Judy Kerr, age 11, of Oromocto, N.B., Canada, for her question:

How come the world is round?

Some Of the thinkers of ancient Greece suggested that our world might be round. But most of our ancestors would state that the earth was a big, flat table top. Some of the crew that sailed with Columbus feared their journey would take them to the edge of the world, where they would fall into a bottomless abyss.

It was the bold mariner Magellan who actually proved that the world is a round ball. Columbus and Gallileo were sure that this is so. Same of the ancient Greeks suspected it. But Magellan proved that the world is a globe by sailing a ship all the way around it. After this event, no sensible person could go on insisting that the world is flat.

But it was some time before humanity grasped the fact that we live on a round globe. And, Of course, when we find the answer to one question, it always leads to another question. Nowadays, it is natural to wonder why this world of ours is a round ball. And the scientists are fairly sure that they have the answer for us.

Our planet, they tell us, is round because it rotates. It spins around on its axis once every 24 hours and this spinning around has been going on for billions of years. The particles from which the earth is made feel this spin, and throughout the ages all of them have been moved by it. When you swirl a flat pan half filled with peas, they too swing around and around.

    If the peas are moist or gooey, sooner or later they will stick together in a round mass. But this spinning motion only partially explains the shape of the earth. The gravity of our massive planet exerts a pull on all of its particles. And the pull comes from the Center of the planet's mass. The Center of gravity at the very center of the planet’s core exerts a pull in every direction.

In time, this would tend to arrange the particles of our planet in a ball and hold them in that shape. Gravity also accounts for the distribution of the earth's weight. A pebble sinks in a pool because stone is heavier than water. This is because gravity pulls a more massive material down through a lighter material. The heaviest or most massive materials are in the dense core of our planet. Layers of lighter and still lighter rocks are wrapped around the dense core like the skin of an onion.

All heavenly bodies rotate, and the stars and planets arc all roughly spherical in shape. The spinning moons are also round. Any sizeable rotating body tends to become a ball. Some Of the smaller asteroids seem to be shapeless, but the larger ones are roughly sphere shaped, and these little bodies are merely a few hundred miles wide.