Sphere two poles is called the axis of rotation. Circles on the sphere that are parallel to the equator are lines of latitude. This terminology is also used for such approximately spheroidal astronomical bodies as the planet Earth (see geoid). Any plane that includes the center of a sphere divides it into two equal hemispheres. Any two intersecting planes that include the center of a sphere subdivide the sphere into four lunes or biangles, the vertices of which all coincide with the antipodal points lying on the line of intersection of the planes. The antipodal quotient of the sphere is the
N-sphere a central point, where the radius may be any positive real number. Thus, the -sphere would be defined by: In particular: An "n"-sphere embedded in an -dimensional Euclidean space is called a hypersphere. The -sphere of unit radius is called the unit -sphere, denoted , often referred to as "the" -sphere. When embedded as described, an -sphere is the surface or boundary of an -dimensional ball. For , the -spheres are the simply connected -dimensional manifolds of constant, positive curvature. The -spheres admit several other topological descriptions: for example, they can be constructed by gluing two -dimensional Euclidean spaces together,