Editing Sphere (section)

===Properties of the sphere===
[[File:Sphere section.png|thumb|A normal vector to a sphere, a normal plane and its normal section. The curvature of the curve of intersection is the sectional curvature. For the sphere each normal section through a given point will be a circle of the same radius: the radius of the sphere. This means that every point on the sphere will be an umbilical point.]]

In their book ''Geometry and the Imagination'', [[David Hilbert]] and [[Stephan Cohn-Vossen]] describe eleven properties of the sphere and discuss whether these properties uniquely determine the sphere.<ref>{{cite book
|author1=Hilbert, David
|author-link=David Hilbert
|author2=Cohn-Vossen, Stephan
|title=Geometry and the Imagination
|url=https://archive.org/details/geometryimaginat00davi_0|url-access=registration|edition=2nd
|year=1952
|publisher=Chelsea
|isbn=978-0-8284-1087-8|chapter=Eleven properties of the sphere|pages=215–231}}</ref> Several properties hold for the [[plane (mathematics)|plane]], which can be thought of as a sphere with infinite radius. These properties are:
#''The points on the sphere are all the same distance from a fixed point. Also, the ratio of the distance of its points from two fixed points is constant.''
#: The first part is the usual definition of the sphere and determines it uniquely. The second part can be easily deduced and follows a similar [[Circle#Circle of Apollonius|result]] of [[Apollonius of Perga]] for the [[circle]]. This second part also holds for the [[plane (mathematics)|plane]].
#''The contours and plane sections of the sphere are circles.''
#: This property defines the sphere uniquely.
#''The sphere has constant width and constant girth.''
#: The width of a surface is the distance between pairs of parallel tangent planes. Numerous other closed convex surfaces have constant width, for example the [[Meissner body]]. The girth of a surface is the [[circumference]] of the boundary of its orthogonal projection on to a plane. Each of these properties implies the other.
#''All points of a sphere are [[umbilic]]s.''
#: At any point on a surface a [[Normal (geometry)|normal direction]] is at right angles to the surface because on the sphere these are the lines radiating out from the center of the sphere. The intersection of a plane that contains the normal with the surface will form a curve that is called a ''normal section,'' and the curvature of this curve is the ''normal curvature''. For most points on most surfaces, different sections will have different curvatures; the maximum and minimum values of these are called the [[principal curvature]]s. Any closed surface will have at least four points called ''[[umbilical point]]s''. At an umbilic all the sectional curvatures are equal; in particular the [[principal curvature]]s are equal. Umbilical points can be thought of as the points where the surface is closely approximated by a sphere.
#: For the sphere the curvatures of all normal sections are equal, so every point is an umbilic. The sphere and plane are the only surfaces with this property.
#''The sphere does not have a surface of centers.''
#: For a given normal section exists a circle of curvature that equals the sectional curvature, is tangent to the surface, and the center lines of which lie along on the normal line. For example, the two centers corresponding to the maximum and minimum sectional curvatures are called the ''focal points'', and the set of all such centers forms the [[focal surface]].
#: For most surfaces the focal surface forms two sheets that are each a surface and meet at umbilical points. Several cases are special:
#: * For [[channel surface]]s one sheet forms a curve and the other sheet is a surface
#: * For [[Cone (geometry)|cones]], cylinders, [[torus|tori]] and [[Dupin cyclide|cyclides]] both sheets form curves.
#: * For the sphere the center of every osculating circle is at the center of the sphere and the focal surface forms a single point. This property is unique to the sphere.
#''All geodesics of the sphere are closed curves.''
#: [[Geodesics]] are curves on a surface that give the shortest distance between two points. They are a generalization of the concept of a straight line in the plane. For the sphere the geodesics are great circles. Many other surfaces share this property.
#''Of all the solids having a given volume, the sphere is the one with the smallest surface area; of all solids having a given surface area, the sphere is the one having the greatest volume.''
#: It follows from [[isoperimetric inequality]]. These properties define the sphere uniquely and can be seen in [[soap bubble]]s: a soap bubble will enclose a fixed volume, and [[surface tension]] minimizes its surface area for that volume. A freely floating soap bubble therefore approximates a sphere (though such external forces as gravity will slightly distort the bubble's shape). It can also be seen in planets and stars where gravity minimizes surface area for large celestial bodies.
#''The sphere has the smallest total mean curvature among all convex solids with a given surface area.''
#: The [[mean curvature]] is the average of the two principal curvatures, which is constant because the two principal curvatures are constant at all points of the sphere.
#''The sphere has constant mean curvature.''
#: The sphere is the only [[Embedding|embedded]] surface that lacks boundary or singularities with constant positive mean curvature. Other such immersed surfaces as [[minimal surface]]s have constant mean curvature.
#''The sphere has constant positive Gaussian curvature.''
#: [[Gaussian curvature]] is the product of the two principal curvatures. It is an intrinsic property that can be determined by measuring length and angles and is independent of how the surface is [[embedding|embedded]] in space. Hence, bending a surface will not alter the Gaussian curvature, and other surfaces with constant positive Gaussian curvature can be obtained by cutting a small slit in the sphere and bending it. All these other surfaces would have boundaries, and the sphere is the only surface that lacks a boundary with constant, positive Gaussian curvature. The [[pseudosphere]] is an example of a surface with constant negative Gaussian curvature.
#''The sphere is transformed into itself by a three-parameter family of rigid motions.''
#: Rotating around any axis a unit sphere at the origin will map the sphere onto itself. Any rotation about a line through the origin can be expressed as a combination of rotations around the three-coordinate axis (see [[Euler angles]]). Therefore, a three-parameter family of rotations exists such that each rotation transforms the sphere onto itself; this family is the [[rotation group SO(3)]]. The plane is the only other surface with a three-parameter family of transformations (translations along the {{mvar|x}}- and {{mvar|y}}-axes and rotations around the origin). Circular cylinders are the only surfaces with two-parameter families of rigid motions and the [[Surface of revolution|surfaces of revolution]] and [[helicoid]]s are the only surfaces with a one-parameter family.