of a sphere]]
A sphere (from Greek language|Greek
—, "globe, ball") is a geometrical
object in three-dimensional space
that is the surface of a ball
(viz., analogous to the circular objects in two dimensions, where a "circle
" circumscribes its "disk"
Like a circle in a two-dimensional space, a sphere is defined mathematically as the set of points
that are all at the same distance from a given point in a three-dimensional space.
This distance is the radius
of the ball, which is made up from all points with a distance less than (or, for a closed ball, less than ''or equal to'') from the given point, which is the center
of the mathematical ball. These are also referred to as the radius and center of the sphere, respectively. The longest straight line segment through the ball, connecting two points of the sphere, passes through the center and its length is thus twice the radius; it is a diameter
of both the sphere and its ball.
While outside mathematics the terms "sphere" and "ball" are sometimes used interchangeably, in mathematics
the above distinction is made between a ''sphere'', which is a two-dimensional closed surface embedded
in a three-dimensional Euclidean space
, and a ''ball'', which is a three-dimensional shape that includes the sphere and everything ''inside'' the sphere (a ''closed ball''), or, more often, just the points ''inside'', but ''not on'' the sphere (an ''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. This is analogous to the situation in the plane
, where the terms "circle" and "disk" can also be confounded.
Equations in three-dimensional space
In analytic geometry
, a sphere with center and radius is the locus
of all points such that
Let be real numbers with and put
Then the equation
has no real points as solutions if
and is called the equation of an imaginary sphere. If
, the only solution of
is the point
and the equation is said to be the equation of a point sphere. Finally, in the case
is an equation of a sphere whose center is
and whose radius is
If in the above equation is zero then is the equation of a plane. Thus, a plane may be thought of as a sphere of infinite radius whose center is a point at infinity
The points on the sphere with radius
can be parameterized via
can be associated with the angle counted positive from the direction of the positive ''z''-axis through the center to the radius-vector, and the parameter
can be associated with the angle counted positive from the direction of the positive ''x''-axis through the center to the projection of the radius-vector on the ''xy''-plane.
A sphere of any radius centered at zero is an integral surface of the following differential form
This equation reflects that position and velocity vectors of a point, and , traveling on the sphere are always orthogonal
to each other.
A sphere can also be constructed as the surface formed by rotating a circle
about any of its diameter
s. Since a circle is a special type of ellipse
, a sphere is a special type of ellipsoid of revolution
. Replacing the circle with an ellipse rotated about its major axis
, the shape becomes a prolate spheroid
; rotated about the minor axis, an oblate spheroid.
In three dimensions, the volume
inside a sphere (that is, the volume of a ball
, but classically referred to as the volume of a sphere) is
where is the radius and is the diameter of the sphere. Archimedes
first derived this formula by showing that the volume inside a sphere is twice the volume between the sphere and the circumscribe
of that sphere (having the height and diameter equal to the diameter of the sphere). This may be proved by inscribing a cone upside down into semi-sphere, noting that the area of a cross section of the cone plus the area of a cross section of the sphere is the same as the area of the cross section of the circumscribing cylinder, and applying Cavalieri's principle
. This formula can also be derived using integral calculus
, i.e. disk integration
to sum the volumes of an infinite number
disks of infinitesimally small thickness stacked side by side and centered along the -axis from to , assuming the sphere of radius is centered at the origin.
At any given , the incremental volume () equals the product of the cross-sectional area of the disk
at and its thickness ():
The total volume is the summation of all incremental volumes:
In the limit as approaches zero,
this equation becomes:
At any given , a right-angled triangle connects , and to the origin; hence, applying the Pythagorean theorem
Using this substitution gives
which can be evaluated to give the result
An alternative formula is found using spherical coordinates
, with volume element
For most practical purposes, the volume inside a sphere inscribed
in a cube can be approximated as 52.4% of the volume of the cube, since , where is the diameter of the sphere and also the length of a side of the cube and ≈ 0.5236. For example, a sphere with diameter 1m has 52.4% the volume of a cube with edge length 1m, or about 0.524 m3
The surface area
of a sphere of radius is:
first derived this formula
from the fact that the projection to the lateral surface of a circumscribe
d cylinder is area-preserving. Another approach to obtaining the formula comes from the fact that it equals the derivative
of the formula for the volume with respect to because the total volume inside a sphere of radius can be thought of as the summation of the surface area of an infinite number of spherical shells of infinitesimal thickness concentrically stacked inside one another from radius 0 to radius . At infinitesimal thickness the discrepancy between the inner and outer surface area of any given shell is infinitesimal, and the elemental volume at radius is simply the product of the surface area at radius and the infinitesimal thickness.
At any given radius , the incremental volume () equals the product of the surface area at radius () and the thickness of a shell ():
The total volume is the summation of all shell volumes:
In the limit as approaches zero
this equation becomes:
Differentiating both sides of this equation with respect to yields as a function of :
This is generally abbreviated as:
where is now considered to be the fixed radius of the sphere.
