Sodré Astrolabe recreation

The instrument shown in the first two images below is based on the earliest known extant mariner's astrolabe, seen in the next two images. The original 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 squall 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

My intention in making this instrument was to recreate the original as it may have appeared before its loss in the 1503 shipwreck. Since the artifact has limited information due to its age and extensive corrosion my recreation is based on what I know of the craft traditions and abilities of the time of its original making as well as what can be seen in the artifact itself. It is widely assumed that mariner's astrolabes were originally derived from the back of planispheric astrolabes, so the first mariner's instruments should be a simple circular sheet of brass with a throne on top for suspension, at least one quadrant graduated in 90 degrees and an alidade for taking sun and polaris sights.  In all of my construction decisions I am looking at the common featues of European planisheric astrolabes as my examples.

Astrolabe body (mater)

 The authors assume the original was 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 and then attached by rivets or solder to a sheet brass body (mater + throne or kursi) as seen in medieval and early modern European planispheric astrolabes. The original artifact was not initially even recognized as an astrolabe as the graduations were not evident under the corroded surface.   The heavy corrosion may also disguise whether the parts were originally a single piece of metal. Though I use power tools and other modern instruments, all of the operations, other than etching, could be readily accomplished, though more slowly, by a 15th century instruments maker. As instrument examples I looked at c. 1400–1600 European planespheric astrolabes, specifically as seen in modern astrolabe books, including catalogs from the Smithsonian, the NMM at Greenwich, the Adler planetarium  and the Time Museum. In all cases the European instrument base is a brass sheet or plate. To create a womb for an astrolabe, a cast ring and throne is riveted or soldered to the base. 

The Sodré artifact is described in the paper by Mearns et.al. as 175mm in diameter and 1.5–2mm thick (6.89" dia. & about 14g),*  so I started with a piece of 14g brass sheet from my scrap metal stock. The astrolabe center was established by heavily scribing the meridian and equator and punching the intersection. The overall circular shape was scribed with a large (10") antique wing-dividers centered on the punch mark. The throne was then approximated as a triangle at the top of the circle with a ruler and scribe, followed by cutting the overall shape slightly oversize with a jeweler's saw. I cleaned up the overall shape, bringing it to the scribed circular outline, with standard 8" bastard and mill files. The desired throne outline was first drawn on paper with a compass, and transferred to brass with small dividers. The throne was achieved with course and fine round files of various diameters (chain saw sharpening files work well for fine work, with jewelers files for finish work). Full circles were next inscribed completely around the astrolabe to enclose graduations (in all of the well preserved examples of mariner's astrolabes that I have seen 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). I often drill a 1/16" hole through the center punch mark to give a more solid bearing for the dividers, especially if I am making multiple passes for deeper lines (a 1/16" hole is also helpful for alignment in my graduation engine). 

Graduations

For this recreation, graduations were 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 as well as figures could have been easily lost to corrosion. All graduation lines were made with my "dividing engine". I first scribed the longer 5° lines, then went back and filled in the shorter 1° lines. I then marked 10° intervals using standard 1/8" figure stamps. The finished scale  can be seen in the close-up below. After graduation I drilled  3/8" hole precisely in the center of the mater to accommodate the pin for mounting the alidade

Suspension

A 7/32" hole was drilled in the upper throne, centered in the upper half-circle and carefully aligned with the center-line of the astrolabe. I made a stepped rivet such that the shackle would have a close but loose fit to either side of the mater when I hammered the rivet to close the shackle.  The stepped rivet was turned on my engine lathe  from 3/8" rod stock with a 3/8" diameter flat head and a 3/16" diameter shaft. The shaft was reduced to 5/32" at 3/16" length to create a stop when the shackle was closed.  The shackle was then made from a 1/2" wide strip of 14g brass bent over a 1/4" diameter steel rod to create an expanded loop with parallel legs to carry the ring. I drilled a 5/32" hole through the center of the aligned legs, then expanded the hole on the obverse side to 3/16" to fit the rivet body. The legs of the shackle were shaped with files to give a half circle shape. A 1 1/2" diameter brass ring recovered from a purse was slipped into the shackle before attaching it to the throne with the rivet and upsetting the rivet over on the obverse side with a planishing hammer to close the shackle, as seen in the close-up images below

Alidade

Since it is generally agreed that the mariner's astrolabe was probably derived from the back of a planispheric astrolabe, the alidade of my instrument is modeled after those found on planispheric astrolabes. Coinsidentally, the planispheric astrolabe I made decades ago is the same diameter as the Sodre´astrolabe, so I used its alidade as a model for this instrument, with the difference that the sights are fixed rather than the folding sights on the planispheric astrolabe. 

I started with a strip of 14g brass a little over 1 1/8" wide and about 7" long, then filed it to 1 1/8" wide with smooth parallel edges, before painting it with  a blue sharpie pen before scibing the shape to be cut out. Using a machinist square and dividers I determined the centerline and center of the alidade and carefully scribed the centerline with a surface gauge and surface plate, holding the brass strip perpendicular to the plate with a 1-2-3 block. I used a punch to mark the center and a small divider to lay out the quarter arcs, as seen in the top image below. The alidade was then cut a bit oversized and finished with files to exact dimensions to create precise and accurate fiducial edges at the centerline of each side and parallel to the outside edges. The arc edges were then filed to give clean, smooth edges following the scribed lines. After filling a 1/16" pilot hole was drilled at the center of the alidade prior to drilling the center out to 3/8" to fit the 3/8" dia pin (see below). The alidade was then mounted on the mater and scribed on both ends to match the outer diameter of the mater before cutting to length, creating a shoulder and then carefully filling to to give a precisely symmetrical alidade. I then scribed lines with a small machinists square set at 1/2" to scribe lines at identical distances from each end and then at two exact distances in from the outer edge to locate the holes that will be used to attache the sights with integral rivets. I then drilled holes with a 1/16" drill, followed by chamferring on the astrolabe side of the alidade to allow the rivets to mushroom out and secure the sights.

