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Mariner's Astrolabe Geocoin 🧭 Mariner's Astrolabe Geocoin

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team-bison66 Send Message to Owner Message this owner
Thursday, 30 April 2020
Nordrhein-Westfalen, Germany
Recently Spotted:
In the hands of the owner.

This is collectible.

Use TB91V7Z to reference this item.

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Only to discover.

About This Item

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 quadrant (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.


How to use it:

Taking a latitude reading from the North Star, Polaris:
1. Grasping the ring, hold the astrolabe up in the air towards the North Star.

2. Line up the holes in the spinning alidade so that you can see Polaris through them.

3. Take the reading from the degree scale and you now have your latitude.

Measuring the declination of the sun: 1. Grasping the top ring, hold the astrolabe to about chest high.


Holding Mariner's Astrolabe at about chest high for a sun declination reading

2. Looking down on the alidade, let the sun shine directly though the upper alidade hole.


How to align the holes of the alidade

3. Adjust the alidade so that the beam of sunlight shinning through the upper hole passes through the lower hole. The picture below shows how the light and shadow from the upper alidade look when slightly misaligned with the lower alidade hole.


How the light from the upper alidade hole looks when misaligned on the lower alidade.

4. Now you can take a reading from the degree scale and see the declination of the sun. In this example, the degree scale shows the sun is at 11 degrees. These example photographs show the angle of the sun during the evening to give better contrast of the sunlight passing though the hole. When using the Mariner's Astrolabe Geocoin™ for navigation, you will want to take a reading at high noon.


Reading this example of the degree scale shows the sun's declination to be 11 degrees.

*Warning* Never look directly at the sun. Sighting the sun directly though the holes into your eye can cause blindness.

Gallery Images related to 🧭 Mariner's Astrolabe Geocoin

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Tracking History () View Map

  • 01-03 of 03 records ·
  • 01
Discovered It 12/22/2020 little-leggs discovered it   Visit Log

Discovered by " My eye's Only "
Thank you to
For sharing this one and some with Me here in sunny




Move To Collection 05/10/2020 team-bison66 moved it to their collection   Visit Log
Discovered It 05/05/2020 Killekalle discovered it   Visit Log

Diese tolle Coin konnte ich heute beim Owner bewundern.

Gruß an team-bison66 und vielen Dank fürs Zeigen.

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  • 01-03 of 03 records ·
  • 01

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