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.
© 2002 Adler Planetarium
and Astronomy Museum, Chicago, IL. Reproduced with permission of the copyright
owner. Further reproduction or distribution is prohibited without permission.