Definition: Def. 11.11: Solid Angle

(Definition 11 from Book 11 of Euclid's “Elements”)

A solid angle is the inclination (constituted) by more than two lines joining one another (at the same point), and not being in the same surface, to all of the lines. Otherwise, a solid angle is that contained by more than two plane angles, not being in the same plane, and constructed at one point.

Modern Formulation

(not yet contributed)

Proofs: 1 2 3 4 5 6 7
Propositions: 8 9 10 11

Thank you to the contributors under CC BY-SA 4.0!



Adapted from (subject to copyright, with kind permission)

  1. Fitzpatrick, Richard: Euclid's "Elements of Geometry"

Adapted from CC BY-SA 3.0 Sources:

  1. Prime.mover and others: "Pr∞fWiki",, 2016