Editing Sphere (section)

==Basic terminology==
[[File:Sphere and Ball.png|thumb|upright=1.1|Two orthogonal radii of a sphere]]

As mentioned earlier {{math|''r''}} is the sphere's radius; any line from the center to a point on the sphere is also called a radius. 'Radius' is used in two senses: as a line segment and also as its length.<ref name="EB" />

If a radius is extended through the center to the opposite side of the sphere, it creates a [[diameter]]. Like the radius, the length of a diameter is also called the diameter, and denoted {{math|''d''}}. Diameters are the longest line segments that can be drawn between two points on the sphere: their length is twice the radius, {{math|1=''d'' = 2''r''}}. Two points on the sphere connected by a diameter are [[antipodal point]]s of each other.<ref name="EB" />

A [[unit sphere]] is a sphere with unit radius ({{math|1=''r'' = 1}}). For convenience, spheres are often taken to have their center at the origin of the [[coordinate system]], and spheres in this article have their center at the origin unless a center is mentioned.

{{anchor|hemisphere}}
A ''[[great circle]]'' on the sphere has the same center and radius as the sphere, and divides it into two equal '''''hemispheres'''''.

Although the [[figure of Earth]] is not perfectly spherical, terms borrowed from geography are convenient to apply to the sphere.
A particular line passing through its center defines an ''[[Axis of symmetry|axis]]'' (as in Earth's [[axis of rotation]]).
The sphere-axis intersection defines two antipodal ''poles'' (''north pole'' and ''south pole''). The great circle equidistant to the poles is called the ''[[equator]]''. Great circles through the poles are called lines of [[longitude]] or [[meridian (geography)|''meridians'']]. Small circles on the sphere that are parallel to the equator are [[circle of latitude|circles of latitude]] (or ''parallels''). In geometry unrelated to astronomical bodies, geocentric terminology should be used only for illustration and noted as such, unless there is no chance of misunderstanding.<ref name="EB" />

Mathematicians consider a sphere to be a [[two-dimensional]] [[closed surface]] [[embedding|embedded]] in three-dimensional [[Euclidean space]]. They draw a distinction between a ''sphere'' and a ''[[Ball (mathematics)|ball]]'', which is a [[solid figure]], a three-dimensional [[manifold with boundary]] that includes the volume contained by the sphere. An ''open ball'' excludes the sphere itself, while a ''closed ball'' includes the sphere: a closed ball is the union of the open ball and the sphere, and a sphere is the [[Boundary (topology)|boundary]] of a (closed or open) ball. The distinction between ''ball'' and ''sphere'' has not always been maintained and especially older mathematical references talk about a sphere as a solid. The distinction between "[[circle]]" and "[[Disk (mathematics)|disk]]" in the [[Plane (geometry)|plane]] is similar.

Small spheres or balls are sometimes called ''spherules'' (e.g., in [[Martian spherules]]).