Carol Dreywood, age 14, of Rush, New York, for her question:

What is a direct circle route?

A straight line is the shortest distance between two points. But when it comes to two points on a map, the shortest distance between them may be a curve. A map, of course, is a flat picture of part of the earth's surface. But the real earth is a globe and its worldwide skin is far from flat. It curves downward at every point to the horizon and if each curve is followed all the way around, it completes a circle around the globe. It may be a great circle of latitude running east and west. It may lead around the two halves of a circle of longitude running north and south through the poles. You can draw a great circle around the globe in any other direction also.

On a map, the shortest distance between San Francisco and London appears to be along a line going eastward across land and sea. But remember, that flat map represents a curve of the globe. A curved line up through the Arctic proves to be a shorter route. Since this curve is an arc of one of those great circles around the globe, we call it a great circle route.