Sphere

Sphere, a three-dimensional object in which every point on its surface is the same distance from a fixed point known as the center. Marbles are examples of spheres. A mathematical sphere has the exact same appearance no matter the viewing angle. A racquetball, with its smooth featureless surface, is very nearly a perfect sphere. A sphere may be described as the rotation of a circle or semicircle about its diameter:

The distance from the surface of a sphere to the center is the sphere’s radius, and the distance across a sphere, passing through the center, is the diameter. The diameter is equal to twice the radius. If r is the radius of the sphere, the volume is given by the formula 4pr³/3 and the surface area by 4pr ². Pi (p) is a constant approximately equal to 3.14159.

The intersection of a plane with the surface of a sphere forms a circle. If the plane passes through the center of the sphere the circle is called a great circle. A great circle has the largest possible radius of any circle that can be drawn on a given sphere.