Spherical cap

In geometry, a spherical cap or spherical dome is a portion of a sphere or of a ball cut off by a plane. It is also a spherical segment of one base, i.e., bounded by a single plane. If the plane passes through the center of the sphere, so that the height of the cap is equal to the radius of the sphere, the spherical cap is called a hemisphere.

Volume and surface area

The volume of the spherical cap and the area of the curved surface may be calculated using combinations of

The radius of the sphere
The radius of the base of the cap
The height of the cap
The polar angle between the rays from the center of the sphere to the apex of the cap and the edge of the disk forming the base of the cap.

These variables are inter-related through the formulas
,,,
and.

	Using and	Using and	Using and
Volume
Area
Constraints

If denotes the latitude in geographic coordinates, then, and.

Deriving the surface area intuitively from the spherical sector volume

Note that aside from the calculus based argument below, the area of the spherical cap may be derived from the volume of the spherical sector, by an intuitive argument, as
The intuitive argument is based upon summing the total sector volume from that of infinitesimal triangular pyramids. Utilizing the pyramid (or cone) volume formula of, where is the infinitesimal area of each pyramidal base and is the height of each pyramid from its base to its apex. Since each, in the limit, is constant and equivalent to the radius of the sphere, the sum of the infinitesimal pyramidal bases would equal the area of the spherical sector, and:

Deriving the volume and surface area using calculus

The volume and area formulas may be derived by examining the rotation of the function
for, using the formulas the surface of the rotation for the area and the solid of the revolution for the volume.
The area is
The derivative of is
and hence
The formula for the area is therefore
The volume is

Moment of inertia

The moments of inertia of a spherical cap about the principal axes of the sphere are:
where m and h are, respectively, the mass and height of the spherical cap and R is the radius of the entire sphere.

Applications

Volumes of union and intersection of two intersecting spheres

The volume of the union of two intersecting spheres
of radii and is
where
is the sum of the volumes of the two isolated spheres, and
the sum of the volumes of the two spherical caps forming their intersection. If is the
distance between the two sphere centers, elimination of the variables and leads
to

Volume of a spherical cap with a curved base

The volume of a spherical cap with a curved base can be calculated by considering two spheres with radii and, separated by some distance, and for which their surfaces intersect at. That is, the curvature of the base comes from sphere 2. The volume is thus the difference between sphere 2's cap and sphere 1's cap,
This formula is valid only for configurations that satisfy and. If sphere 2 is very large such that, hence and, which is the case for a spherical cap with a base that has a negligible curvature, the above equation is equal to the volume of a spherical cap with a flat base, as expected.

Areas of intersecting spheres

Consider two intersecting spheres of radii and, with their centers separated by distance. They intersect if
From the law of cosines, the polar angle of the spherical cap on the sphere of radius is
Using this, the surface area of the spherical cap on the sphere of radius is

Surface area bounded by parallel disks

The curved surface area of the spherical segment bounded by two parallel disks is the difference of surface areas of their respective spherical caps. For a sphere of radius, and caps with heights and, the area is
or, using geographic coordinates with latitudes and,
For example, assuming the Earth is a sphere of radius 6371 km, the surface area of the arctic is =, or = 4.125% of the total surface area of the Earth.
This formula can also be used to demonstrate that half the surface area of the Earth lies between latitudes 30° South and 30° North in a spherical zone which encompasses all of the tropics.

Generalizations

Sections of other solids

The spheroidal dome is obtained by sectioning off a portion of a spheroid so that the resulting dome is circularly symmetric, and likewise the ellipsoidal dome is derived from the ellipsoid.

Hyperspherical cap

Generally, the -dimensional volume of a hyperspherical cap of height and radius in -dimensional Euclidean space is given by:
where is given by.
The formula for can be expressed in terms of the volume of the unit n-ball and the hypergeometric function or the regularized incomplete beta function as
and the area formula can be expressed in terms of the area of the unit n-ball as
where.
A. Chudnov derived the following formulas:
where
For odd :

Asymptotics

If and, then where is the integral of the standard normal distribution.
A more quantitative bound is.
For large caps, the bound simplifies to.