Mariner's Astrolabe

A mariner's astrolabe is a simpler instrument compared to an astronomical astrolabe, having only to measure the height of celestial bodies above the horizon. It is basically a graduated circle of bronze or brass (there were some made of wood too) with large holes in its inner section and a rotating alidade with tiny pinholes called pinnules. To view the sun, the instrument was held aloft and the alidade rotated until the sun's ray passed through the upper pinnule and cast onto the lower.