Charles Jones, age 11, of Stanford, Kentucky, for his question:

Why are the poles and tropics where they are?

The poles and the tropics are caused by the shape of the earth and its tilted axis. The warmest zones are where the sun reaches highest overhead and its rays pierce straight down through the atmosphere. This occurs near the equator, around the bulging waist of the round globe. Toward the poles, the sun is lower in the sky and its rays are cooler because they must slant down a longer path through the atmosphere. The poles are at opposite ends of the axis around which the earth spins. The equatorial tropics are midway between the opposite poles.

The whole routine changes somewhat with the seasons. This is because the axis is tilted to the earth's orbital path. The yearly orbit does not change the positions of the poles and the tropics. But in January, the South Pole is tipped toward the sun and the North Pole is tipped away. In July the North Pole makes its bow. But at all seasons, the position of the axis makes sure that the equatorial tropical belt pets the lion's share of overhead sunshine.