Alternatively, the area element
on the sphere is given in spherical coordinates
by . In Cartesian coordinates
, the area element is
The total area can thus be obtained by integration
The sphere has the smallest surface area of all surfaces that enclose a given volume, and it encloses the largest volume among all closed surfaces with a given surface area. The sphere therefore appears in nature: for example, bubbles and small water drops are roughly spherical because the surface tension
locally minimizes surface area.
The surface area relative to the mass of a ball is called the specific surface area
and can be expressed from the above stated equations as
where is the density
(the ratio of mass to volume).
Curves on a sphere
* The intersection of a sphere and a plane is a circle, a point or empty.
In case of a circle the circle can described by a parametric equation
: see plane section of an ellipsoid
But more complicated surfaces may intersect a sphere in circles, too:
* A non empty intersection of a sphere with a surface of revolution
, whose axis contains the center of the sphere (are ''coaxial'') consists of circles and/or points.
The diagram shows the case, where the intersection of a cylinder and a sphere consists of two circles. Would the cylinder radius be equal to the sphere's radius, the intersection would be one circle, where both surfaces are tangent.
In case of an spheroid with the same center and major axis as the sphere, the intersection would consist of two points (vertices), where the surfaces are tangent.
If the sphere is described by a parametric representation
one gets Clelia curves
, if the angles are connected by the equation
Special cases are: Viviani's curve
) and spherical spiral
), such as Seiffert's spiral
, a rhumb line or loxodrome is an arc crossing all meridians
at the same angle. A rhumb line is not a spherical spiral. There is no simple connection between the angles
Intersection of a sphere with a more general surface
If a sphere is intersected by another surface, there may be more complicated spherical curves.
; Example: sphere – cylinder
The intersection of the sphere with equation
and the cylinder with equation
is not just one or two circles. It is the solution of the non linear system of equations
(see implicit curve
and the diagram)
A sphere is uniquely determined by four points that are not coplanar
. More generally, a sphere is uniquely determined by four conditions such as passing through a point, being tangent to a plane, etc. This property is analogous to the property that three non-collinear
points determine a unique circle in a plane.
Consequently, a sphere is uniquely determined by (that is, passes through) a circle and a point not in the plane of that circle.
By examining the common solutions of the equations of two spheres
, it can be seen that two spheres intersect in a circle and the plane containing that circle is called the radical plane of the intersecting spheres. Although the radical plane is a real plane, the circle may be imaginary (the spheres have no real point in common) or consist of a single point (the spheres are tangent at that point).
The angle between two spheres at a real point of intersection is the dihedral angle
determined by the tangent planes to the spheres at that point. Two spheres intersect at the same angle at all points of their circle of intersection. They intersect at right angles (are orthogonal
) if and only if the square of the distance between their centers is equal to the sum of the squares of their radii.
Pencil of spheres
If and are the equations of two distinct spheres then
is also the equation of a sphere for arbitrary values of the parameters and . The set of all spheres satisfying this equation is called a pencil of spheres determined by the original two spheres. In this definition a sphere is allowed to be a plane (infinite radius, center at infinity) and if both the original spheres are planes then all the spheres of the pencil are planes, otherwise there is only one plane (the radical plane) in the pencil.
A ''great circle
'' on the sphere has the same center and radius as the sphere—consequently dividing it into two equal parts. The plane section
s of a sphere are called ''spheric sections—''which are either great circles for planes through the sphere's center or ''small circles''
for all others.
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 coincide with the antipodal point
s lying on the line of intersection of the planes.
Branches of geometry
Any pair of points on a sphere that lie on a straight line through the sphere's center (i.e. the diameter) are called ''antipodal points''
—on the sphere, the distance between them is exactly half the length of the circumference. Any other (i.e. not antipodal) pair of distinct points on a sphere
* lie on a unique great circle,
* segment it into one minor (i.e. shorter) and one major (i.e. longer) arc
* have the minor arc's length be the ''shortest distance'' between them on the sphere.
shares many analogous properties to Euclidean
once equipped with this "great-circle distance
And a much more abstract generalization of geometry
also uses the same distance concept in the Riemannian circle
The hemisphere is conjectured
to be the optimal (least area) isometric filling of the Riemannian circle.
The antipodal quotient of the sphere is the surface called the real projective plane
, which can also be thought of as the northern hemisphere
with antipodal points of the equator identified.
Terms borrowed directly from geography of the Earth
, despite its spheroid
al shape having greater or lesser departures from a perfect sphere (see geoid
), are widely well-understood. 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.
Poles, longitude and latitudes
If a particular point on a sphere is (arbitrarily) designated as its ''north pole'', its antipodal point is called the ''south pole''. The great circle equidistant to each is then the ''equator
''. Great circles through the poles are called lines of longitude
). A line ''not on the sphere'' but through its center connecting the two poles ''may'' be called the axis of rotation
. Circles on the sphere that are parallel (i.e. not great circles) to the equator are lines of latitude
Spheres can be generalized to spaces of any number of dimension
s. For any natural number
, an "-sphere," often written as , is the set of points in ()-dimensional Euclidean space that are at a fixed distance from a central point of that space, where is, as before, a positive real number. In particular:
* : a 0-sphere is a pair of endpoints of an interval of the real line
* : a 1-sphere is a circle
of radius ''r''
* : a 2-sphere is an ordinary sphere
* : a 3-sphere
is a sphere in 4-dimensional Euclidean space.
