# .

In geometry, a spherical cap or spherical dome is a portion of a sphere cut off by a 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.

The spherical cap is the purple section.

Volume and surface area

If the radius of the base of the cap is a, and the height of the cap is h, then the volume of the spherical cap is

$$V = \frac{\pi h}{6} (3a^2 + h^2),$$

and the curved surface area of the spherical cap is

$$A = 2 \pi r h.$$

The relationship between h and r is irrelevant as long as 0 ≤ h ≤ 2r. The blue section of the illustration is also a spherical cap.

The parameters a, h and r are not independent:

$$r^2 = (r-h)^2 + a^2 = r^2 +h^2 -2rh +a^2,$$
$$r = \frac {a^2 + h^2}{2h}.$$

Substituting this into the area formula gives:

$$A = 2 \pi \frac{(a^2 + h^2)}{2h} h = \pi (a^2 + h^2).$$

Note also that in the upper hemisphere of the diagram, \scriptstyle $$h = r - \sqrt{r^2 - a^2}$$, and in the lower hemisphere $$\scriptstyle h = r + \sqrt{r^2 - a^2}$$; hence in either hemisphere $$\scriptstyle a = \sqrt{h(2r-h)}$$ and so an alternative expression for the volume is

$$V = \frac {\pi h^2}{3} (3r-h).$$

Application

The volume of all points which are in at least one of two intersecting spheres of radii r1 and r2 is [1]

$$V = V^{(1)}-V^{(2)},$$

where

$$V^{(1)} = \frac{4\pi}{3}r_1^3 +\frac{4\pi}{3}r_2^3$$

is the total of the two isolated spheres, and

$$V^{(2)} = \frac{\pi h_1^2}{3}(3r_1-h_1)+\frac{\pi h_2^2}{3}(3r_2-h_2)$$

the sum of the two spherical caps of the intersection. If d <r1+r2 is the distance between the two sphere centers, elimination of the variables h1 and h2 leads to[2] [3]

$$V^{(2)} = \frac{\pi}{12d}(r_1+r_2-d)^2[d^2+2d(r_1+r_2)-3(r_1-r_2)^2].$$

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 (having an axis of rotation), and likewise the ellipsoidal dome is derived from the ellipsoid.

Hyperspherical cap

Generally, the n-dimensional volume of a hyperspherical cap of height h and radius r in n-dimensional Euclidean space is given by [4]

$$V = \frac{\pi ^ {\frac{n-1}{2}}\, r^{n}}{\,\Gamma \left ( \frac{n+1}{2} \right )} \int\limits_{0}^{\arccos\left(\frac{r-h}{r}\right)}\sin^n (t) \,\mathrm{d}t$$

where $$\Gamma$$ (the gamma function) is given by $$\Gamma(z) = \int_0^\infty t^{z-1} \mathrm{e}^{-t}\,\mathrm{d}t$$.

The formula for V can be expressed in terms of the volume of the unit n-ball $$C_{n}={\scriptstyle \pi^{n/2}/\Gamma[1+\frac{n}{2}]}$$ and the hypergeometric function $${}_{2}F_{1}$$or the regularized incomplete beta function $$I_x(a,b)as$$

$$V = C_{n} \, r^{n} \left( \frac{1}{2}\, - \,\frac{r-h}{r} \,\frac{\Gamma[1+\frac{n}{2}]}{\sqrt{\pi}\,\Gamma[\frac{n+1}{2}]} {\,\,}_{2}F_{1}\left(\tfrac{1}{2},\tfrac{1-n}{2};\tfrac{3}{2};\left(\tfrac{r-h}{r}\right)^{2}\right)\right) =\frac{1}{2}C_{n} \, r^n I_{(2rh-h^2)/r^2} \left(\frac{n+1}{2}, \frac{1}{2} \right) ,$$

and the area formula A can be expressed in terms of the area of the unit n-ball $$A_{n}={\scriptstyle 2\pi^{n/2}/\Gamma[\frac{n}{2}]}$$ as

$$A =\frac{1}{2}A_{n} \, r^{n-1} I_{(2rh-h^2)/r^2} \left(\frac{n-1}{2}, \frac{1}{2} \right) ,$$

where $$\scriptstyle 0\le h\le r .$$

Earlier in [5] (1986, USSR Academ. Press) the formulas were received: $$A=A_n p_ { n-2 } (q), V=V_n p_n (q)$$, where $$q= 1-h/r (0 \le q \le 1 ), p_n (q) =(1-G_n(q)/G_n(1))/2,$$

$$G _n(q)= \int \limits_{0}^{q} (1-t^2) ^ { (n-1) /2 } dt.$$

For odd n=2k+1:

$$G_n(q) = \sum_{i=0}^k (-1) ^i \binom k i \frac {q^{2i+1}} {2i+1}.$$

It is shown in [6] that if $$n \to \infty$$ and q/n = const., then $$p_n (q) \to 1- F(\sqrt {q/n})$$ where F() is the integral of the standard normal distribution.

Circular segment — the analogous 2D object.
Solid angle — contains formula for n-sphere caps
Spherical segment
Spherical sector
Spherical wedge

References

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Petitjean, Michel (1994). "On the analytical calculation of van der Waals surfaces and volumes: some numerical aspects". Int. J. Quant. Chem. 15 (5): 507–523. doi:10.1002/jcc.540150504.
Grant, J. A.; Pickup, B. T. (1995). "A Gaussian description of molecular shape". J. Phys. Chem. 99 (11): 3503–3510. doi:10.1021/j100011a016.
Busa, Jan; Dzurina, Jozef; Hayryan, Edik; Hayryan, Shura (2005). "ARVO: A fortran package for computing the solvent accessible surface area and the excluded volume of overlapping spheres via analytic equations". Comp. Phys. Commun. 165: 59–96. Bibcode:2005CoPhC.165...59B. doi:10.1016/j.cpc.2004.08.002.
Li, S. (2011). "Concise Formulas for the Area and Volume of a Hyperspherical Cap". Asian J. Math. Stat. 4 (1): 66–70. doi:10.3923/ajms.2011.66.70..

Weisstein, Eric W., "Spherical cap", MathWorld., derivation and some additional formulas
Online calculator for spherical cap volume and area
Summary of spherical formulas