A marine sextant is used to measure the angle between the horizon and a celestial object. To use the sextant, we look through a small telescope at the horizon mirror.

The horizon mirror allows some light directly through so that on one side we see the horizon. It also has a reflective surface so on the other side we see an image from the index mirror. The index mirror is adjusted, using the index bar, until the image of the object is visible. A fine adjustment is made with the micrometer drum to bring the image of the object to line up with the horizon. The graduated arc and vernier is read to get our sextant altitude. In the animation, you see the image of the lower limb of the Sun is brought to the horizon.