### 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, 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.

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

Referenced by pages:
Anglo-Australian Telescope (AAT)
Atacama Cosmology Telescope (ACT)
APM Galaxy Survey (APMUKS)
BlackGEM
CFHTLS
Cosmic Evolution Survey (COSMOS)
cross section
deep field (DF)
deep survey
Dark Energy Survey (DES)
Deep Lens Survey (DLS)
Euclid
FIRST
4MOST
field of view (FOV)
Fsky (fsky)
GAMA
GOODS
gravitational wave (GW)
Hamburg/ESO Survey (HE)
Herschel Redshift Survey (HeRS)
High Energy Transient Explorer (HETE)
intensity
inverse square law
jansky (Jy)
Kepler Telescope
Lockman hole
Livermore Optical Transient Imaging System (LOTIS)
LaSilla-QUEST Variability Survey (LSQ)
Millennium Galaxy Catalogue (MGC)
Munich Near-Infrared Cluster Survey (MUNICS)
Multiwavelength Survey by Yale-Chile (MUSYC)
NOAO Deep Wide-field Survey (NDWFS)
Outer Solar System Origins Survey (OSSOS)
Palomar 48 Inch Telescope (P48)
Planck function
Pre-OmegaTranS (POTS)
PRIMUS