Transitional Mariner's Astrolabe

Since the complex planispheric astrolabe is much earlier than the Mariner's astrolabe, the Mariner's astrolabe is considered to be derived from it. The transitional form of astrolabe would then have the altitude reading scales and an alidade. Only later, with experience, would the cutouts appear to reduce the effects of wind when taking readings on the open sea. The instrument here is modeled after one claimed to have belonged to Christopher Columbus. It is known that he had an astrolabe, but he apparently found it unusable and navigated without it. Details about the original instrument, including scale drawings, may be found in Alan Stimson's book in References. His scale drawings were used in making this instrument, after scaling them up to life-size. The resulting instrument has the same dimensions and appearance of the original, excepting the thickness of the metal: the mater of mine is made from 14 G copper sheet rather than the thick castings normally used in mariner's astrolabes. A recent article describing the earliest known Portuguese astrolabe found in a 1503 shipwreck from the Fleet of Vasco da Gama shows an instrument only 1.5 mm thick (close to 14 gauge), so my replica may be more accurate than I thought!*