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A **solid angle** (or **angular area**) is a size measurement for
a region of possible directions from a point,
which is equivalent to regions on the surface of a sphere.
For example, a region of a celestial sphere has
a size, and the concept of solid angle quantifies
that size.
One unit of a solid angle is the **steradian** (for **square radian**,
which is adopted as a SI unit): the area on the surface of a
sphere of its radius squared. 4π (roughly 12.6)
steradians cover a whole sphere.
Another unit, the **square degree** is similarly defined for
a degree: a degree has a length along a circle of
π/180 radii and a square degree is that value squared.
A square-like shape on the surface of a sphere,
where each edge is a degree in length following a great circle,
is actually slightly more than a
*square degree* in size because of the curvature of the surface.
Roughly 41253 square degrees cover a whole sphere and roughly
13283 square degrees cover a *steradian*.

Solid angles are used in the definition of some electromagnetic radiation measurements, and equations modeling spherical objects such as stars. They are also cited to describe the apparent size of astronomical objects (galaxies or nebulae), fields of view, and the areas covered by surveys. Square degrees are often used for the latter, but steradians are as well.

http://en.wikipedia.org/wiki/Solid_angle

http://en.wikipedia.org/wiki/Steradian

http://en.wikipedia.org/wiki/Square_degree

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