Spherical sector

A spherical sector (blue)

A spherical sector

In geometry, a spherical sector, also known as a spherical cone, is a portion of a ball that is bounded by a spherical cap and the cone that connects the centre of the sphere to the boundary of the cap. It is the three-dimensional analogue of the sector of a circle.

Volume

If the radius of the sphere is denoted by Template:Mvar and the height of the cap by Template:Mvar, the volume of the spherical sector is $$V=\frac{2\pi r^{2}h}{3}.$$

This may also be written as $$V=\frac{2\pi r^3}{3}(1-\cos \phi ),$$ where Template:Mvar is half the cone aperture angle, i.e., Template:Mvar is the angle between the rim of the cap and the axis direction to the middle of the cap as seen from the sphere center. The limiting case is for Template:Mvar approaching 180 degrees, which then describes a complete sphere.

The height, Template:Mvar is given by $$h=r(1-\cos \phi ).$$

The volume Template:Mvar of the sector is related to the area Template:Mvar of the cap by: $$V=\frac{rA}{3}.$$

Area

The curved surface area of the spherical cap (on the surface of the sphere, excluding the cone surface) is $$A=2\pi rh.$$

It is also $$A=\mathrm{\Omega }{r}^2$$ where Template:Math is the solid angle of the spherical sector in steradians, the SI unit of solid angle. One steradian is defined as the solid angle subtended by a cap area of Template:Math.

Derivation

The volume can be calculated by integrating the differential volume element $$dV={\rho }^2\sin \varphi d\rho d\varphi d\theta$$ over the volume of the spherical sector, $$V={\int }_0^{2\pi }{\int }_0^{\phi }{\int }_0^r{\rho }^2\sin \varphi d\rho d\varphi d\theta ={\int }_0^{2\pi }d\theta {\int }_0^{\phi }\sin \varphi d\varphi {\int }_0^r{\rho }^{2}d\rho =\frac{2\pi r^3}{3}(1-\cos \phi ),$$ where the integrals have been separated, because the integrand can be separated into a product of functions each with one dummy variable.

The area can be similarly calculated by integrating the differential spherical area element $$dA=r^2\sin \varphi d\varphi d\theta$$ over the spherical sector, giving $$A={\int }_0^{2\pi }{\int }_0^{\phi }{r}^2\sin \varphi d\varphi d\theta =r^2{\int }_0^{2\pi }d\theta {\int }_0^{\phi }\sin \varphi d\varphi =2\pi r^2(1-\cos \phi ),$$ where Template:Mvar is inclination (or elevation) and Template:Mvar is azimuth (right). Notice Template:Math is a constant. Again, the integrals can be separated.

See also

• Circular sector — the analogous 2D figure.
• Spherical cap
• Spherical segment
• Spherical wedge