Timothy Warnick, age 8, f Westford, Massachusetts, for his question:
Why is the earth round?
Our wonderful world is a round globe with oceans of space on every side. Around it, the starry heavens stretch far and wide. In the sky, the most fashionable shape is round, round as a ball, and our earth is right in style. Even if it tried, it could not be any other shape.
Suppose the baby earth had been shaped like a box or a baseball bat. It could not have stayed in ei her of these shapes. In time, its form would have changed. In time, the earth would have molded itself into a round ball. The moon also is a round ball arid so is the giant sun. o are the other planets of the Solar System. Even if they started out like boxes or other shapes, they would have to change into round globes. .
Our planet, of course, has a built in force called gravitation. This force pulls and tugs at ever thing around it. It is the force of gravity that hugs us to the surface of the earth. It i~ the force of gravity that pulls down a falling stone and showery drops of rain. The earth is a huge chunk of solid materials. It has so much material and no more. And the pulling force of its gravity seems to come from the very center of this material. It comes fro this central point with equal force in all directions.
It hugs us to the ground and with the same strength it hugs the people to the ground on the opposite side of the globe. This pulling power of the earth's gravity makes us feel heavy. In fact, it gives us weight. It pulls harder on heavy objects. A can of water weigh less than a can of stones because the stones are made of heavier materials. Gravity pulls harder on the stones.
If we drop a stone in the water, it sinks.
This is because gravity pulls it down with more force. Heavier materials tend to sink down through lighter materials down, down to the center of the earth's gravity. The solid earth is made of many different materials; some are much lighter than others. Iron and nickel are heavier than sand and stones. In time, these heavy metals sank down, down below the lighter materials, just as a stone sinks through water and the bottom is the center of gravity in the middle of all the earth's solid materials. In time, a great ball of heavy metals sank down to the center of the earth. It is ball shaped because the force of gravity pulls on it equally in all directions. The other materials were left on top. They were arranged like the skins of an onion in lighter and lighter layers. The lightest materials formed a rocky crust on the outside of the round globe. And if our round earth were bashed out of shape, its gravitation would gradually mold it back again into the shape of a globe.
A ball shape is called a sphere. There is a point in its exact center and every line from this point to the surface is the same length. This object is a perfect sphere. Our earth is not quite a perfect sphere. It bulges a bit around the equator. This is because the equator spins around faster than the rest of the globe. It is not easy to measure its exact shape and the earth may have other bulges that we cannot explain. The latest reports suggest that our round globe may be very slightly egg shaped.