Use
"In the late 18th century, navigators began to use octants to determine their latitude. The octant helped navigators find the precise height of the sun (or, at night, the height of a star) above the horizon. Holding the instrument upright, a navigator would look through the sight, which consisted of one transparent glass window and one mirror. He would first line the instrument up with the horizon by looking through the glass window. Then, he would slowly move the measuring arm, which was attached to the mirror, until the mirror reflected the sun. The resulting measurements on the octant allowed the navigator to calculate the altitude of the sun. By looking up the date and the height of the sun in an almanac, the navigator could then find his ship’s latitude.
