| Instrument | : | Mariner's astrolabe |
| Manufacturer | : | Ars Mechanica (G. Oestmann) |
| Country of origin | : | Germany (Original: Iberian) |
| Manufacturing year | : | 2010 (Original: 1580s) |
In 2006 I met clockmaker Günther Oestmann of Ars Mechanica at the National Maritime Museum in Greenwich. He then had a talk on casting an astrolabe, which he had recently done. After the talk we discussed the possibility to create an Iberian astrolabe for my collection. The museum kindly supplied details of the Valentia astrolabe (NAV0022) in their collection, which I used to create an AutoCAD drawing (see fig. 17). This I subsequently sent to Günther who then had the body cast and built the instrument. During the A Sense Of Direction conference in May 2010 we met again and I was presented the astrolabe.
The mariner's astrolabe was one of the first actual altitude measuring devices at sea. Its predecessors were the quadrant and the kamal. Earlier quadrants were however not yet used with a limb divided in degrees, but - alike the kamal - were used in a relative way with marks for the individual ports. The mariner's astrolabe developed during the 15th century out of the astronomical astrolabe and its earliest recorded use at sea dates from 1481 when it was used in a voyage by Diogo d'Azambuja down the west coast of Africa.1 Eventually the instrument was replaced by the cross-staff and backstaff instruments like the Davis quadrant, the demi-cross and the hoekboog (double triangle).
Mariner's astrolabes were generally made of bronze or brass, but wooden ones are known to have existed.2 The one created for me was cast from a modern bronze alloy (CuSn10-C) to distinguish it from an original (see fig. 4), has a bronze alidade with brass vanes and a brass hinge (see fig. 5) and weighs approximately 2.7 kilograms, slightly more than the original. The alidade is held in place by a pin (see fig. 8) that is locked by a key or horse (see fig. 9) similar to 16th century examples (in the 17th century a wing-nut was more commonly used).3 The vanes are pierced for sun observations only. The limb is marked for zenith distance (see fig. 7).
As observations are done relative to the local vertical (see fig. 11), there is no need for a horizon to function. This allows the instrument to be used at any place as long as the sun or polar star can be seen from it. In practise this meant that - for sun observations - the instrument could be used below deck near the vessel's centre of gravity with the sun visible through a hatch. In this way the astrolabe (and the observer) was less affected by lateral movement, thus making observing easier and more accurate. The observer would sit down, rest his arms on his knees, suspend the instrument between his legs and take his observations.
The first observations with the replica were done outside the National Maritime Museum at the colonnades to the Queens House on May 7th 2010 around noon. From these observations a 20 arc minute instrumental error was found (between face left and face right), which was later corrected. The average was no more than 3 arc minutes off (3 nautical miles). On May 14th 2010 another test, consisting of 100 observations in my sheltered back garden, showed that the instrumental error was adequately dealt with. Around 3 arc seconds, the instrumental error between face left and face right is now negligible, although an error between the pointers remains of about 3.6 arc minutes. The latter - just as well as the instrumental error between the faces - will however average out when observing in two faces and with both pointers as one would do when observing with a theodolite or transit. The only errors left are scale and observer's errors (see fig. 13), in this case still around 10 arc minutes (0.2mm average displacement of the projected sun on the lower vane). The standard deviation calculated over all observations was 10 arc minutes as well (1σ, 68%).
In order to investigate the cause of this average error I took another series of measurements. This time I observed in two different fashions: forward and backward, e.i. I started with facing the sun and looking at the astrolabe from above, taking eight observations. Then I took eight observations with my back towards the sun and holding the astrolabe at about eye-level so I could look at the alidade from an angle mirrored to the one in the forward fashion (see fig. 15). What I suspected came out: when observed backward I get readings too low, when observed forward they are too high (see fig. 14). The cause of this deviation lies in the shape of the pinnule. Due to the drilling process a small burr will appear around the pinnule. In order to get the hole smooth again the burr was taken off using a countersink. This resulted in a small bevel along the edge of the hole (see fig. 16). Although minute and not visible to the naked eye at only 0.15mm, this is large enough to cause a 7 to 8 arc minute observer's error. Whether or not period instrumentmakers used the same technique to get rid of the burr is (yet) unknown, but if they did it in the same way (the alternative is to sand it off) then period instruments would have the same problem. As the 16th and 17th century navigators would only observe in a forward fashion, this would then cause them to get values too high for the sun's altitude. Which on itself would result in latitudes too low and hence a course steered too high when running down the latitude. From this second test an average error of 4 arc minutes with a standard deviation of 13 arc minutes (1σ, 68%) was calculated. The instrument error was again found to be less than one arc minute, with 3.4 arc minutes between the pointers.
After much deliberation I decided to remove the bevelled edge by drilling the hole to 1.3 millimetres (it was 1.0 millimetre originally). It immediately became clear that the instrument performed much better. A series of 40 forward observations resulted in an average error of 4 arc minutes with a standard deviation of 8 arc minutes (1σ, 68%). This proofs that John Davis was right that he stated that '...it is an excellent Instrument, being rightly understood and operated...', at least on land that is.4
I'm very grateful to Richard Dunn National Maritime Museum in Greenwich) as without his details of the Valentia astrolabe in their collection the reproduction of this instrument would not have been possible. Of course I want to thank Günther Oestmann of Ars Mechanica for creating the astrolabe.
If you like to know more, don't hesitate to contact me.
[1]: A. Stimson, The Mariner's Astrolabe, A Survey of Known, Surviving Sea Astrolabes, (Utrecht, 1988), p. 16.
[2]: idem, pp. 15-16.
[3]: idem, p. 24.
[4]: J. Davis, The Seamans Secrets, Devided into two parts, wherein is taught the three kindes of Sayling, Horizontal, Paradoxal, and Sayling upon a Great Circle. Also an Horizontle Tyde-Table for the easie finding of the Ebbing and Flowing of the Tydes, with a Regiment newly Calculated for the finding of the Declination of the Sun, and many other most necessary Rules and Instruments not hereforte set by any. Newly Corrected and ammended, and the Eigth time printed., (London, 1657).