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Why do the planets orbit the Sun? If it's because the gravity of Sun then why don't they just fall in and burn up?

Qmechanic
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VVK
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    As I told my 8 year old, they not so much orbit as fall. Just that they keep missing the sun because of how fast they are moving. And they keep falling and missing, falling and missing, ... for billions of years. – John Alexiou Mar 19 '14 at 14:10
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    Essentially a duplicate of http://physics.stackexchange.com/q/5905/2451 , http://physics.stackexchange.com/q/9049/2451 , http://physics.stackexchange.com/q/12140/2451 , and links therein. – Qmechanic Mar 19 '14 at 14:16
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    For a visceral understanding of this I highly recommend the game Osmos (http://www.osmos-game.com) – aepryus Mar 19 '14 at 14:33

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The planets are attracted towards the sun, as you would expect from the gravitational force. The planets don't fall into the sun, though, because their velocities are at right angles to that force. The planets end up being pulled by the sun into a circle. A planet's speed is constant, but its direction changes. I think it's easiest with a picture:

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Gravity pulls the planet in a circular orbit.

innisfree
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