Astrophysics (index) about

### 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 (fsky)
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)
VIRMOS-VLT Deep Survey (VVDS)
XMM-Newton

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