This instrument, signed 'IOANNES BOS. FECIT 1591', was identified as a fake in the 1950s. It falls among a batch of instruments of questionable authenticity that emerged on the market in Amsterdam between the World Wars from about 1925.
The throne and limb of this instrument are soldered to the mater. There is only one silver plate, engraved on one side only, with a stereographic projection for latitude 39º. There are multiple repetitions of some of the star names on the rete, which should have alerted potential buyers that this is not an authentic 16th century piece. On the back of the astrolabe is a combined scale for degrees and the zodiac and a calendar scale. The rest of the back is divided into two halves: the upper showing the arcs for the unequal hours and the lower containing a shadow square. The (fake) signature and date appear underneath this. The sights on the alidade are attached with pivots so that they can be folded flat.
Credit: National Maritime Museum, Greenwich, London, Caird Collection