How to calculate the solid angle of a spherical rectangle from astronomical angles Say I have 2 astronomical angle pairs defining a confined region on the visible hemisphere:
(minAzimuth, minElevation) & (maxAzimuth, maxElevation)
How can we calculate the solid angle of the projected area of the spherical rectangle on the unit sphere?
I assume that azimuth makes little difference and results in a linear multiple: (maxAzimuth - minAzimuth)/(2*Pi).
My intuition tells me that the elevation factor can be computed as: Sin(maxElevation) - Sin(minElevation)
Thus the answer as a proportion of 1 would be:
(Sin(maxElevation) - Sin(minElevation)) * (maxAzimuth - minAzimuth)/(2*Pi).
and for the answer in steradians multiply by 2*Pi.
Is this correct?
Many thanks for your efforts.
 A: "Yes", the formula in your original question appears to be correct.
Let $S$ be a sphere, $\ell$ a line through the center of $S$, and $C$ the circumscribed cylinder with axis $\ell$. A remarkable theorem of Archimedes asserts that axial projection away from $\ell$ is area preserving.

Assume $S$ is a unit sphere and the region in question is defined by
$$
\theta_{1} \leq \text{Azimuth} \leq \theta_{2},\qquad
\phi_{1} \leq \text{Elevation} \leq \phi_{2}.
$$
The area of the "rectangular" patch on the cylinder, and hence of the "rectangular" patch on the sphere, is
$$
(\theta_{2} - \theta_{1})(\sin\phi_{2} - \sin\phi_{1}).
$$
The fraction of the upper hemisphere covered is, as you initially suspected,
$$
(\theta_{2} - \theta_{1})(\sin\phi_{2} - \sin\phi_{1})/(2\pi).
$$
When you obtained the extra factor of three, is it possible you were computing a volume?
A: Having done the volume integration I have found that the actual answer to this question is:
(Sin(maxElevation) - Sin(minElevation)) * (maxAzimuth - minAzimuth)*(1/3) as a solid angle.
So my intuitive answer was correct. As the volume of a hemisphere with radius 1 comes out as 2*Pi/3, so as a proportion of 1 the answer would be:
(Sin(maxElevation) - Sin(minElevation)) * (maxAzimuth - minAzimuth)*(1/3)/(2*Pi/3)) = (Sin(maxElevation) - Sin(minElevation)) * (maxAzimuth - minAzimuth)/(2*Pi) as a proportion of 1. Or simply (Sin(maxElevation) - Sin(minElevation)) * (maxAzimuth - minAzimuth) in steradians.
