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# Steradian

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The **steradian** (symbol: **sr**) or **square radian** is an SI unit. It is like a radian, but for smooth 3D objects, for example, a sphere. A steradian is about one twelfth the surface area of a sphere, regardless of how big the sphere is.^{[1]}

## Sphere vs Steradian

- The surface area of a sphere is [math]4\pi r^2[/math]
- The surface area of a steradian is just [math]r^2[/math]

So a sphere measures 4π steradians, or about 12.57 steradians. Likewise a steradian is 1/12.57, or about 8% of a sphere. And because we measure an angle, it doesn't matter what size the sphere is, it will always measure 4π steradians.^{[2]}

## References

- ↑ Weisstein, Eric W.. "Steradian" (in en). https://mathworld.wolfram.com/Steradian.html.
- ↑ "Steradian". https://www.mathsisfun.com/geometry/steradian.html.