Put a dot on a piece of paper. Draw a line and make it curve so that it is always the same distance from the dot. The line will finally reach the place where it started. On the paper there will be a circle with the dot in the center. An easy way to draw a circle is to use a drawing tool called a compass. Every part of a perfect circle is exactly the same distance from the center. This distance is the radius.

Anyone hunting for circles will find them almost everywhere. Bracelets and rings are circles. So are wheels and tires. Polka dots are circles with their centers filled in. The faces of most clocks and watches are solid circles, too. The full moon is a solid circle as we see it in the sky.

The equator is an imaginary circle around the earth halfway between the South Pole and the North Pole. The center of this circle is deep inside the earth. The path of the earth around the sun is almost a circle.

The easiest way of telling how big a circle is, is to tell its diameter. The diameter of a circle is the distance across it through its center. The diameter of the equator is nearly 8,000 miles. It is 500,000,000 times as great as the diameter of one quarter coin.