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How can I calculate the solid angle that a sphere of radius R subtends at a point P? I would expect the result to be a function of the radius and the distance (which I'll call d) between the center of the sphere and P. I would also expect this angle to be 4π when d < R, and 2π when d = R, and less than 2π when d > R.

I think what I really need is some pointers on how to solve the integral (taken from wikipedia) $\Omega = \iint_S \frac { \vec{r} \cdot \hat{n} \,dS }{r^3}$ given a parameterization of a sphere. I don't know how to start to set this up so any and all help is appreciated!

Ideally I would like to derive the answer from this surface integral, not geometrically, because there are other parametric surfaces I would like to know the solid angle for, which might be difficult if not impossible to solve without integration.

*I reposted this from mathoverflow because this isn't a research-level question.

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This is too simple a situation to apply calculus to. You know that the area (solid angle) of a circular cap of (angular) radius $\rho$ is $2\pi(1-\cos\rho)$, and you know that your ball of radius $R$, at a distance $d$ has an angular radius of $\rho=\arcsin(R/d)$ as seen from your point $P$. The proper formula falls out, valid only when $R\le d$.

You do realize, I hope, that what you need to parametrize is, in the words of the Wikipedia entry, “the projection of the surface $S$ to the unit sphere with center $P$”, and not anything on your ball of radius $R$. This projection is always a circular cap, as I say above; for a more complicated surface in space, it may be harder to describe the projection on the unit sphere.

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    Welcome to the site Professor Lubin!2011-10-17
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    I don't see why I have to parameterize in terms of the projection... Doesn't the integral sum up little projected areas, of a surface with arbitrary shape in space? Thanks for your help.2011-10-17
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    I think my question is actually very similar to [this one](http://mathoverflow.net/questions/63129/how-do-you-calculate-the-solid-angle-of-a-rectangular-axis-aligned-section-of-a) but for a different shape. It sure looks like the surface, not it's projection, is parameterized.2011-10-17
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    That paraboloid example involves a different shape, already given with a parametrization. In your case, you'd have to get a parametrization of the visible part of the viewed sphere. Much messier, don't you agree?2011-10-17
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Solid angle $(\omega)$ subtended by a sphere, having a radius $R$ at any external point lying at a distance $d$ from the center, is given as $$\omega=2\pi\left(1-\frac{\sqrt{d^2-R^2}}{d}\right)$$ Where, $d\geq R$

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Just to say, that the derivation of the formula given by Harish Chandra Rajpoot is here

https://www.academia.edu/8468361/HCRs_Derivations_of_Solid_Angle_subtended_by_the_Rotatory_Solids_at_a_point