeds way up, step out to the edge again and it slows down. this is “conservation of angular momentum”, the tighter the circle the higher the speed, the smaller the orbit the faster it goes!

    • Photo: Shikha Sharma

      Shikha Sharma answered on 14 Nov 2014:

      Hi Zobo64 and xX_FaZe_FiNnErS_Xx.,
      Well, this is very interesting and fundamental question of physics. Even after discovering gravitational force, Issac Newton was worried about the fact!!!!! This is the 1st question came to his mind as well. If all planets, stars and other matters are pulling each other with gravitational force then why they are not falling on each other!!!! Science has found the explanation for this mystery long back. If Earth would have standing at one place, it would have definitely fallen into Sun due to Sun’s strong gravitational pull. However, the fact is that Earth is moving. It is continuously trying to run away from Sun. This motion is actually balancing the Sun’s gravitational pull and finally Earth is moving in an elliptical path. So is the case of other planets. This is the same as if you have a weight on the end of a string and you are swinging it around. You are constantly pulling it toward you, but the motion keeps it swinging around.