In parallel with the alidade I cut two pieces of 14g brass 1 1/8"+ by 7/8"+ and co-filed them to exactly 1 1/8" x 7/8", precisely square and parallel to create the two sights. Using the same set up of surface gauge and surface plate as above, I scribed a line on each sight that would align with the fiducial edge when the sight outer edge is matched with the outer edge of the alidade, this line will later be used to establish the location of each sight hole. I then used the small machinist square set as above to scribe lines to locate the two rivet posts that will be created on each sight by filing. Using mill bastard and needle files I first removed material to make 1/16" square posts about 3/32" high, checking alignment with the alidade holes as the work progressed. I then used needle and riffler files to round the posts to fit the holes in the alidade. After carefully fitting the sights the posts were lightly mushroomed with a planishing hammer and then filed flat to the surface of the alidade as seen in the image below.

Horse and Pin

I turned the center pin with a 5/8" dia, 1/8" high head a 3/8" dia shaft about an inch long from 5/8" brass roundstock on my engine lathe. I then drilled a 1/16" hole carefully centered on the shaft about 1/4" (the combined thicknesses of the washer, mater and alidade) from the head and another in the same line 1/4" further along. Using a jewelrs saw and then needle files I connected the holes to give a 1/16" wide slot to fit the horse wedge to hold the alidade to the astrolabe. The slot was filed to fit the horse when the astrolabe was assembled to give a tight, stable fit.

The horse was initially cut with a jewelers saw from 14G brass, then finished with needle files to a satisfactory shape. After the pin  was placed into the astrolabe, with a modified commercial brass washer from my stash, the horse was file to create a flat or slight hump to ensure a secure fit when the instrument is fully assembled. 

My granddaughter had found a reference to 2026 being the year of the fire horse in the Chinese calendar, so I thought it would be appropriate to give my horse ruby eyes, since I already had a few  0.118" dia ruby spheres recovered from HPLC check valves years ago.  The ruby sphere eye was press-fit into a slightly undersized #32 (0.116" dia) drilled hole.

Armillary device and Portuguese Coat of Arms

A unique and surprising aspect of this early instrument was the presence of the two medallions on the astrolabe back. Because of the poor condition of the Sodré astrolabe I put out a call on rete, a scientific instrument web group, to find contemporary images of King Manuel I's coat of arms and armillary device. The images I used to create my versions of the coat of arms and armillary are shown below.***

Consistent with European astrolabe manufacturing traditions, the medallions and heavy washer would have been added after being cast and then riveted or soldered onto the mater. Unfortunately I do not currently have access to brass casting, so as a substitute I determined that I would fabricate blanks for all three from 1/8" flattened brass sheet I had in stock (originally 4" brass pipe) left over from the torquetum I made in 1999, then use etching (which would not have been available in Europe in 1500) as a substitute technique.

The circles for the armillary and washer were cut with an adjustable arm circle cutter on my drill press (for the armillary I removed the center drill, requiring a slower cutting rate and cutting from one side only) to give a 1 3/8" 8G washer with a 1/4" center hole a 1 7/8" 8G brass disk. Both were then edge finished and lightly chamfered with a mill bastard file and flattened with rubber and dead-blow hammers to assure a seamless fit to the back of the astrolabe. The washer was finished by drilling out the center hole to 3/8" and chamfering both sides of the hole by hand with a  45° countersink. 

For the armillary I then drew the curves and lines to approximate the pattern on the Sodré artifact with Sharpie™ permanent markers as a resist, referring to the illumination from the Gradual of Manuel I, seen below, to clarify what the original image may have looked like. Having covered the back and sides with electrical tape, I then suspended the disk in a commercial ferric chloride PCB etchant solution as per bottle instructions. The etched armillay is seen below.

The blank for the coat of arms was rough cut from the brass sheet with a power hand jig saw using a metal cutting blade.  I then used bastard, nmill bastard, and course and fine round files to achieve the final shape seen tin the first image below. The layout of the shield was then transferred with a punch from images found on the web for the c. 1500 Portuguese coat of arms. The crown was hand drawn using web images appropriate too Manuel I's rule with the design on a Portugues coin (image below) as a guide to level of acceptable detail. The result before etching is seen in the third image below. 

After ethcing and post etch cleanup, I affixed the medallions to the back of the astrolabe with rivets made from short lengths of 1/16" brass rod. Since the pin would hold it in place I did not bother permanently affixing the washer. The final step for my project was to sign and date it, using standard 1/8" stamps (punches), as seen in the image below. 

A final note: I have not polished or heavily cleaned the brass used in fabricating this project. This is an aesthetic/artistic choice. I enjoy the appearance of brass in its many states from rough to polished to corroded, and I recognize that cleanup/polishing is an irreversible process that can always be accomplished later.

*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.

**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. 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 and intuition as a recreator of early navigational and astronomical instruments to determine its final appearance.

***I would like to thank the various colleagues from the scientific instrument community who responded to my request on rete for help in finding contemporary images of Manuel's coat of arms and personal device on the internet. Their comments and directions significantly helped my search for relevant images I used to create medallions on the back that reflect what were likely on the original astrolabe.