# eli5 : How does the earth not collide with the sun when orbiting

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eli5 : How does the earth not collide with the sun when orbiting

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It’s because the Earth is moving so fast that it moves sideways around the Sun exactly the same amount that it falls towards the Sun.

Because it is moving really, really, really fast.

Yeah, if the Earth were created, and just dropped with no speed, it would fall into the Sun. But the Earth is moving relative to the Sun, at a speed of ~30 km/second. That is enough that even though it’s falling towards the sun, it always misses.

Well, the earth is moving at such speed at a constant distance (in planetary scale) from the sun that its gravity pull doesn’t have enough time to act fast enough to pull Earth in. It’s only when Earth starts slowing down that the sun has time to pull on us and when that happens the earth picks up speed very quickly again and keeps the orbit.

This is a constant dance between stars, planets, moons, etc..

The movement of objects in space (and in general) are conserved. Basically this means if something is moving in a direction it will continue to do so unless something acts on it to stop it. (See Newton’s Laws of Motion)

An orbiting body such as a planet has a velocity tangent to the surface of the object it is orbiting. To simplify imagine that the sun is at 0X, 0Y on a grid. The Earth is at 0X, 10Y on the grid, but it is moving at a speed along the X axis.

The sun is pulling on the Earth so it will fall towards the sun along the Y axis. But in the time it takes to fall those 10 units Earth will have moved 10 units along the X axis! Now Earth is at 10X, 0Y and the steady pull of gravity has slowed it’s movement along the X axis to zero. But Earth has picked up speed along the Y axis from falling!

Basically you are now right back to where you started; rotate your grid 90 degrees and start again. The result is a continual orbit.

Because the Earth is moving sideways. By the time the Earth would have fallen 93 million miles (150 million km, 1 AU) into the sun, it has already moved 93 million miles to the side of the sun. This results in the circular movement around the sun. For circular motion, you need a force toward the center such that a = v^2 /r. That force is provided by the sun’s gravity.

Newton had a through experiment that basically explained how orbit starts. If you fired a cannon on top of a mountain fast enough, the cannonball could move fast enough that the curved surface of the Earth would curve away from the cannonball before the cannon ball can be pulled towards the ground, and thats essentially how orbit works, just much further away from the planet/star