For the sailor, it was first used to measure the height of Polaris, the Pole star. Sailors had learned from the Arabs that the Pole Star could also help with navigation; the height of Polaris above the horizon changed depending on a person's Latitude, the distance above or below the equator. Knowing how high Polaris was at a particular port city you wanted to reach would help you navigate your way there.

The Mariners' quadrant is simple to use. Sight the star through the two sight holes. The piece of rope hangs down because of the weight attached (called a plumb bob), and the scale crossed by the rope gives the angular height of the body. With that information, you can obtain your latitude.