Despite their resemblance, the nautical astrolabe has very little in common with the plane astrolabe, the most versatile instrument for astronomical calculation, in use since the sixth century. The nautical astrolabe is very heavy and carries perforations to keep it from oscillating with the ship's movements or with the wind. It features a circular graduated scale and a rotating arm with two perforated sights. The instrument's sole function is to measure the altitude of celestial bodies above the horizon. For the Sun, the observer rotates the arm until the shadow of the upper sight is cast on the lower sight. This alignment causes the ray of sunlight to pass through the large hole on the upper sight and strike the much smaller hole exactly at the center of the lower sight. For the North Star, the observer aims at the star through the holes in the sights. The star's altitude is roughly equal to the latitude reached during the navigation.