The planispheric astrolabe is made of gilded brass. All letters and numbers are engraved. The instrument is suspended from a ring passing through a swivelling pin fastened to the throne bearing the coat of arms of a descendant of the Antwerp merchant Antonio Anselmo (b. 1536).
The rete, which rotates freely around the central pin, carries 25 marked star-pointers fashioned as flames or as thorns for the following stars: 'Cap And:', 'Venter c?ti', 'Pecte Cassiopea', 'Oculus [Taurus]', 'Hircus', 'Dex hum: Orionis', 'Canis maior', 'Canicula', 'Cor [Leo]', 'Princip: caud Vrse mai:', 'Fund: Vasis', 'Sin: hum: Bootis', 'Spica [Virgo]', 'Arcturus', 'Luc: Coron', '[Scorpio] Cor', 'Caput Herculis', 'Caput Serpent', 'Lyra', 'Aquila', 'Cauda [Capricorn]', 'Crus [Aquarius]', 'Cauda Cygni', 'Hum: Pegasi' and 'Crus Pegasi'. An unmarked (26th) star-pointer indicates the position of Rigel. Most of the star-pointers are numbered on the reverse of the rete. The ruler, rotating over the rete, bears a declination scale and the inscriptions 'Declinatio Sept' and 'Decl. Merid'.
Under the rete there are three central plates engraved on both sides for the latitudes of 40?, 42?, 44?, 48?, 52? and 54?. They are fixed in orientation by a projecting lug and on each side carry a set of almucantars at 2? intervals, a set of azimuth circles at 10? intervals and other positional circles. They all further show the boundary curves for the astrological houses following the system known in the West as that of Regiomontanus.
The mater is engraved with a 'nautical quadrant' after the design of Gemma Frisius bearing the Latin names of the cardinal directions ('SEPTENTRIO', 'ORIENS', 'MERIDIES' and 'OCCIDENS') and the Italian names of the winds ('Tramontana', 'Greco', 'Leuante', 'Syrocho', 'Mezzodie', 'Lebecchio', 'Ponente' and 'Maestrale').
The reverse features a degree scale on the outer rim for reading the altitude of a celestial body with the aid of a counter-changed alidade, marked 'Hore Ortus vel Occasus O' and 'Hypothenuse', which is equipped with sighting vanes at both ends. Further inwards is a combined zodiac/calendar scale for the Gregorian calendar. The dates for the equinoxes and the solstices are respectively March 20.9, June 21.3, September 23.0 and December 21.8. The upper central section depicts a diagram with hyperbolic spiral segments for converting equal hours into unequal hours and vice versa marked for every hour and inscribed 'HOR? IN?:QVA:LES.', 'HORAE ?QVALES.', 'Hor? ante meridiem.' and 'Hor? post meridiem'. The lower central section has a symmetrical pair of shadow boxes for the horizontal and the vertical shadows (twice marked 'Vmbra recta' and 'Vmbra versa')
Below the shadow boxes the instrument is signed 'Elaborabat Antu:erpie Michael Coignet, Anno ? Chri:sto nato 1601.'
The instrument (IC 533, previously catalogued as Obs A 3) has been on loan from the Leiden Observatory since 1931. It was presented to the Leiden University in 1743 by the widow of Samuel Jeremias Garama from The Hague.
See R. H. van Gent, The Portable Universe: Two Astrolabes of the Museum Boerhaave (Leiden, 1994), pp.29-38 and p. 40.
Robert van Gent