### Solid Angle

**(Ω, Angular Area)**
(two-dimensional analog of an angle)

A **Solid Angle** (or **Angular Area**) is a size measurement for
a region of possible directions from a point.
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 the radius squared. 4π 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 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 a degree long is actually slightly more than a
*square degree* in size because of the curvature of the surface.

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), Field-of-views, and the areas covered by
Surveys.
Square degrees are often used for the latter, but
steradians are as well.

(*mathematics,measurement*)
http://en.wikipedia.org/wiki/Solid_angle

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

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

**Referenced by:**

Black-Body Radiation

Field of View (FOV)

Intensity

Inverse Square Law

Planck Function

Radiance

Equation of Radiative Transfer (RTE)

Surface Brightness

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