The astrolabe is made of brass. All letters and numbers are punched. There is no throne or ring for suspending the instrument in the vertical position.
The rete, which rotates freely around the central pin, carries 31 marked star-pointers fashioned as vine leaves for the following stars: 'arios', 'humerus eque', 'Ala equi', 'caput algol', 'aldebora', 'rygyl', 'cygnul orionis' (3 star-pointers), '[...]alhayot[...]', 'ca[...] oion', 'albabor', 'algomiysa', 'ultima rote', 'secunda rote', 'alhore', 'secund eque', 'penulta rote', 'cor leonis', 'pm eque', 'azymech', 'alramech', 'spica', 'corona elfeca', 'cor scorpionis', 'vulur cadens', 'vult volas', '[...]s aque', 'spatula sinist', and 'spatula ale'. Three unmarked star-pointers indicate the positions of Canopus, Rastaban/Eltanin and Enif. There is no ruler.
Under the rete there are two central plates engraved on both sides with a set of almucantars at 3? intervals, azimuth circles at 10? intervals and other positional circles for the latitudes of 44?, 46?, 49? and 52?. On the lower half of each side there is a set of hour lines for determining the time from the sun's altitude. The lugs, for fixing the plates in the correct orientation, have broken off.
The reverse features an altitude scale subdivided into degrees and marked every 10?. Further inwards is a combined zodiac/calendar scale for the Julian calendar. The dates for the equinoxes and the solstices are respectively March 12.1, June 13.8, September 14.8 and December 12.8. The upper central section depicts a diagram for determining the time from the sun's altitude. The lower central section has a mirrored pair of shadow boxes for the horizontal and the vertical shadows (twice labelled 'Vmbra recta' and 'Vmbra versa'). The counter-changed alidade has two upright sighting vanes at each end.
The hour-conversion diagram on reverse contains some faint markings which appear to make out '51g 1Vp', probably the latitude (51 17') for which the astrolabe was once used.
The instrument is unsigned but has all the characteristics of the astrolabes made around the middle of the 15th century after the school of the Paris instrument maker Jean Fusoris (circa 1365-1436). The intermediate form of the numerals '4' and '7' and a comparison of the calendar dates of the equinoxes and the solstices with contemporary solar tables both suggest a date near to circa 1450.
The instrument (IC 536, previously catalogued as Obs A 1) has been on loan from the Leiden Observatory since 1932. Of its history, prior to its discovery around 1868 in the attic of the Leiden University Library, nothing is known.
See R. H. van Gent, The Portable Universe: Two Astrolabes of the Museum Boerhaave (Leiden, 1994), pp.20-28 and p. 39.
Robert van Gent