Classic Mariner's Astrolabe

This is a replica of an astrolabe found off the coast of Ireland and now held at the Greenwich National Maritime Museum in England. This is the mature Spanish/Portuguese design, c. 1560–1800. Note the heavy open cast body, weighted on the bottom to improve stability with wind and motion. The short distance between sights makes it easier to make observations in a tossing ship.

I had wanted to make a proper cast version of the Mariner's astrolabe for many years, and in fact started this project nearly a decade ago with a different faculty sculpture instructor. Now, two faculty members later, I have finally completed it! (Though I still retain the right to modify it further in the future for artistic reasons.) I have summarized some aspects of the process of making this replica below.

Based on existing mariner's astrolabes and scale drawings in Alan Stimson's The Mariner's Astrolabe, I laid out the astrolabe on a sheet of 3/4" plywood.

The pattern for the casting was then cut out using a jig saw. Since I was originally planning on using sand casting, the edges were cut with 5° relief to allow the pattern to be removed without ruining the edges.

As it turned out, the current sculpture faculty member at our university suggested a lost-wax casting would be more successful and offered to make a mold and wax copies of my pattern. (My heart-felt thanks to Art Professor Sandra Schwetman of Humboldt State University for all of her help in creating the casting.) The wax model was then incased in investment, as seen in the picture.

The wax was then burned out in an oven and the mold filled with molten bronze as seen in the picture taken at the HSU foundry.

After breaking the investment away with a hammer, the main sprue (left in image) was cut off with a hacksaw and the smaller vents on the sides were removed with a bolt cutter (the top vent can still be seen in this image).

After finding the center with dividers and rule, the casting was carefully centered and mounted on a four-jaw chuck for turning the two surfaces flat.The top sprue was cut off before turning.

After mounting the four-jaw chuck on the lathe and checking the centering with the tail-stock center, the surfaces were turned until the oxidized layer was removed.

Partially turned astrolabe still mounted on lathe. A couple of additional passes removing 5-10 thousandths/pass gave a clean and flat surface. The graduated surface was further treated with very fine sandpaper to give a surface where the scribed graduations would be readily visible.

The astrolabe has now been "polished" with fine sandpaper near the rim. The alidade, cut from 1/4" cast bronze from a grave marker, is seen laying across the body. The circles were laid out with trammel points taking multiple light cuts. The trammel points were then used to scribe 60° marks around the final circle as reference points to check the later graduation. Finally the upper 180° was graduated to 1° as described in a photo essay on my dividing engine.

Note the visible layout lines and the center punch-prick on the alidade. They will later be removed during sanding. The basic shape of the alidade was cut out with a heavy cut jeweler's saw blade. The inward curves were created by hand with various round files, while the two slots for site-vanes were cut with a file-edge. The heavy sheet brass vanes were soldered in place with 'easy' silver solder and an oxy-acetylene torch to provide enough heat (the large mass of bronze made it impossible to use a propane torch as I have done in the past with less massive pieces. The color from the heat of soldering (coppery) was left.

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