Mariners astrolab, bronze

In the 1400s, the mariner's astrolabe was developed as the replacement for planispheric astrolabes used at sea and saw continued use until the 1700s. It was a brass disk that was marked in degrees and had a spinning alidade. The distinctive large holes in the disk were to reduce its movement in light winds. It was designed to have a much simpler function and did not contain several moving disks which the planispheric astrolabe had that were difficult to utilize on a tossing ship.

Even though the mariner's astrolabe never fully replaced the quadran> (the quadrant was much too versatile) one of its biggest benefits is that you did not have to look directly at the sun to take a measurement like was needed with cross staffs and some quadrants.



The design of this coin:
The image of the navigator on the front of the Mariner's Astrolabe Geocoin™ shows how this tool was used when taking a reading from the sun. We also incorporated a raised metal edge around the coin. A real antique mariner's astrolabe would not have this raised edge, but we thought it was important to provide some protection to the spinning alidade. Instead of attaching a large metal ring for the handle, which were typical of old style astrolabes, we attached a rotating clip to make it easier for hooking the astrolabe to a stable item such as a post when taking a reading. The two holes on the spinning alidade are small enough to get a fairly precise reading from sunlight, but are large enough to see through when taking a reading of the North Star, Polaris.