Roger Lee, aged 7, of Tulsa, Okla, for his question:

What is the horizon?

Imagine yourself on an old time sailing ship. Your job is to keep a lookout. You climb up high in the ropes and rigging and hold on tight. You look this way and that way across the blue ocean. The sky is clear, the sun is. bright. Your ship is in the middle of the vast Pacific Ocean. There is no land, not even an island. in sight. The heaving ocean stretches out in all directions. Overhead is the blue sky.

You may feel dizzy and a bit scared up there. But you have just about the best view of the horizon you can get. It is, of course, the line where the sea seems to meet the sky. The ocean looks like a round blue lake with your ship exactly in the middle.

Before you climb up there again, the ship has sailed 500 miles. But the view from your lookout post seems exactly the same, The ship still seems to be in the exact middle of a round ocean. The horizon girdles around it the same as before. The sky still fits over the round ocean in a high blue dome.

But you know this cannot be exactly the same view. You have traveled 500 miles. You are in a different part of the ocean under a different sky. Even the horizon must be a different one, This is no time to wonder about it because just look over there in the southwest. Ship Ahoy! you yell down to your mates on the deck.

‑All you have seen is the top of a mast. But you know from times past that a ship is coming into view, First the top of her mast shows above the horizon. Then comes the top half of her sails. Gradually, as she nears, the bottom half of her sails creep up. Last, the hull, the body of the ship comes over the horizon.

You have seen a proof that the world is round. The ship coming over the horizon seemed to be climbing a hill ‑ your hill. And in a way it was. You were looking out over the round globe. In every direction, the surface of the sea sloped down and away from you. Your ship sailed always on the top of a round hump. You could see down the sloping sides of the hump as far as the horizon.

The higher you climbed in the rigging, the further down the hump you could see. The higher you climb anywhere, the further away the horizon is. This is true on land and sea. You cant see beyond your horizon because the big earth curves around into a globe, As you travel, your horizon travels with you. It will go with you all over the world. It will always be ahead and you will never be able to catch up with it.