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Dr. Christopher S. Baird

Why doesn't the earth fall down?

Category: Space      Published: July 1, 2013

The earth does fall down. In fact, the earth is constantly falling down. It's a good thing too, because that is what keeps the earth from flying out of the solar system under its own momentum. Gravity is a centrally attractive force, meaning that objects in a gravitational field always fall towards the source of the gravity. Gravity is caused by mass, so objects with more mass, such as planets and stars, exert a lot of gravity. The earth and everything on it is constantly falling towards the sun because of the sun's immense gravity. This statement is not a metaphor or a play on words. The earth is literally falling towards the sun under its immense gravity.

So why don't we hit the sun and burn up? Fortunately for us, the earth has a lot of sideways momentum. Because of this sideways momentum, the earth is continually falling towards the sun and missing it. Scientists use fancy phrases for this effect such as "stable orbit" or "closed trajectory", but fundamentally what they mean is "falling and missing". All gravitational orbits are actually cases of falling and missing. Astronauts on the International Space Station are not in a no-gravity environment. They are surrounded by the earth's and the sun's immense gravity. More correctly, the astronauts are in a state of free fall. Astronauts in orbit are constantly falling towards the earth and missing it.

newton cannonball diagram
Newton's cannonball thought experiment demonstrating that orbits are just objects falling but traveling sideways fast enough to never strike the source of the gravity. Public Domain Image, source: Christopher S. Baird.

Newton had a clever way of explaining the nature of orbits. Consider a cannon on the surface of the earth that shoots a cannonball straight forward. As the ball speeds forward, earth's gravity pulls on it and it falls to the earth until it hits the ground. But the cannonball does not strike the earth at the exact spot it was fired because its forward momentum carries it forward a ways before striking the earth. Now shoot the cannonball again, this time with a higher forward speed. The ball still falls and eventually strikes the earth, but because it has a higher forward speed (sideways, relative to the earth) the ball can cover more distance before striking the earth. If you shoot the ball fast enough, as shown in the picture on the right, it will still fall but will never manage to strike the earth. The earth will curve away faster than the ball can fall towards it. As a result, the ball will continually fall and miss and will end up circling the earth. This is exactly what satellites do. To get an object to orbit the earth, you just have to give it enough sideways speed that it will miss the earth as it falls.

If the earth was not falling around the sun, it would fly wildly out of orbit under its own inertia. The falling trajectory of the earth around the sun, combined with earth's tilt, is what causes the different seasons. All the planets in our solar system are falling around the sun but have enough speed to not hit it. Why are there no objects that do fall right into the sun? There were such objects, put once they fall into the sun, they burn up and become part of the sun. Our solar system is so old, that all rocks and dust clouds without enough speed to miss the sun have long since burned up in the sun.

All objects in the universe are constantly falling. You fall to the earth every time you jump. You and the earth are constantly falling around the sun. You, the earth, and the sun are constantly falling around the center of the galaxy. Why don't we feel all this falling motion? We do experience all this falling, we just don't notice it. The sun is so far away compared to humans, that our falling motion around the sun is very close to a constant speed in a straight line. Interestingly, you can't feel a constant speed in a straight line. Similarly, the galactic center is so far away that our falling motion around the galactic center is very close to a constant speed in a straight line. Our actual trajectory around the galactic center is curved, but the curve is so huge that it is essentially straight on human scales.

Topics: falling, gravity, orbit, sun