An example of a spherical cap in blue (and another in red.)
3D model of a 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 (the pole) and the edge of the disk forming the base of the cap
The relationship between and is relevant as long as . For example, the red section of the illustration is also a spherical cap for which .
The formulas using and can be rewritten to use the radius of the base of the cap instead of , using the Pythagorean theorem:
Substituting this into the formulas gives:
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 (located on the surface of the sphere) and is the height of each pyramid from its base to its apex (at the center of the sphere). 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
Rotating the green area creates a spherical cap with height and sphere radius .
The volume and area formulas may be derived by examining the rotation of the function
Volumes of union and intersection of two intersecting spheres
The volume of the union of two intersecting spheres
of radii and is
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
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 (with height ) and sphere 1's cap (with height ),
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 (north of the Arctic Circle, at latitude 66.56° as of August 2016) is 2π·63712|sin 90° − sin 66.56°| = 21.04 million km2, or 0.5·|sin 90° − sin 66.56°| = 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.
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