### 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 (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 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.
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.

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

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

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

**Referenced by:**

Anglo-Australian Telescope (AAT)

Atacama Cosmology Telescope (ACT)

APM Galaxy Survey (APMUKS)

black-body radiation

BlackGEM

CFHTLS

Cosmic Evolution Survey (COSMOS)

deep field (DF)

deep survey

Dark Energy Survey (DES)

Deep Lens Survey (DLS)

Euclid

FIRST

4MOST

field of view (FOV)

Fsky (f_{sky})

GAMA

GOODS

gravitational wave (GW)

Hamburg/ESO Survey (HE)

Herschel Redshift Survey (HeRS)

intensity

inverse square law

Kepler Telescope

Lockman hole

NOAO Deep Wide-field Survey (NDWFS)

Outer Solar System Origins Survey (OSSOS)

Palomar 48 Inch Telescope (P48)

Planck function

Pre-OmegaTranS (POTS)

radiance

radio source (RS)

equation of radiative transfer (RTE)

Spitzer HETDEX Exploratory Large Area Survey (SHELA)

SkyMapper

STAR-X

stellar association

SuperWASP (SWASP)

surface brightness

Subaru XMM-Newton Deep Survey (SXDS)

2dF Galaxy Redshift Survey (2dFGRS)

VLT Survey Telescope (VST)

XMM-Newton

index