Douglas Doe, age 7, of Toronto, Canada, for his question:

Why do we have an equator?

The earth has a North Pole and an opposite South Pole. These two pinpoints are just as far apart as they can get. Exactly halfway between the two poles is the equator, a great circle around the wide waist of the world. There is no mark or line there    but we do need to know where it is. For one thing, it divides the globe into two halves called the Northern Hemisphere and the Southern Hemisphere.

And certain things happen at the equator that don't happen anywhere else. It is the place where the sun marks the changing seasons between summer and winter. And on opposite sides of the equator, certain winds blow in different directions. But most important, the equator is a great global circle called Latitude 0. Pilots use the lines of latitude and longitude to find their way. So they need to know where all of them are    especially Latitude 0, which is the equator.