Construction of Regular Icosahedron within Given Sphere/Porism

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Porism to Construction of Regular Icosahedron within Given Sphere

In the words of Euclid:

From this it is manifest that the square on the diameter of the sphere is five times the square on the radius of the circle from which the icosahedron has been described, and that the diameter of the sphere is composed of the side of the hexagon and two of the sides of the decagon inscribed in the same circle.

(The Elements: Book $\text{XIII}$: Proposition $16$ : Porism)


Proof

Immediately apparent from the construction.

$\blacksquare$


Historical Note

This proof is Proposition $16$ of Book $\text{XIII}$ of Euclid's The Elements.


Sources