A sphere is a solid bounded by a curved surface, every point of which is equally distant from a point within called the center.

The diameter is a straight line drawn through the center and having its extremities in the curved surface. The radius - 1/2 diameter - is the straight line from the center to a point on the surface.

A plane is tangent to a sphere, when it touches the sphere in only one point. A plane perpendicular to a radius at its outer extremity is tangent to the sphere, Fig. 83.

Fig. 83. Plane Tangent to Sphere

Fig. 83. Plane Tangent to Sphere.

Fig. 84. Large Circle

Fig. 84. Large Circle.

Fig. 85. Small Circle

Fig. 85. Small Circle.

An inscribed polyedron is a polyedron whose vertices lie in the surface of the sphere.

A circumscribed polyedron is a polyedron whose faces are tangent to a sphere.

A great circle is the intersection of the spherical surface and a plane passing through the center of a sphere, Fig. 84.

A small circle is the intersection of the spherical surface and a plane which does not pass through the center, Fig. 85.