Navigation Teacher Question:

If sight of the natural Horizon is not available, an Artificial Horizon is some means by which an observer can refer to a horizontal plane through his position. With a marine sextant, it is common to use a mirror in the horizontal plane. The angle between the celestial body as seen in the sky and as reflected in the mirror is measured with the sextant. Halving this angle gives the Altitude of the celestial body above the Horizon.