The mariner's astrolabe was a brass ring used to observe the altitude of the North Star (or the altitude of the sun if it was daytime). This was a simple wooden or brass disk with degrees marked around its edge. It had a rotating arm with small holes at either end. The disk would be hung vertically from a ring. The user could move the arm until the sunlight shone through the hole at one end and fell on the hole on the other end. The arm then would indicate the altitude by the degrees marked around
the edge of the disk.