Circle

A circle is a flat, round shape made of all the points that sit the same distance from one center point. That single rule, every point the same distance from the middle, is what gives a circle its perfect roundness. Circles are one of the most basic shapes in math, and they show up everywhere in nature, from the full moon to the ripples in a pond.

Every circle has a few important parts. The center is the point in the middle. The radius is the distance from the center to the edge. The diameter is the distance straight across the circle through the center, and it is always twice the radius. The edge itself is called the circumference. If you walk all the way around a circular track, the distance you walked is the circumference.

Circles have a special number tied to them called pi. If you measure the circumference of any circle and divide by the diameter, you always get pi, which is about 3.14. This works for a circle the size of a coin and for a circle the size of the Sun. Pi never ends and never repeats. It is one of the most famous numbers in math.

People have studied circles for thousands of years. The ancient Greek mathematician Euclid wrote about them around 300 BCE. Even older cultures used circles in art, calendars, and buildings. The wheel, one of the most important inventions in human history, depends on the circle. A wheel rolls smoothly because every point on its edge stays the same distance from the center, so the axle never bumps up or down.

Circles are everywhere in the natural world. Bubbles form circles because that shape uses the least surface to hold the air inside. Tree trunks grow in roughly circular rings. The pupils of your eyes are circles. Planets and stars look like circles in the sky because they are actually spheres, and a sphere from any side looks like a circle.

A circle is not the same as a sphere. A circle is flat, like a drawing on paper. A sphere is solid and round in three directions, like a basketball. If you slice a sphere right through the middle, the cut you see is a perfect circle. So every sphere hides circles inside it.