The basic geometry by which a sextant works is shown. See http://en.wikipedia.org/wiki/Sextant for details on real-life sextants and their components.

The two red dots can be moved to demonstrate the operation of a sextant.
Question: Look at the arc at the bottom of the sextant. The scale along a sextant's arc shows two times the angle between the sextant's mirrors. In what sense is the angle between object 1 and object 2 really two times the angle between the mirrors? Can you prove it?