Terry Carl, age 11, of Peterborough, Ontario, Canada, for his question:

What are the earth, land and water hemispheres?

A sphere is a ball which can be divided into two halves or hemispheres. This sounds simple enough. But in matters pertaining to our spherical earth, let's not expect anything to be as simple as it looks. By using different slicing lines we can sever a north and south hemisphere and an east and zest hemisphere. By using another rather cunning slice, we can divide the spherical earth into a land and a water hemisphere.

For some years now, scientists have been beaming a message across the vast oceans of space. Someday it may be picked up by intelligent beings out there among the 100 billion starry suns of our Galaxy. When translated, our message describes the human family and locates us on our home planet. Who knows, intelligent aliens may get the message and decide to visit us. If and when they approach our little planet, naturally they will see only one side or one hemisphere of our round globe.

What they see of course would depend upon where they come from. Suppose their flight path is aimed to land plumb in the middle of New Zealand. As they approach, they see a hemisphere covered almost entirely by water    with precious little dry land on the map. This is what earth geographers call our water hemisphere.

Let's return to our alien visitors and re route their approach from the very opposite direction. This time they are homing in on London, England. This hemisphere holds most of the dry land, with only_smallish sections of ocean. Geographers call this our land hemisphere. Actually this is a sort of geographical gimmick to show that most of the land is crowded into one half of our watery planet.

Geographers are undecided about where to divide the east and west hemispheres. One good suggestion is a line along meridians 20 degrees west and 160 degrees east. This global circle runs from pole to pole through the Atlantic and Pacific Oceans. It places the New World of the Americas in the western hemisphere, with the other major land masses in the eastern hemisphere.

The poles, as we know, are at opposite ends of the earth's axis    and the equator is halfway between them. These are natural markers and perhaps the most natural method to sever the earth's sphere is at the equator. This gives us the familiar northern and southern hemisphere. However, these two natural hemispheres are not identical twins. Far from it. The Arctic tapers slightly, very slightly. What's more, the northern half has the lion's share of the land and most of the southern hemisphere is under water.

Seasons of the northern and southern hemispheres alternate at opposite times of the year. Their prevailing winds veer at pposite angles and even their whirlpools swirl in opposite directions.