Under Construction

This instrument is based on the earliest known extant mariner's astrolabe. It was discovered in 2014 as part of the archeological excavation of the wreck of two  Portuguese naus (heavy, 3-4 masted sailing vessels) the Esmeralda and the Sao Pedro, lost to a heavy wind in May 1503 off the coast of modern Oman.* The two ships were part of Vasco da Gama's fourth armada, departing Portugal for India in 1502. From the markings on the astrolabe and the date of departure for the armada, the authors of the description of this instrument date its manufacture between 1496 and 1501. This artifact is the only solid mariner's astrolabe with solid provenance and date. It is the earliest known mariner's astrolabe and the only instrument of this type decorated with national symbols.

Maker's Notes

The instrument is 175mm in diameter and 1.5–2mm thick (6.89" dia. & about 14G). In the paper describing this instrument the authors assume it is a casting due to the thickness at the decorations, however, I am assuming for my recreation that the decorations and center area were made separately from the body and later attached as seen commonly in European planispheric astrolabes made during the medieval period.

Laser scans of the original astrolabe found graduations at 5° intervals in the upper right quadrant of the disc. There were no minor graduations, nor was there evidence of circular arcs or figures. However, the poor condition of the surface due to corrosion leaves it uncertain as to the original  layout.l As a result, in my recreation I am "engraving" my astrolabe based on my literature research on both planispheric and mariner's astrolabes using images and descriptions to guess what the original astrolabe was likely to have looked like. I am also using my experience intuition as a recreator of early navigational and astronomical instruments to determine its final appearance. 

Design constraints/decisions:

• In this recreation, I am working with an artifact which was initially not even recognized as an astrolabe. As my philosophy in building it ’m going to act as if I am a maker of instruments in the 15th century. So how would I make an Astrolabe in the 15th century ? This is obviously an instrument that was made as a token of recognition (King Manual 1's, sponsor of deGama's Indian ocean expeditions,  coat of arms and armillary sphere royal standard are depicted on the back) as well as as an instrument of navigation. It would probably have been made by a classic Astrolabe maker, so I’m going to look at the c. 1500 planespheric astrolabes. I have gone through a number of my astrolabe books, including catalogs from the Smithsonian, the NMM at Greenwich, the Adler planetarium  and the Time Museum. In nearly all cases the instrument was made from a brass sheet or plate. For thicker instruments a cast ring is riveted or soldered on to create the womb of the mater. I’m thus assuming that in the creation of this early Astrolabe, it would’ve been made with a sheet of brass, and then the decorations on the back, the coat of arms and armillary, would have been added as castings or engravings on heavier sheet brass riveted to the main plate. Since I don’t have currently easy access to casting, I’m going to make all of my pieces from brass sheet or plate and then use various techniques to finish/engrave them, etc. 

• Full circles are inscribed completely around the astrolabe to enclose graduations—in all of the examples of mariner's astrolabes the I have been able to look at good pictures of, the arcs for the graduations have extended to full circles whether the instrument is graduated around the full circle, just the upper two quadrants or only one of the quadrants. In addition, circles are traditional if only because of their special status as "perfect" shapes.

• Graduations are made to five degrees for major graduations (about 1/2" long) and to one degree for minor graduations (about 1/8" long). In this case on the original artifact only five degree marks are discernible, with graduation lines up to about 13 mm (1/2") long. Minor graduations along the edge could have been easily lost to corrosion.

• Since it is generally agreed that the mariner's astrolabe was probably derived from the back of a planispheric astrolabe, I decided to  model the alidade of my instrument on those found on planispheric astrolabes. Remarkably, the planispheric astrolabe I made decades ago is the same diameter as the Sodre´astrolabe, so I am using its alidade as a model for this instrument, with the difference that the sights will be fixed rather than the folding sights I made for for my planispheric astrolabe.

• Although figures for the graduations are not discernible on the original instrument I have decided to add them at ten degree intervals, starting with 90° at the vertical position, again using planispheric astrolabes as a model.

• See Mearns, David L., Warnett, Jason M. and Williams Mark A. "An Early Portuguese Mariner's Astrolabe from the Sodre Wreck-site, Al Hallaniyah, Oman. The International Journal of Nautical Archeology (2019) 00.0: 1–12.