Spheres for are sometimes called hypersphere
The -sphere of unit radius centered at the origin is denoted and is often referred to as "the" -sphere. Note that the ordinary sphere is a 2-sphere, because it is a 2-dimensional surface (which is embedded in 3-dimensional space).
The surface area of the unit ()-sphere is
where is Euler's gamma function
Another expression for the surface area is
and the volume is the surface area times or
General recursive formulas also exist for the volume of an -ball
More generally, in a metric space
, the sphere of center and radius is the set of points such that .
If the center is a distinguished point that is considered to be the origin of , as in a normed
space, it is not mentioned in the definition and notation. The same applies for the radius if it is taken to equal one, as in the case of a unit sphere
Unlike a ball
, even a large sphere may be an empty set. For example, in with Euclidean metric
, a sphere of radius is nonempty only if can be written as sum of squares of integer
, an -sphere is defined as a space homeomorphic
to the boundary of an -ball
; thus, it is homeomorphic to the Euclidean -sphere, but perhaps lacking its metric
* A 0-sphere is a pair of points with the discrete topology
* A 1-sphere is a circle (up to
homeomorphism); thus, for example, (the image of) any knot
is a 1-sphere.
* A 2-sphere is an ordinary sphere (up to homeomorphism); thus, for example, any spheroid
is a 2-sphere.
The -sphere is denoted . It is an example of a compact topological manifold
. A sphere need not be smooth
; if it is smooth, it need not be diffeomorphic
to the Euclidean sphere (an exotic sphere
The Heine–Borel theorem
implies that a Euclidean -sphere is compact. The sphere is the inverse image of a one-point set under the continuous function . Therefore, the sphere is closed. is also bounded; therefore it is compact.
Remarkably, it is possible to turn an ordinary sphere inside out in a three-dimensional space
with possible self-intersections but without creating any crease, in a process called sphere eversion
on a sphere]]
The basic elements of Euclidean plane geometry
. On the sphere, points are defined in the usual sense. The analogue of the "line" is the geodesic
, which is a great circle
; the defining characteristic of a great circle is that the plane containing all its points also passes through the center of the sphere. Measuring by arc length
shows that the shortest path between two points lying on the sphere is the shorter segment of the great circle
that includes the points.
Many theorems from classical geometry
hold true for spherical geometry as well, but not all do because the sphere fails to satisfy some of classical geometry's postulate
s, including the parallel postulate
. In spherical trigonometry
s are defined between great circles. Spherical trigonometry differs from ordinary trigonometry
in many respects. For example, the sum of the interior angles of a spherical triangle
always exceeds 180 degrees. Also, any two similar
spherical triangles are congruent.
Eleven properties of the sphere
[[Image: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. Several properties hold for the 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 result
of Apollonius of Perga
for the circle
. This second part also holds for the 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
#: At any point on a surface a normal direction
is at right angles to the surface because 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 cones
, cylinders, tori
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.''
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 imbedded
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 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 - and -axes and rotations around the origin). Circular cylinders are the only surfaces with two-parameter families of rigid motions and the surfaces of revolution
s are the only surfaces with a one-parameter family.
File:Einstein gyro gravity probe b.jpg|An image of one of the most accurate human-made spheres, as it refracts the image of Einstein in the background. This sphere was a fused quartz gyroscope for the Gravity Probe B experiment, and differs in shape from a perfect sphere by no more than 40 atoms (less than 10nm) of thickness. It was announced on 1 July 2008 that Australian scientists had created even more nearly perfect spheres, accurate to 0.3nm, as part of an international hunt to find a new global standard kilogram.New Scientist | Technology | Roundest objects in the world created
File:King of spades- spheres.jpg|Deck of playing cards illustrating engineering instruments, England, 1702. King of spades: Spheres
* Spherical cap
* Spherical polygon
* Spherical sector
* Spherical segment
* Spherical wedge
* Spherical zone
* Affine sphere
* Alexander horned sphere
* Celestial spheres
* Directional statistics
* Dome (mathematics)
* Dyson sphere
* Hand with Reflecting Sphere
, M.C. Escher
self-portrait drawing illustrating reflection and the optical properties of a mirror sphere
* Hoberman sphere
* Homology sphere
* Homotopy groups of spheres
* Homotopy sphere
* Lenart Sphere
* Napkin ring problem
* Orb (optics)
* Riemann sphere
* Solid angle
* Sphere packing
* Spherical coordinates
* Spherical Earth
* Spherical helix, tangent indicatrix
of a curve of constant precession
* Spherical shell
* Tennis ball theorem
* Zoll sphere
Notes and references
* Mathematica/Uniform Spherical Distribution
Surface area of sphere proof