- published: 25 Feb 2016
- views: 3071
The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point. A small object nearby may subtend the same solid angle as a larger object farther away (for example, the small/near Moon can totally eclipse the large/remote Sun because, as observed from a point on the Earth, both objects fill almost the same amount of sky). An object's solid angle is equal to the area of the segment of unit sphere (centered at the vertex of the angle) restricted by the object (this definition works in any dimension, including 1D and 2D). A solid angle equals the area of a segment of unit sphere in the same way a planar angle equals the length of an arc of unit circle.
The SI units of solid angle are steradian (abbreviated "sr"). From the point of view of mathematics and physics a solid angle is dimensionless and has no units, thus "sr" might be skipped in scientific texts. The solid angle of a sphere measured from a point in its interior is 4π sr, and the solid angle subtended at the center of a cube by one of its faces is one-sixth of that, or 2π/3 sr. Solid angles can also be measured in square degrees (1 sr = (180/π)2square degree) or in fractions of the sphere (i.e., fractional area), 1 sr = 1/4π fractional area.
Hey now, it is high time we moved onward.
Yeah, we have seen it a thousand times.
Is it so sweet to the taste, this load of junk?
Hey now, this is something special.
Can't you feel it is something special?
Are you anaesthetised by the volume of waste?
TO GET OFF I WOULD BEND INTO ANY SHAPE AT ALL.
We are solid with each other.
Easy to make, hard to destroy:
The make-up of the world we think we know so well.