Editing Solid angle (section)

{{short description|Measure of how large an object appears to an observer at a given point in three-dimensional space}}
{{distinguish|spherical angle}}
{{Infobox physical quantity
| name = Solid angle
| width =
| background =
| image = Angle solide coordonnees.svg
| caption = Visual representation of a solid angle
| unit = [[steradian]]
| otherunits = [[Square degree]], [[spat (angular unit)]]
| symbols = Ω
| baseunits = m<sup>2</sup>/m<sup>2</sup>
| dimension = wikidata
| conserved = No
| transformsas =
| derivations = <math>\Omega = A/r^2</math>
}}
In [[geometry]], a '''solid angle''' (symbol: {{math|Ω}}) is a measure of the amount of the [[field of view]] from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point.
The point from which the object is viewed is called the ''apex'' of the solid angle, and the object is said to ''[[Subtended angle|subtend]]'' its solid angle at that point.

In the [[International System of Units]] (SI), a solid angle is expressed in a [[dimensionless quantity|dimensionless]] [[unit of measurement|unit]] called a ''[[steradian]]'' (symbol: sr), which is equal to one square radian, sr = rad<sup>2</sup>. One steradian corresponds to one unit of area (of any shape) on the [[unit sphere]] surrounding the apex, so an object that blocks all rays from the apex would cover a number of steradians equal to the total [[surface area]] of the unit sphere, <math>4\pi</math>. Solid angles can also be measured in squares of angular measures such as [[Square degree|degrees]], minutes, and seconds.

A small object nearby may subtend the same solid angle as a larger object farther away. For example, although the [[Moon]] is much smaller than the [[Sun]], it is also much closer to [[Earth]]. Indeed, as  viewed from any point on Earth, both objects have approximately the same solid angle (and therefore apparent size). This is evident during a [[solar eclipse]].